-Highway Drainage Guidelines-American Association of State Highway and Transportation Officials (AASHTO) (2007)

HIGHWAY DRAINAGE
GUIDELINES
2007
American Association of State
Highway and Transportation Officials
444 North Capitol Street, N.W., Suite 249
Washington, DC 20001
(202) 624-5800
www.transportation.org
© 2007, by American Association of State Highway and Transportation Officials. All rights
reserved. This book, or parts thereof, may not be reproduced in any form without written
permission of the publisher. Printed in the United States of America.
ISBN 978-1-56051-292-9
ii
Preface
As early as 1866, Congress first authorized the use of the metric system, devised in France about the
time of the French Revolution, for measuring weights in the United States. International standardization
began in Paris in 1875 when the International Bureau of Weights and Measures (IBWM) was established
under the jurisdiction of the General Conference on Weights and Measures (CGPM). In 1960, the CGPM
adopted an extensive revision and simplification called “System International d’Unites”, which is universally
known by its abbreviation of SI.
In 1968, Congress adopted an act requiring a United States metric study. The report to Congress in 1971
recommended that the nation change to the SI system deliberately and carefully. Two factors mandated this
change—the adoption of the metric system by the remainder of the world and the changing global economy.
The resultant Metric Conversion Act of 1975 (15 U.S.C. 2056) declared a national policy of coordinating
and encouraging the increased use of the metric system and provided for a U.S. Metric Board to coordinate
the voluntary conversion to the metric system. As the trend of U.S. industries losing their share of world
markets continued, Congress attempted to keep the United States competitive in the international trade arena
by adopting the Omnibus Trade and Competitiveness Act of 1988. Section 5164(b) of the Trade and
Competitiveness Act amended the Metric Conversion Act of 1975 to declare that the metric system is the
preferred system of weights and measures for U.S. trade and commerce. It also required each Federal
agency to convert to the metric system by the end of fiscal year 1992.
On May 3, 1990, the U.S. Department of Transportation issued Order 1020.1C which established policy
and administrative procedures for the transition to metric. As a result, the Federal Highway Administration
(FHWA) formed a Metric Work Group which developed a conversion plan and a timetable. The Metric Work
Group found the implementation deadline of 1992 to be impracticable and proposed a five-year schedule. On
this basis, an NCHRP Project 20-40 was established by the American Association of State Highway and
Transportation Officials (AASHTO) in order to convert documents published by the association to the
International System of Units (SI) of measurement by October 1, 1996, to comply with the FHWA mandates.
Since the federal conversion date for these mandates has been left optional for States, this guideline is
being provided in dual units. The SI number and unit are shown first followed by the U.S. Customary number
and unit in parentheses, e.g., 1 m (3.3 ft). If the U.S. Customary number is not a direct conversion and is a
comparable value to the SI measurement, a bracket is used, e.g., 1 mm [1 inch].
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GLOSSARY OF
HIGHWAY-RELATED DRAINAGE TERMS
Glossary of
Highway-Related Drainage Terms
G.1 OVERVIEW
This glossary is divided into three parts:
Overview,
Glossary, and
References.
It is not intended that all the terms in this glossary be rigorously accurate or complete. Realistically,
this is impossible. Depending on the circumstance, a particular term may have several meanings; this
can never change.
The primary purpose of this glossary is to define the terms found in the Highway Drainage
Guidelines and Model Drainage Manual in a manner that makes them easier to interpret and
understand. A lesser purpose is to provide a compendium of terms that will be useful for both the
novice and the more experienced hydraulics engineer. This glossary may also help those who are
unfamiliar with highway drainage design to become more understanding and appreciative of this
complex science and facilitate communication between the highway hydraulics engineer and others.
Where readily available, the source of a definition has been referenced. For clarity or format
purposes, cited definitions may have some additional verbiage contained in double brackets [ ].
Conversely, three “dots” (. . .) are used to indicate where some parts of a cited definition were
eliminated. Also, as might be expected, different sources were found to use different hyphenation and
terminology practices for the same words. Insignificant changes in this regard were made to some
cited references and elsewhere to gain uniformity for the terms contained in this glossary, e.g.,
“groundwater” vs. “ground-water” or “ground water,” and “cross section area” vs. “cross-sectional
area.”
Cited definitions were taken primarily from two sources—W.B. Langbein and K.T. Iseri’s “General
Introduction and Hydrologic Definitions” (36) and a draft of a glossary being developed by the
Interagency Hydrology Committee. A few cited definitions were considered to be partially outdated;
corrections were suggested where this occurred. Future plans of the AASHTO Task Force on
Hydrology and Hydraulics are to integrate the current American Society of Civil Engineers’ Glossary
of hydrologic terms with this glossary.
Highway Drainage Guidelines
G-2
Many terms are explained and defined in several ways—sometimes with considerable detail. This was
done intentionally in order to:
facilitate understanding for those who respond better to a particular written format or detail of
explanation,
highlight actual or apparent contradictions in current terminology,
avoid or minimize litigation problems from overly restrictive definitions, and
selectively augment certain subject matter in the AASHTO Highway Drainage Guidelines and
Model Drainage Manual.
Some terms included in this glossary will have limited application to highway drainage design. They
have been included in order to:
facilitate communication with other, related sciences; and
preclude confusion with similar but different hydraulics-related terms.
As often happens in any science, some practitioners have different names for the same thing. Every
attempt has been made to sort out these colloquialisms and synonyms and assign all the definitions to
one term. Cross referencing of these terms was attempted but is unlikely to ever be complete or to
satisfy all practitioners. It is anticipated that errors and oversights will be resolved with revisions of
this glossary. The reader is encouraged to submit their experience with this glossary to the AASHTO
Task Force on Hydrology and Hydraulics. In particular, the following information is solicited:
proposed terms and definitions,
proposed revisions of present terms to include reasons where appropriate, and
problems (particularly legal) with the present definitions.
In cross referencing, two terms are used—“See” and “Compare with.” The term “see” usually means
one of two things: (1) The definition is provided under a different term, or (2) the cross-referenced
term provides additional information. The term “compare with” infers that the cross-referenced term
may be at variance with or an antonym of the defined term; judgmental decisions may be needed in
such cases.
This glossary also attempts to “package” similar terms having two or more words. This provides the
added advantage of facilitating a comparison of terms without flipping back and forth between pages.
As an example, see the section of this glossary that addresses “streams,” “gages,” “probability
distribution,” or “weir.” In some instances, this might prove inconvenient and add some length to the
glossary, but it was felt the advantages outweighed the disadvantages. An exception was made where
a particular term might best be left with another, smaller grouping of similar terms.
The scientific and, in particular, the regulatory world is inundated with acronyms and abbreviations.
This glossary attempts to define some of the more familiar acronyms and abbreviations likely to be
encountered by the highway hydraulics engineer. In a few instances, they have different meanings
(see “TW,” for instance). A few hydraulic variables and equations are also included.
Glossary
G-3
And, finally, because this is a glossary of transportation-related hydraulic terms, an attempt has been
made to provide this unique focus without distorting a term’s meaning.
G.2 GLOSSARY
The following terms are provided to facilitate the application of the AASHTO Highway Drainage
Guidelines and Model Drainage Manual.
AASHTO. Acronym for American Association of State Highway and Transportation Officials.
ablation. The process by which ice and snow waste away from melting and evaporation or by which
land wears away by the action of surface water.
abrasion. Removal of stream bank material due to entrained sediment, ice, or debris rubbing against
the bank. Compare with erosion, scour, and mass wasting.
absorption. The assimilation or taking up of water or other solutions by soil or other material, i.e.,
the entrance of water into the soil or rocks by all natural processes. It includes the infiltration of
precipitation or snowmelt; gravity flow of streams into the valley alluvium (see storage, bank), sinks,
or other large openings; and the movement of atmospheric moisture. The process by which substances
in gaseous, liquid, or solid form dissolve or mix with other substances. Compare with adsorption.
abstraction. That portion of rainfall that does not become runoff. It includes interception, infiltration,
and storage in depressions. It is affected by land use, land treatment and condition, and antecedent
soil moisture.
abutment. The superstructure support at either end of a bridge or similar type structure, usually
classified as spillthrough or vertical. Considered part of the bridge substructure. See spillthrough
abutment and vertical abutment.
acre-foot. The quantity of water required to cover 1 acre to a depth of 1 foot and equal to 43,560
cubic feet or approximately 326,000 gallons or 1233 m3. Abbreviated as af. See hectare-meter.
accretion. Build-up of beach due to wave and wind action.
act (acts). Written law, such as an Act of Congress.
Act of God. In law, a direct, sudden, or irresistible action of natural forces that could not reasonably
have been foreseen and prevented.
action (highway). A highway action as it pertains to drainage design is any construction,
reconstruction, rehabilitation, repair, or improvement in a watershed or on the transportation system
that measurably changes such things as floodplain limits, established flood patterns, and runoff
characteristics or that requires a design or review storm system to accommodate storm runoff.
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adsorption. The adhesion in an extremely thin layer of molecules (such as gases, solutions, or
liquids) to the surface of solid bodies or liquids with which they are in contact. Compare with
absorption.
adverse slope. See slope, adverse.
aeration zone. See zone of aeration.
aerobic. A condition in which molecular oxygen is a part of the environment.
af. Acronym for acre-foot. Common abbreviation is ac-ft.
afflux. Backwater or height by which water levels are raised at a stated point, owing to presence of a
constriction or obstruction, such as a bridge.
aggradation. General and progressive upbuilding of the longitudinal profile of a channel or within a
drainage facility by the deposition of sediment. Compare with sedimentation. Permanent or
continuous aggradation is an indicator that a change in the stream’s discharge and sediment load
characteristics is taking place.
AHW. Acronym for allowable headwater. Compare with HW.
ALERT. Acronym for Automated Local Evaluation in Real Time. An automated, local flood warning
system consisting of automatic self-reporting river and rainfall gages, a communications system based
on line-of-sight radio transmission of data, and a base station. The base station consists of radio
receiving electronic equipment and a microprocessor. Data analysis software is available to collect,
quality control, and display data. A hydrologic model to provide simulation of streamflow is also
available.
alfalfa. A deep-rooting plant, Medicago Sativa, native to Eurasia, having compound leaves with three
leaflets and a cluster of small purple flowers. It is widely cultivated for forage and is used as a
commercial source of chlorophyll. Its deep-rooting characteristics, commercial value, and
environmental (forage and bee habitat) qualities often make it desirable as a ground cover to
control erosion.
algae. Any of various primitive, chiefly aquatic, one-celled or multi-cellular plants that lack true
stems, roots, and leaves but usually contain chlorophyll.
allowable headwater. See allowable headwater depth.
allowable headwater depth. The depth or elevation of the flow impoundment for a drainage facility
above which damage, some other unfavorable result, or a significant flood hazard could occur.
Compare with headwater depth.
alluvial. Referring to deposits of silts, sands, gravels, or similar detrital material that has been
transported by running water.
Glossary
G-5
alluvial channel. Channel formed wholly in alluvium with no bedrock exposed in the channel at low
flow or likely to be exposed by erosion. A channel whose processes are controlled by the flow and
boundary interactions.
alluvial fan. A landform shaped like a fan in plan view and deposited where a stream issues from a
narrow valley of high slope onto a plain or broad valley of low slope. Compare with debris cone.
alluvium. Unconsolidated material such as clay, silt, sand, or gravel deposited by water in a channel
or on a floodplain, alluvial, fan, or delta.
alpha (α). The kinetic-energy, velocity head coefficient. See velocity head coefficient.
alternate bar. See bar, alternate.
alternate depth. See depth, alternate.
AMC. Acronym for antecedent moisture condition.
amphibian. Any of the various cold-blooded, smooth-skinned vertebrate (with backbone) organisms
such as toads, frogs, and salamanders, characteristically hatching as an aquatic larvae that breathe by
means of gills and metamorphosing to an adult form having air-breathing lungs.
anabranch. Individual channel of an anabranched stream. A diverging branch of a river that reenters
the main stream.
anabranched stream. See stream, anabranched.
anaerobic. A condition in which molecular oxygen is absent from the environment.
analysis, hydraulic. An evaluation of a drainage-related circumstance or condition based on
measured or computed findings coupled with prudent judgment. Compare with assessment, hydraulic.
analysis, economic. See economic analysis. Compare with economic assessment.
anastomosing stream. See stream, anastomosing.
anchor ice. Ice in the bed of a stream or upon a submerged body or structure (55).
angle of flare. Angle between direction of wingwall and the centerline of a culvert barrel.
angle of repose. The maximum angle, as measured from the horizontal, at which granular particles
can stand.
angularity. The acute angle between the plane of the highway centerline along the bridge, and a line
normal to the thread of the stream, i.e., the acute angle between the thread of the stream and a line
normal to the centerline along bridge. Angle of skew if abutments are parallel to the flow line.
annual flood. See flood, annual.
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annual (flood) series. A list of annual events such as annual maximum floods and minimum flows.
See flood, annual.
A general term for a set of any kind of data in which each item is the maximum, minimum, average,
or some other consistent value in a year (33).
A frequency series in which only the largest value for a particular series of data in each year is used,
such as the annual floods (67).
annual runoff. See runoff, annual.
annual yield. See runoff, annual.
antecedent moisture. See antecedent moisture condition.
antecedent moisture condition (AMC). The degree of wetness of a watershed’s surface soils at the
beginning of a storm.
antecedent precipitation. Rainfall that occurred prior to the particular rainstorm under consideration.
antecedent precipitation index. An index of moisture stored within a drainage basin before a storm
(36), (43).
antidune. A particular type of bed form caused by water flowing over a mobile material such as sand.
A sand wave indicated on the water surface by a regular undulating wave. The ridges may move
upstream and the surface waves become gradually steeper on the upstream sides until they break like
surf and disappear. These surface waves are usually in series and often reform after disappearing. See
bed form. Compare with dune.
apex. The highest point, the vertex.
approach channel. The reach of channel upstream from a dam, bridge constriction, culvert, or other
drainage structure. See approach section.
approach section. A cross section of the stream channel, normal to thread of current and for the
discharge of interest, located in the approach channel. See approach channel.
apron. Protective material laid on a streambed to prevent scour commonly caused by some drainage
facility. More specifically, a floor lining of such things as concrete, timber, and riprap, to protect a
surface from erosion, such as the pavement below chutes, spillways, at the toes of dams, or at the
outlet of culverts. Material placed on the banks is commonly termed a blanket. Compare with channel
lining and blanket.
apron, launching. A flexible apron designed to settle and protect the side slopes of a scour hole or
eroded channel bed both during and after settlement.
aquatic life. Wildlife living or growing on, in, or adjacent to water.
aqueduct. See flume.
Glossary
G-7
aqueous form. Dissolved in the water.
aquifer. A porous, water-bearing geologic formation. Generally restricted to materials capable of
yielding an appreciable supply of water.
area. The size, in square meters [feet], of a flow section, channel, or waterway opening. Also, the size
in square kilometers [miles], or hectares [acres], of drainage area or basin, catchment area, or drainage
area. See drainage area.
area curve. When plotted against some other variable such as discharge or velocity, a graph of:
(1) the cross-sectional area of a stream at a gaging station or other section, (2) the surface area of a
reservoir plotted against water-surface elevations, (3) the flow area of a drainage structure.
area, hydrologic. See hydrologic area.
area rainfall. The average rainfall over an area, usually as derived from or discussed in contrast with
point rainfall (67).
area-capacity curve. A graph showing the relation between the surface area of the water in a
reservoir and the corresponding volume.
arid. Geographic areas that are dry, lacking moisture. Compare with desert and semi-arid.
armor. Artificial surfacing of channel beds, banks, or embankment slopes to resist streambed scour
and/or lateral bank erosion. Compare with apron, blanket, channel lining, and revetment.
armoring. A natural process whereby an erosion-resistant layer of relatively large particles is formed
on a channel bank and/or channel bed due to the removal of finer particles by streamflow, i.e., the
concentration of a layer of stones on the bed of the stream that are of a size larger than the transport
capability of the recently experienced flow—the winnowing out of smaller material capable of being
transported while leaving the larger sizes as armor that, for discharges up to that point in time, cannot
be transported. Armoring may also refer to the placement of a covering on a channel bank and/or
channel bed to prevent erosion.
array. A list of data in order of magnitude; in flood-frequency analysis it is customary to list the
largest value first, and in a low-flow frequency analysis the smallest first (33).
arroyo. Term of regional geographic usage. See channel.
artesian. Pertains to groundwater that is under pressure and will rise to a higher elevation if
unconstrained.
articulated concrete mattress. Rigid concrete slabs that can move without separating as scour
occurs, usually hinged together with corrosion-resistant fasteners; primarily used for a lower bank
protection armor blanket.
asphalt, end dumped. Mass of uncompacted, asphalt chunks of various sizes that are usually dumped
from a truck for upper bank protection or from a barge for lower bank protection. The purpose is to
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Highway Drainage Guidelines
protect the bank against erosion. Note: There may be environmental prohibitions against using asphalt
for such purposes.
assessment, hydraulic. Primarily a prudent, judgmental evaluation of a drainage-related
circumstance or condition using observations and experience but without a large amount of measured
or computed information. Compare with analysis, hydraulic.
assessment, economic. See economic analysis or economic assessment.
augmentation, flow. The addition of water to a stream, especially to meet water quality or fish and
wildlife needs.
augmented flow. The increased volume of water entering a channel, or allowed to run overland as
waste waters from the diversion of surface flow or as water from another stream or watershed; or
from waters withdrawn or collected upstream and released after use.
average discharge. See discharge, average.
average velocity. See velocity, average.
avulsion. A sudden change in channel course that occurs when a stream suddenly breaks through its
banks; usually associated with a large flood or a catastrophic flood or event.
backfill. The material used to refill a ditch or other excavation, material placed adjacent to or around
a drainage structure, or the process of doing so.
backwater. The increase in water surface elevation induced upstream from such things as a bridge,
culvert, dike, dam, another stream at a higher stage, or other similar structures or conditions that
obstruct or constrict a channel relative to the elevation occurring under natural channel and floodplain
conditions. Stated another way, water backed up or retarded in its course as compared with its normal
or natural conditions of flow. Also applies to the water surface profile in a channel or conduit. See
backwater curve.
The difference between the observed stage and that indicated by the stage-discharge relation, is
reported as backwater (36).
Four other similar definitions and examples are offered: (1) in a general sense, a flow retarding
influence due to a dam, other constrictions such as a bridge or culvert, or another stream; (2) the
increase in water-surface elevation due to a bridge constriction above the normal unconstricted
elevation at an approach section located one bridge length upstream from the bridge constriction;
(3) water backed up or retarded in its course as compared with its normal or natural condition of flow;
(4) in stream gaging, a rise in stage produced by a temporary obstruction such as ice or weeds, or by
the flooding of the stream below.
backwater area. The low-lying lands adjacent to a stream that may become flooded due to backwater
effects. Compare with floodplain.
Glossary
G-9
backwater curve. A particular form (or profile) of the surface curve (water surface) of a stream of
water that is concave upward. It is caused by an obstruction in the channel such as those that cause
backwater. The depth is greater at all points under the curve than the critical and the normal depth,
and the velocities diminish downstream. The term is sometimes used in a generic sense to denote all
water surface curves or profiles. Compare with backwater and water surface profile.
backwater flooding. The backup of water into a stream from a river, lake, or ocean having a higher
water elevation (32).
backwater ratio. The ratio (Cb) of the amount of backwater (h1*) created by a bridge constriction to
the total drop (∆h) through the constriction.
backwater reach. The reach of channel influenced by the backwater curve.
baffle. A structure constructed on the bed of a stream or drainage facility to deflect or disturb the
flow. Vanes or guides, a grid, grating, or similar device placed in a conduit to check eddy currents
below them, and effect a more uniform distribution of velocities. Also a device used in a culvert or
similar structure to facilitate fish passage.
baffle piers. Obstructions set in the path of high-velocity water, such as piers on the apron of a
stilling basin, to dissipate energy and prevent bank erosion and streambed scour.
bank. The side slopes or margins of a channel between which the stream or river is normally
confined. More formally, the lateral boundaries of a channel or stream, as indicated by a scarp, or on
the inside of bends, by the streamward edge of permanent vegetal growth.
The margins of a channel. Banks are called right or left as viewed facing in the direction of the flow
[commonly downstream] (36).
See bank, upper; bank, lower; left bank of a channel; and right bank of a channel.
bank caving. See mass wasting.
bankfull discharge. Discharge that, on the average, fills a channel to the point of overflowing.
Commonly considered as the mean annual discharge (Q2.33) or two- to three-year discharge (Q2, Q3) in
a channel that has been relatively stable for a number of years without the occurrence of a large,
bank-destroying flood. Sometimes used as the dominate discharge. See discharge, dominant.
bankfull stage. Stage at which a stream first overflows its natural banks. Compare with flood stage.
Bankfull stage is a hydraulic term whereas flood stage implies damage (36).
The water elevation that reaches the top of the banks along a stream (32). Compare with
bankfull discharge.
bank, guide. See guide bank.
bank, left. See left bank of a channel.
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bank, lower. That portion of a channel bank having an elevation less than the mean water level of the
stream or river. Compare with bank and bank, upper.
bank, middle. That portion of a channel bank having an elevation approximately the same as that of
the mean water level of the stream or river.
bank, right. See right bank of a channel.
bank storage. See storage, bank.
bank, stream. See bank, right bank of a channel, and left bank of a channel.
bank, upper. The portion of a channel bank having an elevation greater than the average (mean
annual or dominant flow) water level of the stream. Compare with bank and bank, lower.
bar. An elongated deposit of alluvium, not permanently vegetated, within or along the side of a
channel. See bar, point; bar, middle; and bar, alternate.
bar, alternate. An alluvial deposit of sand and gravel lacking permanent vegetal cover occurring in
an alternating pattern from bank to bank in a relatively straight channel reach.
bar, middle. A bar lacking permanent vegetal cover that divides the flow in a channel at
normal stage.
bar, point. An alluvial deposit of sand or gravel lacking permanent vegetal cover occurring in a
channel at the inside of a meander loop bendway and usually somewhat downstream from the apex of
the loop. The longer, tapered portion, when present, commonly points upstream.
barrage. See check dam.
barrel width. Commonly the inside, horizontal extent of a drainage facility.
base. The layers of specified material placed on the subbase or subgrade to support the pavement,
surface course, or a drainage facility.
base discharge. See base flow.
base flood (or storm). See flood, base; and flood (or storm), review.
base floodplain. Surface area flooded by the base flood.
base flow. In the U.S. Geological Survey’s annual reports on surface-water supply, the discharge
above which peak discharge data are published. The base discharge at each station is selected so that
an average of approximately three peaks a year will be presented (36). See runoff, base.
base flow recession. The declining rate of discharge of a stream fed only by base flow for an
extended period. Typically, a base flow recession will be exponential (20). Compare with
recession curve.
Glossary
G-11
base flow recession hydrograph. A hydrograph that shows a base flow recession curve.
base runoff. See runoff, base.
base station. The central location for receiving flood data from sensors in the ALERT [or IFLOWS]
system. Gaging station providing basic hydrologic data (32).
baseline monitoring. The establishment and operation of a designed surveillance system for
continuous or periodic measurements and recording of existing and changing conditions that will be
compared with future observations.
basic hydrologic data. Includes inventories of land and water features that vary only from place to
place (topographic and geologic maps are examples) and records of processes that vary with both
place and time (records of precipitation, streamflow, groundwater, and quality-of-water analyses are
examples). Basic hydrologic information is a broader term that includes surveys of the water
resources of particular areas and a study of their physical and related economic processes,
interrelations, and mechanisms.
basic-stage flood series. See partial-duration flood series.
basin, debris. A basin constructed in a channel to store debris, such as sand, gravel, silt, and
driftwood. See debris; drift barrier; and dam, debris. Compare with basin, sedimentation; basin,
recharge; basin, detention; and basin, retention.
basin, detention. A basin or reservoir incorporated into the watershed whereby runoff is temporarily
stored, thus attenuating the peak of the runoff hydrograph. A stormwater management facility that
impounds runoff and temporarily impounds runoff and discharges it through a hydraulic outlet
structure to a downstream conveyance structure. Compare with basin, retention; basin, recharge;
basin, debris; and basin, sedimentation.
basin drainage. See drainage area.
basin lag. The amount of time from the centroid of the rainfall hyetograph to the hydrograph peak.
basin, recharge. A basin or pit excavated to provide a means of allowing water to soak into the
ground at rates exceeding those that would occur naturally (20). A basin excavated in the earth to
receive the discharge from streams or storm drains for the purpose of replenishing groundwater
supply and/or attenuating flood discharge.
A basin or reservoir wherein water is stored for recharging the groundwater. It does not have an
uncontrolled outlet. The stored water is disposed by such means as infiltration or injection (dry
wells) designed for such purposes. See recharge and recharge area. Compare with floodwater
retarding structure; reservoir, retarding; basin, detention; basin, sedimentation; basin, debris;
and basin, retention.
basin, retention. A basin or reservoir wherein water is stored for regulating a flood. It does not have
an uncontrolled outlet. The stored water is disposed by such means as infiltration, injection (or dry)
wells, or by release to the downstream drainage system after the storm event. The release may be
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through a gate-controlled gravity system or by pumping. Compare with floodwater retarding
structure; reservoir, retarding; basin, detention; basin, recharge; basin, sedimentation; and
basin, debris.
basin, sedimentation. A basin or tank in which floodwater or stormwater containing settleable solids
is retained to remove by gravity or filtration a part of the suspended matter. Compare with basin,
recharge; basin, detention; basin, retention; and basin, debris.
basin, stilling. See stilling basin.
Beard distribution. See probability distribution.
bed. The bottom of a channel. The part of a channel not permanently vegetated which is bounded by
banks and over which water normally flows. Compare with substrate.
bed form. A recognizable and commonly transient relief feature on the bed of a channel, such as a
ripple, dune, or bar. A relief feature caused by water flowing over a mobile material such as sand
and gravel.
bed, indurated. A channel bed that has been made hard or has hardened. See control.
bed layer. A flow layer, several grain diameters thick (usually two) immediately above the bed.
bed load. Sediment that is transported in a stream by rolling, sliding, or skipping (saltating) along the
bed or very close to it; considered to be within the bed layer. The quantity of silt, sand, gravel, or
other detritus rolled along the bed of a stream, often expressed as weight or volume per time.
Compare with contact load sediment and wash load.
bed load discharge. The quantity of bed load passing a cross section of a stream in a unit of time.
Compare with sediment discharge, suspended; and wash load.
bed material. The sediment mixture of which a streambed, lake, pond, reservoir, or estuary bottom is
composed (50). Sediment consisting of particle sizes large enough to be found in appreciable
quantities at the surface of a channel bed. Material found on the bed of a channel (may be transported
as bed load discharge or sediment discharge). See bed load discharge.
bedrock. The scour-resistant material underlying erodible soils and overlying the mantle rock,
ranging from surface exposure to depths of several hundred kilometers [miles].
bed sediment discharge. The part of the total sediment discharge that is composed of grain sizes
found in the bed and is equal to the transport capability of the flow occurring at the time. Compare
with bed load; bed load discharge; and sediment discharge, suspended.
bed shear. See tractive force.
bed slope. The longitudinal inclination of a channel bottom. Compare with superelevation.
bench-flume. A conduit on a topographical bench, cut into sloping ground. Compare with flume.
Glossary
G-13
bendway. A point of distinct curvature in a stream, river, or channel where the bankfull flow changes
direction. Reach of sharpest curvature in a sinuous channel.
bendway scour. See scour, bendway.
beneficial use. Relates to the beneficial use of a water and flood right. See water right and flood
right. Beneficial use is such a thing as municipal use, agricultural use, recreation, and environmental
purposes (instream flows).
berm. A narrow shelf or ledge; also a form of dike. A more detailed description might be: (1) the
space left between the upper edge of a cut and the toe of an embankment; (2) a horizontal strip or
shelf built into an embankment to break the continuity of an otherwise long slope.
Also, a berm may be the top surface or plane of a shoulder, ledge, or bank constructed in connection
with the roadway embankment at bridge abutments, waste along fill slopes, and canal or ditch banks.
Compare with dike.
Bernoulli’s Theorem. A proposition advanced by Daniel Bernoulli that the energy head at any
section in a flowing stream is equal to the energy head at any other downstream section plus the
intervening losses.
best management practices (BMPs). Erosion and pollution control practices employed during
construction to avoid or mitigate damage or potential damage from the contamination or pollution of
surface waters or wetlands from a highway action.
beta (β). The momentum velocity head coefficient. See momentum coefficient.
biomass. The amount of living matter (as in a unit area or volume of habitat) according to Webster’s
New Collegiate Dictionary.
biostimulation. To rouse any living organism to activity.
biotic. Pertaining to life or specific life conditions.
biotic region. A geographic area having similar biotic conditions. See mitigation alternatives.
bituminous mattress. An impermeable rock-, mesh-, or metal-reinforced layer of asphalt or other
bituminous material placed on a channel bank to prevent erosion. Note: There may be environmental
prohibitions for such material.
blanket. Material covering all or a portion of a channel bank to prevent erosion. Stream bank surface
covering, usually impermeable, designed to serve as protection against erosion. Common pavements
used on channel banks are concrete, compacted asphalt, and soil-cement. Compare with apron, apron
launching, armor, and channel lining.
blanket, clay. See clay blanket.
blanket, filter. See filter blanket.
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Highway Drainage Guidelines
BLM. Acronym for the Bureau of Land Management.
Blom distribution. See probability distribution.
BMP (BMPs). Acronym for Best Management Practices.
bog. A shrug peatland dominated by ericaceous shrubs, sedges, and peat moss and usually having a
saturated water regime or a forested peatland dominated by evergreen trees (usually spruces and firs)
and/or larch. See wetlands.
boreal region. The geographical area just below the arctic tundra and usually characterized by the
evergreen forests.
bore, hydraulic. A wave of water having a nearly vertical front, such as a tidal wave, advancing
upstream as a result of high tides in certain estuaries; a similar wave advancing downstream as the
result of a “cloudburst,” or the sudden release of a large volume of water from a reservoir, as in the
Johnstown (PA) flood. The bore is analogous to the hydraulic jump in that it represents the limiting
condition of the surface curve wherein it tends to become perpendicular to the bed of the stream.
bottom contraction. A channel contraction resulting from some protrusion across the bottom of
a channel.
bottom material. See bed material.
boulder. A rounded or angular fragment of rock, the diameter of which is in the size range of
250 mm to 4000 mm (10 in. to 160 in.) according to FHWA’s Highways in the River
Environment Manual.
braid. A subordinate channel of a braided stream. See stream, braided. Compare with anabranch.
braided stream. See stream, braided.
braiding of river channels. See stream, anastomosing.
branch. Term of local geographic usage. See channel.
breakers. The surface discontinuities of waves as they breakup. They may take different shapes
(spilling, plunging, surging). Zone of break-up is called the surf zone.
bridge. A structure including supports erected over a depression or an obstruction, such as water,
highway, or railway, and having a tract or passageway for carrying traffic or moving loads, and, for
definition purposes (AASHTO), having an opening measured along the center of the roadway equal
to or more than 6.1 m (20 ft) between undercopings of abutments or spring lines of arches, or extreme
outside ends of openings for multiple boxes; it may also include multiple pipes, where the clear
distance between openings is less than half of the smaller contiguous opening. Also a structure that,
while designed hydraulically using the principles of open-channel flow to operate with a free water
surface, may be inundated under flood conditions. The structure generally consists of a deck or
Glossary
G-15
superstructure supported on two or more abutments and often includes intermediate piers. Compare
with culvert.
bridge opening. The total cross section area beneath a bridge superstructure that is available for the
conveyance of water. Compare with bridge waterway.
bridge, relief. An opening through an embankment located on a floodplain for the purpose of
permitting passage of overbank flow.
bridge waterway. The area of a bridge opening available for flow as measured below a specified
stage and normal to the principal direction of flow. Compare with bridge opening.
broad-crested weir. See weir, broad-crested.
broken record. A systematic record [of events such as annual floods] that is divided into separate,
continuous segments because of a deliberate, discontinuation of recording for significant periods of
time (32).
broken-back culvert. See culvert, broken-back.
bubble gage. See gage, bubble.
buffer zone. Areas of trees, grass, or other vegetation located between the top of a channel bank and
the adjacent highway; also called greenbelt.
bulkhead. A steep or vertical wall that supports a natural or artificial embankment and may also
serve as a protective measure against bank erosion.
bypass flow. See flow, bypass.
caisson. A chamber, usually sunk by excavating from within, for the purpose of gaining access to the
subsurface geology selected to support a structure’s substructure (the foundation). If the chamber is
closed on top and the water excluded by air pressure, it is called a pneumatic caisson. Oftentimes in
highway construction a caisson is filled with concrete or masonry to serve as the pier or abutment
substructure. Compare with cofferdam.
calibration. The process of fitting a [computational] model to a set of observed data by changing
unknown or uncertain model parameters systematically within their allowable ranges until a “best fit”
of the model to the observed data is achieved (1).
California distribution. See probability distribution.
canal. A constructed open conduit or channel for the conveyance of irrigation water that is
distinguished from a ditch or lateral by its larger size. It is usually excavated in natural ground,
although lined canals on berms are not uncommon. Compare with channel and swale.
capacity. A measure of the ability of a channel or conduit to convey water. Compare with
conveyance.
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Highway Drainage Guidelines
capacity curve. A graph of the volume of such things as a reservoir, tank, or detention pond as a
function of elevation. The capacity of a reservoir is defined by reference to an elevation.
carryover. See flow, bypass.
catastrophic flood (or storm). See flood (or storm), catastrophic.
catch basin. A structure, sometimes with a sump, for inletting drainage from such places as a gutter
or median and discharging the water through a conduit. In common usage it is a grated inlet, curb
opening, or combination inlet with or without a sump. Note that sumps in catch basins may cause
environmental hazards by further polluting “first flush” runoff and subsequent runoff passing through
the catch basin.
catchment. See drainage area.
catchment area. See drainage area.
catchment basin. See drainage area.
cation exchange capacity (CEC). The sum total of exchangeable cations that a soil can absorb;
expressed in milliequivalents per gram, or per 100 g of soil.
Cauchy distribution. See probability distribution.
causeway. Rock or earth embankment carrying a roadway across water.
caving bank. See mass wasting.
cavitation. A phenomenon associated with the vaporization of a flowing liquid in a zone of low
pressure, wherein cavities filled with liquid vapor (vapor bubbles) alternately develop and collapse.
Significant cavitation is unusual in highway drainage structures but, if present, may cause structural
deterioration. Put another way, a condition wherein a vacuum, to any degree, exists as a result of
flowing water. Complete cavitation occurs when the pressure within the affected part is reduced to
that of the vapor pressure of the water.
CEC. Acronym for cation exchange capacity.
celerity, wave. The velocity of a wave measured relative to the liquid, known as celerity C, is equal
to the wave length divided by the period [of the wave], or C = L/t (13).
cellular-block mattress. Regularly cavitated, interconnected concrete blocks placed directly on a
bank (or shore) to prevent erosion. The cavities can permit bank (or shore) drainage and the growth of
either volunteer or planted vegetation when synthetic filter fabric is not used between the mattress and
bank (or shore). See apron and blanket.
CFR. Acronym for Code of Federal Regulations.
cfs. Acronym for cubic feet per second (36). See cubic feet per second.
Glossary
G-17
cfs-day. Often [erroneously] called a “second-foot-day.” The volume of water represented by a flow
of 1 cubic foot/s/day (49).
chain or tape gage. See gage, chain or tape.
channel. The term “channel” has been defined numerous ways: (1) the bed and banks that confine the
flow of surface water in a natural stream or artificial channel; also see river and stream; (2) the course
where a stream of water runs or the closed course or conduit through which water runs, such as a
pipe; (3) an open conduit either naturally or artificially created that periodically or continuously
contains moving water or which forms a connecting link between two bodies of water. River, creek,
run, branch, anabranch, [arroyo, draw, wash] and tributary are some of the terms used to describe
natural channels. Natural channels may be single or braided (see stream, anastomosing). Canal [ditch,
lateral] and floodway are some of the terms used to describe artificial channels (36).
Channel has also been defined as an elongated, open depression in which water may, or does, flow.
An elongated depression, either naturally or artificially created and of appreciable size, which
periodically or continuously contains moving water or which forms a connecting link between two
bodies of water. It must have a definite bed and bank, which serve to confine the water up to some
bankfull discharge amount.
Local convention may use river, stream, arroyo, or branch. With constructed canals, the term ditch or
lateral may be used. See field ditch.
Compare with canal, swale, and waterway.
channel, alluvial. See alluvial channel.
channel, approach. See approach channel.
channel, bed. See bed.
channel coefficient. A roughness factor in the Kutter, Manning, Bazin, and other formulas expressing
the character of a channel as it affects the friction slope of water flowing therein. More specifically,
for highway drainage design in the United States, the roughness factor, n, in Manning’s equation. See
Manning’s Equation.
channel, collateral. Channels that are side-by-side, accompanying, from the same source, or similar
but of subordinate nature.
channel contraction. The degree of contraction imposed by a bridge-type constriction on the river
channel for a given discharge. It is measured by the channel-contraction ratio (M) that is defined as
the ratio of that portion of flow directed at the contracted opening divided by the total flow that must
pass through the contracted opening. Sometimes the inverse of this ratio is used to define the channel
contraction, so caution is warranted. Compare with constriction, contracted section, contraction, and
coefficient of contraction.
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Highway Drainage Guidelines
channel diversion. The taking of water from a stream or other body of water into a canal, pipe, or
other conduit (36). The removal of all or a portion of the flow from a natural or artificial (canal, ditch,
field ditch, or lateral) channel.
channel, incised. See incised channel; incised reach; and stream, incised.
channelization. Straightening and/or deepening of a channel by such things as artificial cutoffs,
grading, flow-control measures, river training, or diversion of flow into an artificial channel. See river
training structure.
channel lining. The material applied to the bottom and/or sides of a natural or constructed channel.
Material may be such things as concrete, sod, grass, rock, or any of several other types of commercial
linings. Covering of stones on a channel bed or bank (used in the AASHTO Model Drainage Manual
with reference to natural covering). Compare with apron, blanket, and armor.
channel, low-flow. Lower portion of natural or artificial watercourse often of perceptible extent with
a definite bed and banks that confines and conducts continuously or periodically flowing water. A
low-flow channel is considered as that portion of a channel commonly lying below the plane of the
ordinary highwater (OHW). A low-flow channel may be adjoined by a floodplain. The bankfull
capacity is often associated with the dominate discharge or mean annual discharge.
channel, meandering. See meandering channel.
channel migration. Change in position of a channel by lateral erosion of one bank and the
simultaneous accretion of the opposite bank. Systematic channel shifting in the direction of flow.
channel, natural. A surface or underground watercourse created by natural agents and conditions.
The principal stream channel or channels and, if the stream is braided, its natural and customary
overflow channels.
channel, non-uniform. A channel where the flow streamlines are not straight and parallel. The
velocity vector varies significantly with distance along a flow streamline at a given instance. With
non-uniform channels, resistance and gravity forces are not in balance, the water surface is not
parallel to the channel bottom or friction slope, Sf. Also, the rate of loss of total head will not equal
the average channel bottom slope, So. Compare with channel, uniform.
channel, open. A channel having a water surface exposed at all points to atmospheric pressure. Any
conveyance in which water flows with a free surface. Compare with open-channel flow.
channel pattern. The aspect of a stream channel in plan view, with particular reference to such
things as the degree of sinuosity, braiding, or anabranching.
channel process. Behavior of a channel with respect to channel migration, erosion, and
sedimentation.
channel, regime. See regime channel.
Glossary
G-19
channel routing. The process whereby a peak flow and/or its associated streamflow hydrograph
is mathematically transposed to another site downstream taking into account the effect of
channel storage.
channel slope. Fall per unit length along the channel centerline.
channel, stable. A condition that exists when a channel has a bed slope and cross section that allows
it to transport the water and sediment delivered from the upstream watershed without significant
aggradation, deposition, or bank erosion. See regime, regime channel, and regime of a stream.
channel storage. See storage, channel.
channel, tortuous. A winding channel that is not free to shift its alignment. Compare with
flow, turbulent.
channel, uniform. Channel with a uniform cross section and constant roughness. A constant slope is
also a requirement for uniform flow and depth. See depth, normal.
[For highway drainage design,] a channel where the flow streamlines are [essentially] straight and
parallel. Lack of [significant] variation in the velocity vector with distance along a streamline at a
given instant. If a channel is uniform and resistance and gravity forces are in near balance, the water
surface will be parallel to the bottom of the channel and to the friction slope, Sf, and the rate of loss of
total head will equal the slope, So, of the channel bottom. This is the condition of uniform flow. The
depth from the water surface to the bottom grade line of the channel, which because of the parallelism
of the water surface and grade line is the same at all sections, is known as normal depth, Yo [Yn]. The
normal depth is a function of the shape and roughness of the channel, its slope, and the rate of
discharge. For a given discharge, channel shape, and slope, the normal depth will vary according to
the total effect of boundary resistance (54). Compare with channel, non-uniform.
channel, wandering. A channel exhibiting a more or less non-systematic process of channel shifting,
erosion, and deposition, with no definite meanders or braided pattern.
check dam. A relatively low dam or weir across a channel for the diversion of irrigation flows from a
small channel, canal, ditch, or lateral. A check dam can also be a low structure, dam, or weir, across a
channel for such things as the control of water stage or velocity or the control of channel bank erosion
and channel bed scour from such things as headcutting. Sometimes termed a barrage in Britain and
former British Commonwealth countries.
check flood. See flood (or storm), review.
check flood for bridge scour. See flood (or storm), review.
Chegodoyev distribution. See probability distribution.
chemical gaging. A process of measuring the flow of water by ascertaining the resulting degree of
dilution of a chemical solution of known saturation introduced at a known rate into the stream.
chemical-hydrometry. See chemical gaging.
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Highway Drainage Guidelines
chemical stabilization. Channel bank protection technique involving the application of chemical
substances to increase particle cohesiveness and to shift the size distribution toward the coarser
fraction. The net effect is to improve the erosion resistance of the material.
chemocline. A salinity or other chemical gradient within surface waters. Chemoclines are usually
thought of for lakes, but might include some ponds. See lake and pond.
Chezy formula. An empirical formula for uniform flow expressing the relation between velocity of
water, hydraulic radius, and friction slope; thus, V = C(RS)1/4 where V = velocity; R = hydraulic
radius; Sn = energy or friction slope; and C = a coefficient.
choking (of flow). Severe backwater effect resulting from excessive constriction.
chute. An open or closed channel used to convey water, usually situated on the ground surface.
Compare with cutoff. Two other definitions are: (1) a high-velocity conduit for conveying water to a
lower level; (2) an inclined drop or fall. See drop.
chute cutoff. See cutoff.
Cipolletti weir. See weir, Cipolletti.
civil action. Action presenting an issue to be resolved under civil law, as distinguished from criminal
law, and/or brought to establish or recover private and civil rights or redress for damage; tort action.
civil law. The system of jurisprudence established by a nation, state, or commonwealth peculiarly for
itself; the division of law regulating ordinary private matters, as distinct from laws regulating
criminal, political, or military matters. The civil laws regarding the management of naturally
occurring waters established the rights or easements, both favorable and restrictive, of the riparian
owners individually and with respect to others, and are directed toward equitable use and the
preservation and continuation of natural drainage conditions. Compare with common law.
Civil Law Doctrine or Rule. A rule of law pertaining to the disposal of drainage waters, under which
the owner of higher land has the right or easement to dispose of the surplus or excess waters from his
lands to lower lands, unobstructed by the owners thereof. Compare with Common Enemy Doctrine or
Rule, Natural Drainage Doctrine or Rule, and Reasonable Use Doctrine or Rule.
clay. Material passing the No. 200 (0.074-mm) U.S. Standard Sieve that exhibits plasticity (putty-like
properties) within a range of water contents and has considerable strength when air-dry (Unified Soil
Classification System) (FHWA Highways in the River Environment Manual).
clay blanket. Layer of compacted clay placed over cohesionless bank soils to protect them against
erosive streamflow.
clay plugs. See meander plugs.
clearance. An unobstructed horizontal or vertical space. Compare with freeboard.
Glossary
G-21
climate. The sum total of the meteorological elements that characterize the average and extreme
condition of the atmosphere over a long period of time at any one place or region of the earth’s
surface. The collective state of the atmosphere at a given place or over a given area within a specified
period of time (36).
climatic year. A continuous 12-month period during which a complete annual cycle occurs,
arbitrarily selected for the presentation of data relative to hydrologic or meteorologic phenomena. The
climatic year is usually designated by the calendar year during which most of the 12 months occur
(36). See also water year.
A year selected for the presentation of data on water supply, precipitation, etc.; the climatic year of
the USGS extends from October 1 to September 30 following.
cloudburst. A torrential downpour of rain, which by its spottiness and relatively high intensity
suggests the bursting and discharge of a whole cloud at once (74).
coast line. The line or interface forming the boundary between the land and water. See shore.
coastal zone. The strip of land that extends inland from a coast (or shore) line to the first major
change in terrain features.
cobble. A fragment of rock the diameter of which is in the size range of 64 to 250 mm (2.5 to 10 in.)
(21, FHWA Highways in the River Environment Manual).
Code of Federal Regulations (CFR). Codifies and publishes at least annually Federal regulations
currently in force. The CFR is kept up to date by individual issues of the Federal Register. The
two publications must be used together to determine the latest version of any given rule. See
Federal Register.
COE. Commonly used acronym for the U.S. Army Corps of Engineers. The official government
acronym for the U.S. Army Corp of Engineers is USACE.
coefficient of contraction. The ratio of the smallest cross section area of the flow after passing the
constriction to the nominal cross section area of the constriction. Compare with channel contraction.
coefficient of discharge. Ratio of observed to theoretical discharge. Also the coefficient used for
orifice or other flow processes to estimate the discharge past a point or through a reach.
coefficient of roughness. See channel coefficient.
coefficient of skew (skewness). See skew coefficient.
cofferdam. A barrier built in the water so as to form an enclosure from which the water is pumped to
permit free access to the area within. Compare with caisson.
collateral channel. See channel, collateral.
combination inlet. See inlet, combination.
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Highway Drainage Guidelines
combined sewer. See sewer, combined.
Common Enemy Doctrine or Rule. A common law rule recognized by some states, pertaining to the
disposal of surplus or excess surface waters, which holds that such waters are a “common enemy,”
and, therefore, the landowner has the right to protect his lands from such waters coming from higher
lands. Under this rule, surface waters are regarded as a common enemy that each landowner may fight
as he deems best and without regard to the harm that may be caused to others. Compare with Civil
Law Doctrine or Rule, Natural Drainage Doctrine or Rule, and Reasonable Use Doctrine or Rule.
common law. As distinguished from “Roman” or “civil” law, the body of unwritten law, especially
of England, based on long-standing usages and customs and the court decisions and decrees
recognizing, affirming, and enforcing such usages and customs. Compare with civil law.
common law, federal. A body of decisional law developed by the Federal courts, unencumbered by
State court decisions.
Common Law Rules. Principles or maxims established under the common law doctrine or rule.
community species level. The living part of the ecosystem having an ecological structure that exists
in dynamic equilibrium with its environment.
composite hydrograph. See hydrograph, composite.
concentration time. See time of concentration.
concordant flows. See flow, concordant.
concrete paving. Plain or reinforced concrete slabs poured or placed on the surface to be protected.
conduit. An artificial or natural channel, usually a closed structure such as a pipe or culvert. A
general term for any channel intended for the conveyance of water, whether open or closed; any
container for flowing water. With highways, conduits are often considered as being a pipe, culvert,
flume, channel, chute, or similar drainage facility.
confidence limits. Computed [statistical] values on both sides of an estimate of a parameter that show
for a specified probability the range in which the true value of the parameter lies.
confluence. The junction of two or more streams.
conjugate depth. See depth, conjugate.
conservation storage. See storage, conservation.
constriction. A compressed or constricted section or reach of a channel may be a natural condition or
one produced by raising the bottom (as a sill or dam), or contracting the width (as a highway
embankment on a floodplain), or both. A control section, such as a bridge crossing, channel reach,
sill, or dam, with limited flow capacity in which the discharge is related to the upstream water surface
elevation; a constriction may be either natural or artificial. See contraction, stream contraction, and
stream constriction. Compare with channel contraction, contracted section, and contraction.
Glossary
G-23
constriction geometry. A measure of the size, shape, and character of such things as the
embankments and abutments of a bridge-type constriction, expressed as dimensionless length ratios.
consumptive use. A term used mainly by irrigation engineers to mean the amount of water used in
crop growth plus evaporation from the soil (67). See evapotranspiration.
The quantity of water absorbed by the crop and transpired or used directly in the building of plant
tissue together with that evaporated from the cropped area (U.S. Bureau of Reclamation). The
quantity of water transpired and evaporated from a cropped area or the normal loss of water from the
soil by evaporation and plant transpiration (8). The quantity of water discharged to the atmosphere or
incorporated in the products of the process in connection with vegetative growth, food processing, or
an industrial process (44).
consumptive use, net. The consumptive use decreased by the estimated contribution of rainfall
toward the production of irrigated crops (60). See precipitation, effective. Net consumptive use is
sometimes called crop irrigation requirement.
consumptive waste. The water that returns to the atmosphere without benefiting man (61).
contact load sediment. Particles that roll or slide along in almost continuous contact with the
streambed. Compare with bed load.
contents. The volume of water in a reservoir [or lake]. Unless otherwise indicted, reservoir content is
computed on the basis of a level pool and does not include bank storage (36) (50).
contiguous. Touching, adjacent, adjoining, bordering on. Adjacent things may or may not be in
actual contact, but they are not separated by similar things. That which is adjoining something and
touches it at some point or along a line. Things are contiguous when they touch along the whole or
most of one side.
continuity equation. Discharge equals velocity times cross section area (Q = V × A). For steady flow
there is a continuity of discharge through succeeding sections of channel, expressed as: Q = (A1)(V1) =
(A2)(V2) = (AnVn) = a constant.
continuous models. Models that simulate the rainfall-runoff process over long periods of time
(months or years) including dry periods between events (1).
continuous stream. See stream, continuous.
contracted section. A cross section within a constriction; for example, at the downstream side of a
bridge opening, or at a culvert entrance. Compare with contraction, channel contraction, and
constriction.
contracting reach. A reach of channel wherein flow is accelerating; where the velocity head at lower
cross section exceeds the velocity head at the upper cross section.
contraction. The effects of a channel constriction on flow. Also, see stream contraction and stream
constriction. Synonymous with constriction.
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Highway Drainage Guidelines
contraction coefficient. See coefficient of contraction.
contraction method (of flow measurement). A method of indirect measurement of peak discharge
following a flood by field survey of highwater marks and channel and bridge geometry at a
constriction, such as at a bridge. Discharge is computed on the basis of an evaluation of energy
changes between the approach section and the downstream side of the constriction by methods given
in U.S. Geological Survey Circular 284. See indirect method (of flow measurement).
contraction scour. See scour, contraction.
control. A natural constriction of the channel, a long reach of the channel, a stretch of rapids, or an
artificial structure downstream from a gaging station that determines the stage-discharge relation at
the gage. A control may be complete or partial. A complete control exists where the stage-discharge
relation at a gaging station is entirely independent of fluctuations in stage downstream from the
control. A partial control exists where downstream fluctuations have some effect upon the
stage-discharge relation at a gaging station. A control, either partial or complete, may also be shifting.
Most natural controls are shifting to a degree, but a shifting control exists where the stage-discharge
relation experiences frequent changes owing to impermanent bed or banks (36).
A feature downstream from the gage that determines the stage-discharge relation at the gage. This
feature may be a natural constriction of the channel, artificial structure, or a uniform cross section
over a long reach of the channel (50).
control flume. See flume, control.
controlled river. See river, controlled.
controlled spillways. See spillway, controlled.
control section. A control section, such as a bridge opening, reach of channel, or dam, with a
definable flow capacity, in which the discharge is related to some measurable depth(s) such as the
upstream water surface elevation, tailwater elevation, and/or contracted flow depth. See control.
control structure. A structure [in a riverine environment] on a stream or canal that is used to regulate
the flow or stage of the stream, or to prevent the intrusion of salt water (50). See control.
convention. A short conduit for uniting two others having different hydraulic elements; a transition.
Not a common term.
conveyance. A measure of the ability of a stream, channel, or conduit to convey water. A
comparative measure of the water-carrying capacity of a channel; that portion of the Manning
discharge formula that accounts for the physical elements of the channel. Conveyance is expressed as
(1/n)AR2/3 where n is Manning’s n, A is the cross section area of flow, and R is the hydraulic radius.
See Manning’s Equation.
A measure of the water-transporting capacity of such things as a channel, floodplain, drainage
facility, storm drain, and/or other natural or artificial watercourse feature traversed by flows such as
Glossary
G-25
runoff or irrigation water. With the review flood or storm, conveyance may include that associated
with overtopping flows and inundation of a traveled way at cross-drainages. Compare with capacity.
core-wall. A wall of such things as masonry, sheet-piling, or puddled clay built inside a dam or
embankment to reduce percolation.
Corps. See USACE.
correlation. The process of establishing a relation between a variable and one or more related
variables. Correlation is “simple” if there is only one independent variable and “multiple” if there are
more than one independent variable. For gaging-station records, the usual variables are the short-term
gaging-station record and one or more long-term gaging-station records (57).
correlative estimate. A discharge determined by correlation. A correlative estimate represents a
likely value of the discharge for any particular period according to a specified method of
analysis (35).
corrosion. The deterioration of pipe or structure by chemical action.
cost-effective. A measure of a drainage design strategy’s acceptability is often based on a judgment
where either expected first costs or, when appropriate, the economic analysis costs are weighed
against the selected design criteria.
The relationship between the benefits derived from a system and the cost of purchasing, operating,
and maintaining it (32).
countermeasure. A measure, either incorporated into the design of a drainage facility or installed
separately at or near the facility, that serves to prevent, minimize, or control hydraulic problems.
cover. The vertical extent of soil above the crown of a pipe or culvert. [Depending on the context,
may also be] the vegetation, or vegetational debris, such as mulch, that exists on the soil surface. In
some classification schemes, fallow or bare soil is taken as the minimum cover class (67).
cradle. A footing structure shaped to fit the conduit it supports.
creek. Term of regional geographic usage. See channel.
crest. The maximum elevation of a flood at a specific location (32). Other definitions are: (1) the top
of a dam, dike, spillway, or weir; (2) the overflow portion of a road or embankment; (3) the summit
of a wave; and (4) the peak of a flood.
crest-stage gage. See gage, crest-stage.
crib. See retard.
crib dam. See dam, crib.
criteria. See design criteria.
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Highway Drainage Guidelines
criteria, design. See design criteria.
criterion. A standard, rule, or test on which a judgment can be based. Compare with design criteria.
critical depth. When the energy head is a minimum and the velocity head equals one-half the mean
depth, the corresponding depth is Belanger’s critical depth (4).
The depth at which the specific energy (depth + velocity head) for a particular discharge is a
minimum. It is the depth at which, for a given energy content of the water in a channel, maximum
discharge occurs or the depth at which, in a given channel, a given quantity of water flows with
minimum content of energy.
critical-depth method (of flow measurement). A method for indirect measurement of peak
discharge, following a flood, by field survey of highwater marks and channel, and control-section or
control-structure geometry. Discharge is computed on a basis of critical flow (flow at minimum
energy) theory. See indirect method (of flow measurement).
critical flow. That flow in open channels at which the energy content of the fluid is at a minimum.
Also, that flow which has a Froude number of unity. Flow at critical depth. See critical depth.
critical public services. [Services] typically consisting of police and fire assistance, emergency
medical services, communications, utilities, transportation, and public works (32).
critical shear stress. The minimum amount of shear stress (tractive force) exerted by passing stream
currents required to initiate soil particle motion. Compare with tractive force.
critical slope. That particular slope of a given uniform conduit operating as an open channel at which
normal depth equals critical depth for a given discharge (Qc). The slope required in a conduit to
sustain flow at critical depth for a given discharge; critical slope (Sc) must equal the energy slope (SE)
for the given discharge flowing at critical depth.
critical velocity. Mean velocity (Vc) of flow at critical depth (dc); in open channels the velocity head
equals one-half the mean depth.
cross drainage. The runoff from contributing drainage areas both inside and outside the highway
right-of-way and the transmission thereof from the upstream side of the highway facility to the
downstream side.
crossing. See crossover.
crossover. The relatively short and shallow reach of a stream between bends; also called a crossing.
The point where flows crossover from one low-flow channel bank to the other.
cross section (stream or valley). A diagram or drawing cut across a channel normal to the expected
flow direction [for a particular flood magnitude] that illustrates the banks, bed, [vegetal cover, soils]
and water surface. The shape of a channel, stream, or valley, viewed across the axis. In watershed
investigations and channel analyses it is determined by a line approximately perpendicular to the
main path of water flow [for a particular flood magnitude], along which measurements of distance
Glossary
G-27
and elevation are taken to define the cross-sectional area [conveyance properties] and shape. In
hydraulic analyses, vegetal patterns, floodplain material, and bed material [and any other conveyance
properties] are considered part of the cross section (67).
cross slope. See roadway cross slope.
cubic feet per second. A unit measurement of water flow. Sometimes erroneously called “second
feet,” primarily in oral discussions. See cfs.
A unit expressing rates of discharge. Cubic feet per second is equal to the discharge of a stream of
rectangular cross section, a foot wide and one foot deep, flowing water at an average velocity of one
foot per sec. Abbreviated cfs, cusec, or ft3/s (36).
The rate of discharge representing a volume of one cubic foot passing a given point during one
second and equivalent to 7.48 gallons per second or 448.8 gallons per minute or 0.028 32 m3/s (50).
cubic feet per second per square mile. The average number of cubic feet of water flowing per
second from each square mile of area drained, assuming that the runoff is distributed uniformly in
time and space (50).
cubic foot per second-day. The volume of water represented by a flow of one cubic foot per second
for 24 hours. It is equivalent to 86,400 cubic feet, approximately 1.9835 acre-feet, approximately
646,000 gallons, or 2445 m3.
cubic meters per second. A unit measurement of water flow, primarily in oral discussions.
A unit expressing rates of discharge. Cubic meter per second is equal to the discharge of a stream of
rectangular cross section, a meter wide and one meter deep, flowing water at an average velocity of
one meter per second.
Symbolized by m3/s.
culvert. A structure that is usually a closed conduit or waterway that is designed hydraulically to take
advantage of submergence to increase hydraulic capacity. A structure used to convey surface runoff
through such things as a highway or railroad embankment. Although there are borderline cases, a
culvert is a structure, as distinguished from bridges, which is usually covered with embankment and is
composed of structural material around the entire perimeter, although some are supported on spread
footings with the channel bed serving as the bottom of the culvert. A culvert commonly has a regular,
uniform shape, where a bridge opening may not—in other words, a culvert is a relatively large pipe or
conduit. A culvert usually has a large ratio of length to width. Usually in practice, and in certain
localities by AASHTO’s definition, a structure of less than 6.1-m (20-ft) span as measured along the
road centerline is classified as a culvert. Compare with bridge.
culvert, broken back. A culvert having two or more longitudinal structure profile slopes. Such
culverts are sometimes effective in reducing outflow velocities by energy dissipation from a hydraulic
jump near the outlet.
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Highway Drainage Guidelines
culvert method (of flow measurement). A method of indirect measurement of peak discharge,
following a flood, by field survey of highwater marks and channel and culvert geometry where
temporary upstream flood storage effects can be quantified or are not significant. Discharge is
computed on the basis of an evaluation of energy changes between the approach section (headwater)
and the control section. See indirect method (of flow measurement) and flow type, culvert.
culvert, sag. A culvert where the inlet and outlet flow line is above the barrel flow line. A culvert that
“sags” to pass under a low highway grade line. More commonly used to convey irrigation flows; not
suitable for drainage subject to freezing. In the common but incorrect highway vernacular, a “siphon”
or “inverted siphon.”
A pipeline crossing a depression or under a highway, railroad, canal, etc., that makes use of pressure
flow. A closed conduit, a part of which rises above the hydraulic grade line. It utilizes atmospheric
pressure to affect or control the flow of water through it. The term “inverted siphon” or “siphon” is
commonly and incorrectly used in highway drainage as such structures have none of the properties of
a true siphon; i.e., these two terms are misnomers.
cumulative conveyance. A tabulation or graphical plot of the accumulated measures of conveyance
at various points across a cross section proceeding from the landward edge of one floodplain (or
stream bank) to the landward edge of the other floodplain (or stream bank) so as to encompass the
water surface width of a particular flood. Compare with flow distribution.
curb opening inlet. See inlet, curb opening.
current. Water flowing through a channel. The generally downstream moving portion or vector of
flowing water.
current meter. An instrument for measuring the speed of flowing water. The U.S. Geological Survey
uses a rotating cup meter (32).
A device that is lowered into a stream to record the rate at which the current is moving (20).
An instrument used to measure the flow velocity of a current. It is usually operated by a wheel
equipped with vanes or cups that is rotated by the action of the impinging current. An indicating or
recording device is provided to measure the speed of rotation that is correlated with the velocity of
the current. A device or instrument for determining or measuring the velocity of flowing water by
ascertaining the speed at which a stream of water rotates a vane or a wheel.
cusec. The abbreviation for cubic feet per second is common in the British Commonwealth countries,
but is generally not used in the United States (36). See cubic feet per second.
cut bank. The concave wall of a meandering stream. The outside, eroding bank of a bendway.
cutoff. A natural or artificial channel that shortens the length of a stream; natural cutoffs may occur
either across the neck of a meander loop (neck cutoffs) or across a point bar (chute cutoffs). Compare
with cutoff wall.
cutoff, chute. See cutoff.
Glossary
G-29
cutoff, neck. See cutoff.
cutoff wall. A wall, collar, or other structure intended to reduce percolation of water along otherwise
smooth surfaces, or through porous strata. May also be a wall, usually constructed of such things as
sheet piling or concrete, that extends from the end of a drainage structure and/or flow line downward
to below the expected scour depth, or to scour-resistant material. Compare with cutoff.
cycle. A regularly recurring succession of events such as the cycle of the seasons. Climatic cycle
sometimes is used to describe a group of wet years followed or preceded by a group of dry years (4).
Compare with trend and probability distribution.
D16. The particle diameter at the 16 percentile point on a size versus weight distribution curve.
D50. Median size of riprap or granular material. The particle diameter at the 50 percentile point on a
size versus weight distribution curve such that half of the particles (by weight) are larger and half
are smaller.
D85. The particle diameter at the 85 percentile point on a size versus weight distribution curve.
daily discharge. Discharge averaged over one day.
dam. A barrier to confine or raise water for storage or diversion, or to create a hydraulic head.
damage reach. A length of floodplain or valley selected for damage evaluation (67).
dam-break analysis. The use of a [computer] model to calculate the effects of a flood caused by the
actual or hypothetical failure of a dam (32).
dam, crib. A barrier made of timber, forming bays or cells that are filled with stone or other
suitable material.
dam, debris. The barrier constructed across a channel to form a debris basin.
dam, diversion. A barrier constructed for the purpose of diverting part or all the water from a channel
into a different course.
dam method (of flow measurement). A method of indirectly determining the peak discharge of a
flood by using the field survey of highwater marks in the headwater and tailwater pools, and the dam
or highway-embankment geometry. Discharge is computed by recognized head-on-weir formulas,
adjusted for degree of submergence when necessary. See indirect method (of flow measurement).
datum. Plane of reference for elevations.
dc. See critical depth.
dead storage. See storage, dead.
debris. Any material transported by the stream, either floating or submerged, such as logs, brush,
suspended sediment, bed load, or trash that may lodge against a structure. Compare with detritus.
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Highway Drainage Guidelines
debris barrier. See drift barrier.
debris basin. See basin, debris.
debris cone. A fan-shaped deposit of soil, sand, gravel, and boulders at the point where a steep
stream meets a valley or where its velocity is reduced sufficient to cause such deposits. Compare with
alluvial fan.
debris dam. See dam, debris.
deep water (for waves). Water of such a depth that surface waves are little affected by bottom
conditions; customarily, water deeper than half the wave length.
deflector. Little used alternative term for “spur dike” or “spur.” See dike, spur.
degradation (streambed). General and progressive lowering of the longitudinal profile of the
channel bed due to long-term erosion. A progressive lowering of the channel bed due to scour.
Permanent or continuing degradation is an indicator that a change in the stream’s discharge and
sediment load characteristics is taking place.
demonstrably. Where such things as an engineering analysis or assessment, sometimes coupled with
an economic analysis or assessment, along with prudent judgment is used to support a drainagerelated finding.
density of water sediment mixture. Bulk density (mass per unit volume) including both water
and sediment.
dental. A tooth-like projection on an apron, stilling basin, or other surface, to deflect or break the
force of flowing water; a form of baffle. See apron and baffle piers.
dentated sill. A notched sill at the end of an apron or stilling basin to check the force of flowing
water and thus reduce erosion downstream from the apron. Compare with dental.
dependable yield, n-years. The minimum supply of a given water development that is available on
demand, with the understanding that lower yields will occur once in n-years, on the average (51).
dependent variable. See variable.
depletion. The progressive withdrawal of water from surface or groundwater reservoirs at a rate
greater than that of replenishment. Compare with recession curve and stream flow depletion.
depletion curve. See recession curve.
deposition. The settling of material from the streamflow onto the bed.
depression storage. See storage, depression.
depth, alternate. For a given rate of flow and a given specific head, two depths of flow are possible.
These two depths are alternate depths. These depths, one less than and one greater than critical depth,
Glossary
G-31
may be present in a channel or conduit for any given value of specific energy above the minimum.
Compare with depth, conjugate.
depth-area curve. A graph showing the change in average rainfall depth as the drainage
area changes.
A graph showing the changes in average rainfall depth as the size of the area receiving precipitation
changes (67). It may also be the relationship between water depth and the land surface area inundated.
depth, conjugate. The alternate depth of flow involved with the hydraulic jump, i.e., the depth d1 and
d2 before and after a hydraulic jump. Unlike the alternate depths for a given specific head, the
conjugate depths for a hydraulic jump reflect the energy loss from the hydraulic jump. Compare with
depth, alternate.
depth, critical. See critical depth.
depth, normal. The depth of water in an open conduit that corresponds to uniform velocity for the
given flow. It is a hypothetical depth under conditions of steady non-uniform flow; the depth for
which the water surface and bed are parallel. Normal depth and velocity apply only to uniform flow
with a free water surface. These conditions will be approached with a steady discharge in a length of
uniform channel that is sufficient to establish uniform flow. See channel, uniform.
depth of scour. The vertical distance a streambed is lowered by scour below a reference elevation.
desert. A wild, uninhabited and uncultivated tract. An arid barren tract incapable of supporting any
considerable population without an artificial water supply (Webster’s New Collegiate Dictionary).
Compare with arid and semi-arid.
design/analyses models. Models used for simulation of the detailed performance of particular
elements within a subcatchment (1).
design criteria. Criteria, coupled with prudent judgmental factors, that are used to design a drainage
facility. Compare with policy.
design discharge. The maximum rate of flow (or discharge) for which a drainage facility is designed
and thus expected to accommodate without exceeding the adopted design constraints. Maximum flow
a bridge, culvert, or other drainage facility is expected to accommodate without contravention of the
adopted design criteria. The peak discharge, volume, stage or wave crest elevation, and its associated
probability of exceedance (see exceedance probability) selected for the design of a road culvert or
bridge over a channel, floodplain, or along a shoreline. By definition, the design discharge, or wave,
does not overtop the road. The design discharge headwater, or wave height, may be at an elevation
lower than the road’s profile grade to meet other design criteria such as the protection of property,
accommodating land use needs, lowering of velocities, reducing scour, or complying with
regulatory mandates.
With irrigation facilities, the design discharge is the water right plus any flood right. Where
floodwaters can enter the irrigation ditch or canal, the design discharge would include the water right,
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Highway Drainage Guidelines
flood right, and any floodwaters capable of being conveyed to the point of interest (without first
escaping elsewhere).
Selected flood discharge and corresponding recurrence interval for designing a highway
encroachment. The design flood for highway bridge scour analyses shall be the base (100-year) flood,
and the review (500-year) flood. Compare with flood (or storm), review; flood (or storm),
catastrophic; and superflood.
design fish. See fish, design.
design flood. See design discharge.
design flood for bridge scour. The flood flow equal to or less than the 100-year flood that causes the
deepest scour at bridge foundations. The highway or bridge may be inundated at the stage of the
design flood for bridge scour. The worse case scour condition may occur for the overtopping flood
due to the potential for pressure flow.
design flood (or storm) system. Drainage facility (or storm drain system) that uses natural terrain
and constructed facilities to provide for the conveyance of runoff from a frequently occurring flood
(or storm), up to and including the design flood (or storm runoff), in a practicable and cost-effective
manner so as to avoid a significant increase in the existing flood hazard or causing a flood hazard
greater than allowed by policy and design criteria. By definition this system also precludes
overtopping of the highway and an unacceptable flood hazard due to an encroachment on a floodplain
or spread on the traveled way of guttered highways and streets. See encroachment and spread.
The water conveyance elements (such as gutters, inlets, laterals, trunks, pipes, channels, and ditches)
of a storm drain system. Storm drains generally are that portion of the total drainage system between
the most remote point of interception of runoff, such as a gutter(s), to an outfall(s). Sometimes
erroneously called storm sewer when sewage is not involved. Compare with sewer, storm; drain,
storm; and sewer, combined.
design flood frequency (or storm frequency). The frequency (recurrence interval) for the selected
design discharges (storms) that is expected to be accommodated without contravention of the adopted
design criteria. See design discharge.
design flow. See design discharge.
design highwater elevation. The maximum water levels that can occur through a reach and at a
culvert, bridge-type opening, or other drainage facility without contravention of the adopted design
criteria. May also be the usual term used to describe the estimated water surface elevation or profile in
the stream (or other surface waters) at the project site for the selected design discharge.
design highwater level. See design highwater elevation.
design storm. Selected storm of a given frequency (recurrence interval) used for designing a design
storm system. See design flood (or storm) system.
Glossary
G-33
Hypothetical storm derived from intensity-duration-frequency curves by reading the rainfall intensity
from these curves for various durations for the frequency of interest and rearranging these rainfall
intensities to fit an assumed storm pattern and storm duration (67).
A given rainfall amount, areal distribution, and time distribution, used to estimate runoff. The rainfall
amount is either a given frequency (25-, 50-year, etc.) or a special large [or specific frequency]
value (67).
detention basin. See basin, detention.
deterministic. See hydrology, deterministic.
detour. A temporary change in the roadway alignment. It may be localized at a structure or may be
along an alternative route.
detritus. Loose fragments, particles, grains, grasses, twigs, or similar material comprising the smaller
fraction of debris carried by flowing water. Compare with debris and drift.
development. Refers to any constructed change to improved or unimproved real estate, including but
not limited to buildings or other structures, mining, dredging, filling, grading, paving, excavation, or
drilling operations or storage of equipment or materials. Taken from the 1992 NFIP regulations.
Land use changes from those of an existing condition.
Compare with direct and indirect support (of floodplain development), land treatment measure,
and land use.
dike. An impermeable linear structure for the containment or control of overbank flow; such dikes
trend parallel with a river bank and differ from a levee only in that such dikes extend for a much
shorter distance along the bank. Relatively short dikes are also placed to contain and redirect flow
such as into a culvert or down some other path. Compare with levee.
dike, embankment spur. Relatively short dikes constructed normal to an embankment such as an
approach fill to a bridge or otherwise along a highway. Their purpose is to impede flow and direct it
away from such embankments. An embankment spur dike is placed at an angle to the roadway for the
purpose of shifting the erosion characteristics of stream or floodplain flow away from a drainage
structure or a roadway embankment. Compare with dike, riparian spur.
dike, finger. See dike, embankment spur.
dike, guide. See guide bank.
dike, riparian spur. River training structure used for bank protection. A permeable or impermeable,
linear structure projecting into a channel from a bank. A dike of rock or other material constructed
from the bank into the channel for protection or for channel improvement. A dike extending from a
bank into a channel that is designed: (1) to reduce the stream velocity as the current passes through
the dike, thus encouraging sediment deposition along the bank (permeable dike); or (2) to deflect
erosive currents away from the stream bank (impermeable dike). An embankment or wall constructed
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Highway Drainage Guidelines
more or less perpendicular to a stream bank or shoreline (also termed “groin”). A structure in the form
of a barrier placed oblique to the primary motion of water, designed to control movement of bed load.
Riparian spur dikes (groins) are usually solid, although they may be constructed with openings to
induce sedimentation and control the elevations of such sediments. An elongated, permeable or
impermeable obstruction projecting into a stream to control shoaling and scour by deflection of
currents and waves. A dike constructed of piles, rock, or other material extending into a stream, the
sea, or at the mouths of rivers to induce scouring, bank building, or bank or protection. Compare with
dike, embankment spur; and retard.
dike, spur. An outdated term for a guide bank. See dike; dike, riparian spur; and guide bank.
dike, toe. See dike, training.
dike, training. Embankments constructed to provide a transition from the natural stream channel or
floodplain into and/or away from a channel constriction such as a bridge crossing. Embankments
constructed to prevent lateral flow from scouring the corner of the downstream side of an approach
embankment to a bridge and the bridge abutment.
dimictic lake. See lake, dimictic.
direct and indirect support (of floodplain development). To encourage, allow, serve, or otherwise
facilitate additional base (100-year) floodplain development. Direct support results from an
encroachment. Indirect support results from a highway action outside the base floodplain that leads to
development in the base floodplain. Compare with development.
direct precipitation. Water that falls directly into a lake or stream without passing through any land
phase of the hydrologic cycle (20).
direct runoff. See runoff, direct. Compare with rainfall, effective.
dirt. See soil.
discharge. Volume of water passing a point during a given time. The rate a volume of flow passes a
point per unit of time, usually expressed in cubic meters per second (m3/s) or cubic feet per second
(ft3/s). Four somewhat differently stated definitions are: (1) the quantity of water, silt, or other mobile
substances passing along a conduit per unit of time; (2) rate of flow such as cubic meters [feet] per
second, liters [gallons] per second, millions of liters [gallons] per day; (3) the act involved in water or
other liquid passing through an opening or along a conduit or channel; (4) the water or other liquid
that emerges from an opening or passes along a conduit or channel. See cubic meters per second or
cubic feet per second.
In its simplest concept, discharge means outflow; therefore, the use of this term is not restricted as to
course or location, and it can be applied to describe the flow of water from a pipe or from a drainage
basin. If the discharge occurs in some course or channel, it is correct to speak of the discharge of a
canal [channel] or of a river. It is also correct to speak of the discharge of a canal [channel] or stream
into a lake, a stream, or an ocean (see streamflow and runoff) (36). The volume of water (or more
broadly, volume of fluid plus suspended sediment) that passes a given point within a given period of
time (50).
Glossary
G-35
The data in the reports of USGS on surface water represent the total discharge, and streamflow and
runoff represent water with the solids dissolved in it and the sediment mixed with it. Of these terms,
discharge is the most comprehensive. The discharge of drainage basins is distinguished as follows:
yield—total water runoff or crop; includes runoff plus underflow;
runoff—that part of water yield that appears in streams;
streamflow—the actual flow in streams, whether or not subject to regulation, or underflow. See
underflow.
Each of these terms can be reported in total volumes (such as cubic meters [feet] per second or
hectare-meter [acre-feet] per year). The differentiation between runoff as a volume and streamflow as
a rate is not accepted. Compare with runoff and streamflow.
discharge, average. In the annual series of the USGS reports on surface-water supply, the arithmetic
average of all complete water years of record whether or not they are consecutive. Average discharge
is not published for less than five years of record. The term “average” is generally reserved for
average of record, and “mean” is used for averages of shorter periods, namely, daily mean discharge.
discharge curve. See stage-discharge curve.
discharge, design. See design discharge.
discharge, dominant. The channel forming (morphological sense) discharge in a specific channel for
a specific channel feature. The dominant discharge for hydraulic geometry relationships is sometimes
taken to be the bankfull discharge. With stable banks, the bankfull discharge has a return period of
approximately 1.5 years, whereas the bankfull discharge is sometimes associated with the mean
annual flood (Q2.33) or two-year flood (Q2).
discharge rating curve. See stage-discharge curve.
discharge, mean. See discharge, average.
discharge, peak. See peak discharge.
discharge, sediment. See sediment discharge, suspended and sediment discharge, total.
discharge, unit. Discharge per unit width (may be average over a cross section, or local at a point).
disk, Secchi. See Secchi disk.
distributed system. The opposite of a lumped system. See lumped system.
distribution-free. Requiring no assumptions about the kind of probability distribution a set of data
may have (33).
distribution, frequency, and distribution, graph. See probability distribution.
distribution, hydrograph. See hydrograph, distribution.
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Highway Drainage Guidelines
distribution, probability. Various probability distributions that may be of interest to a hydrologist
and highway hydraulics engineer are California, Cauchy, Exponential, Extremal, Extreme Value,
Gumbel, Logarithmetically Transformed, Normal, Partly Bounded, Pearson, Student’s t, Totally
Bounded, and Weibull. See probability distribution.
distribution system. A [statistical] system in which at least some of the variations in space have been
recognized and established (1). Also used to refer to such things as a drinking water system.
ditch. An artificial channel, usually distinguished from a canal by its smaller size. See canal, channel,
and field ditch.
ditch, field. See field ditch.
diversion. See channel diversion.
diversion dam. See dam, diversion.
diversity. An environmentally related term referring to the range and number of species types in the
biomass of surface waters.
divide. See drainage divide.
doctrine. A rule, principle, theory, or tenet of the law. See rule and rule of law.
dominant discharge. See discharge, dominant.
double-mass curve. A plot on arithmetic cross-section paper of the cumulated values of one variable
against the cumulated values of another or against the computed values of the same variable for a
concurrent period of time (58). A graph in which accumulated amounts of item X are plotted versus
accumulated amounts of item Y, the amounts for given times being used (67).
drain. A conduit for carrying off surplus groundwater or surface waters. Closed drains are
usually buried.
drainage. Four definitions are provided: (1) the process of removing surplus groundwater or surface
waters by artificial means; (2) the manner in which the waters of an area are removed; (3) the area
from which waters are drained; (4) a drainage basin. See drainage area.
drainage area. The catchment area for rainfall [and other forms of precipitation] that is delineated as
the drainage area producing runoff, i.e., contributing drainage area. Usually it is assumed that base
flow in a stream also comes from the same drainage area.
The drainage area of a stream at a specified location is that area, measured in a horizontal plane,
which is enclosed by a drainage divide. Over the years, use of the term to signify drainage basin or
catchment area has come to predominate, although drainage basin is preferred. Used alone, the term
“watershed” is ambiguous and should not be used unless the intended meaning is made clear (36).
According to the National Engineering Handbook (67): (1) the area draining into a stream at a given
point. The area may be of different sizes for surface runoff, subsurface flow, and base flow, but
Glossary
G-37
generally the surface runoff area is used as the drainage area; (2) the area contributing direct runoff to
a stream. Usually it is assumed that base flow in the stream also comes from the same area. However,
the groundwater watershed may be larger or smaller.
The drainage area of a stream at a specified location is that area, measured in a horizontal plane,
enclosed by a topographic divide from which direct surface runoff from precipitation normally
drains by gravity into the stream above the specified point. Figures of drainage area given in [WRD
Data Reports] include all closed basins or noncontributing areas within the area unless otherwise
noted (50).
The area of land drained by a channel. An area confined by drainage divides, often having only one
outlet for discharge; the total drainage area contributing runoff to a single point. The watershed area
to include all other catchment physical characteristics. The term “catchment” is often used
synonymously with other terms such as “drainage area” and implies all physical characteristics,
including the contributing area.
An area surrounded by a continuous ridge [or drainage divide] within which all runoff is expected to
join into a single flow stream and that extends to the point of junction of this flow stream
(downstream) with the ridge. Natural boundaries, constructed boundaries, or minimum size of pipe
are criteria that can be used to define the catchment (1).
Three other definitions of interest are: (1) the area drained by a stream or stream system;
(2) synonymous with drainage area, drainage basins, or catchment area; (3) for the sake of clearness,
the use of watershed to mean catchment basin or drainage basin is avoided by the USGS and others.
Compare with drainage basin.
drainage basin. A part of the surface of the earth that is occupied by a drainage system, which
consists of a surface stream or a body of impounded surface water together with all tributary surface
streams and bodies of impounded surface water (36). The land area from which surface runoff drains
into a stream system (20). A part of the surface of the earth that is occupied by a drainage system,
which consists of a surface stream or a body of impounded surface water together with all tributary
surface streams and bodies of impounded surface water (50). Compare with drainage area.
drainage density. Length of all channels above those of a specified stream order per unit of drainage
area (36).
drainage divide. The rim of a drainage basin. The divide separating one drainage basin from another
and in the past has been generally used to convey this meaning. Drainage divide, or just divide, is
used to denote the boundary between one drainage area and another (36).
drain, storm. A conduit for carrying off stormwaters. See drain; design flood (or storm) system;
sewer, combined; and major storm drain. Compare with sewer.
draw. Term of regional geographical usage. See channel.
drawdown. The difference in elevation between the water surface elevation at a constriction in a
stream or conduit and the elevation that would exist if the constriction were absent. Drawdown also
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occurs at changes from mild to steep channel slopes and at weirs or vertical spillways. Compare with
drawdown, rapid.
drawdown curve. A particular form of the surface curve of a stream of water that is convex upward,
generally a conversion from tranquil flow upstream to rapid flow downstream in which the stream
flow drops through critical depth, such as flow over a weir or spillway or a sharp increase in
streambed slope, proceeding downstream. Compare with backwater curve.
drawdown, rapid. Lowering the water against a bank more quickly than the bank can drain which
can leave the bank in an unstable condition due to excessive pore pressure. Compare with drawdown.
drift. Alternative term sometimes used (perhaps incorrectly) for “debris.” Debris that drifts on or near
the water surface. Sometimes used as general term for all debris, that floated or otherwise, in evidence
following a flood. Compare with detritus and debris.
drift barrier. An open structure constructed across a stream channel to catch drift or debris. It may
be of any form from a simple wire fence to a barrier of massive piers with heavy cables strung
between them. Where acceptable, the material caught is burned in place at low stages of the stream.
drift line. An accumulation of deposited water-carried debris and/or detritus along a contour, at the
base of vegetation, or within the vegetation and other topographic objects that provides direct
evidence of prior inundation and often indicates the directional flow of floodwaters. See
highwater mark.
drift rate. The mass of total macro-invertebrates drifting with the current per unit of time, usually
grams/hour [ounces/hour]. Depending on context, may be the rate (velocity), such as meters [feet] per
second, that floodborne drift or detritus is transported downstream.
drip groove. Linear depression in the bottom of structure components to cause water flowing on the
surface to drop.
drop. Two definitions are offered: (1) a vertical or inclined structure for dropping the water in a
conduit or channel to a lower level and dissipating its surplus energy. An inclined drop in a channel is
often termed a chute; (2) a fall in water-surface elevation between the upstream and downstream (as
between headwater and tailwater) sides of a flow-contacting drainage facility such as at a bridge
constriction or culvert or between two sections of a slope-area reach.
dry weather flow. The flow in sanitary or combined sewers that contains no stormwater (1).
dune. A sand wave of approximately triangular cross section (in a vertical plane in the direction of
flow) formed by moving water or wind, with gentle upstream slope and steep downstream slope.
Dunes travel downstream by the displacement of sediments on the upstream slope and their
subsequent deposition on the downstream slope. See bed form. Compare with antidune.
duration curve. See flow-duration curve for one type (36).
A graph showing the percentage of time that the given flows of a stream will be equaled or exceeded.
It is based upon a statistical study of historic streamflow records (20).
Glossary
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dynamic equilibrium. The delicate balance of many factors that must occur in a stream reach so that
the channel is neither aggrading or degrading. See stream, poised.
EAP. Acronym for Emergency Action Plan (32). See Emergency Action Plan.
earth dam. A barrier to streamflow composed of earth, clay, sand, or sand and gravel, or a
combination of earth and rock.
eccentricity. An expression descriptive of the position of a bridge constriction not centrally located in
the floodplain. Eccentricity, if considerable, has an effect upon the discharge coefficient. High
eccentricity tends to decrease the discharge coefficient and increase the backwater.
eccentricity ratio. The eccentricity ratio (Ke) is defined as the ratio of flow contracted from the
smaller lateral region on one side of a stream to the flow contracted from the larger lateral region on
the other side, and is expressed as Ke = Ka/KbҖ ≤ 1.0, where Ka and Kb are the conveyances of the
smaller and larger lateral regions (approach cross section sub-sections).
economic analysis. A probabilistically based analysis that compares the estimated construction costs
with those expected average flood-related operational costs and risks that can be quantified for the
anticipated service life of a project to identify an optimum design flood frequency. Compare with
economic assessment.
economic assessment. An economic assessment is a less rigorous, more judgmental economic
analysis. Compare with economic analysis.
ecoregion. Continuous geographical areas characterized by distinctive flora, fauna, land forms,
climate, vegetation, and ecological climax.
ecosystem. The living organisms and the nonliving environment interacting in a given area. The
complexity of a community and its environment functioning as an ecological unit in nature.
eddy current. A vortex-type motion of a fluid flowing contrary to the main current, such as the
circular water movement that occurs when the main flow becomes separated from the bank or
emerges from a bridge or culvert opening.
eddy loss. The energy lost (converted into heat) by swirls, eddies, and impact, as distinguished from
friction loss.
effect. According to Webster’s New Collegiate Dictionary: (1) something produced by an agent or
cause; result; (2) an outward sign; appearance; (3) a distinct impression.
What occurs to surface waters as a direct or indirect result of a highway action. See significant.
Compare with impact.
effective duration. The time in a storm during which the water supply for direct runoff is produced.
Also used to mean the duration of excess rain (67).
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effective particle size. The diameter of particles, equivalent to a spherical shape, equal in size and
arranged in a given manner, of a hypothetical sample of granular material that would have the same
transmission constant, shear resistance (see tractive force), erosion/scouring response, and fall
velocity as the actual material under consideration.
effective precipitation. See precipitation, effective.
effective rainfall. See rainfall, effective.
effluent. Sewage, water, or other liquid, partially or completely treated or in its natural state as the
case may be, flowing out of a reservoir, basin, or treatment plant.
effluent stream. See stream, gaining.
electrolytes. Substances that dissociate into ions in solution or when fused, thereby becoming
electrically conducting.
embankment end slope. Conical slope at end of road approach embankment; bridge spillslope. See
spillthrough abutment and compare with fill slope.
embankment spur dike. See dike, embankment spur.
Emergency Action Plan (EAP). A detailed description of the actions that must be taken to reduce
flood losses [and hazards] and to disseminate information about an actual or expected flood
hazard (32).
Emergency Operation Center (EOC). The center [for] decision making, information gathering and
dissemination, initiating emergency actions, and coordinating responses to emergencies. The center
may be part of a public service or safety unit in the local government or may be activated only when
emergencies arise (32).
emergency spillway. See spillway, emergency.
eminent domain. In law, the right of a government to take or to authorize the taking of private
property for public use, just compensation being given to the owner.
encroachment. A highway action within the limits of a base (100-year) floodplain. With a design or
review storm system, encroachment is sometimes used when referring to the width of gutter flow
spreads onto a traveled way as measured perpendicular from either the edge of the traveled way or
from the face of the curb. See spread.
encroachment, longitudinal. An encroachment that roughly parallels a channel or floodplain.
encroachment, transverse. An encroachment that crosses a channel or floodplain. Compare
with spread.
end contractions. The contraction in the area of overflowing water caused by such things as the ends
of a weir notch.
Glossary
G-41
end section. A structure, commonly made of concrete or metal, that is attached to the end of a culvert
for such purposes as retaining the embankment from spilling into the waterway, improving the
appearance, providing anchorage, improving the discharge coefficient, and limiting some scour at the
outlet. Compare with inlet, flared.
energy. The capacity to perform work—kinetic energy is that due to motion, and potential energy is
that due to position. In a stream the total energy at any section is represented by the sum of its
potential and kinetic energies.
energy dissipation. The phenomenon whereby energy is dissipated or used up. In highway drainage
this is the energy in flowing water.
energy dissipator. See stilling basin.
energy equation. The work-energy relationship, reduced to the simplified form from the Bernoulli
equation (Bernoulli’s Theorem that states that P/γ + Z + V2/2g = constant). The equation is:
V12/2g + P1/γ + Z1 = V22/2g + P2/γ + Z2 + hL
where: subscripts 1 and 2 denote the upstream and downstream cross section.
energy grade line. A line joining the elevation of energy heads of a stream; a line drawn above the
hydraulic grade line a distance equivalent to the velocity head of the flowing water at each cross
section along a stream or channel reach or through a conduit. An inclined line representing the total
energy of a stream flowing from a higher to a lower elevation. For open channel flow the energy
grade slope is located (or plotted) a distance equal to the velocity head (V2/2g) plus the flow depth
above the water surface: V = velocity, and g = acceleration due to gravity. Slope of the foregoing line
joining the elevations of total energy through the reach of a stream or channel or through a conduit of
flowing water.
energy grade slope. See energy gradient.
energy gradient. The slope of the energy line with reference to any plane or, more simply, the slope
of the energy grade line. The slope of this line represents the rate of loss of head, and it must always
slope downward in the direction of flow. Compare with hydraulic gradient and friction slope.
energy head. See head, energy.
energy, kinetic. Energy due to motion. The kinetic energy of a given discharge is generally taken as
proportional to the product of its weight per unit of time and the velocity head of its mean velocity.
For a constant discharge, kinetic energy may be represented by a line at a distance above a flowing
water surface proportional to the velocity head of its mean velocity. The elevation of such a line
above any datum represents the total energy (potential plus kinetic) of the given discharge above
that datum. Strictly, the kinetic energy of a given discharge is the integral of the kinetic energies
of its particles.
energy line. See energy grade line.
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energy line, minimum. An energy line corresponding to conditions of critical flow.
energy, potential. Energy due to position. The potential energy of a given volume of immobile water
with reference to any datum is proportional to the product of its weight and the elevation of the center
of gravity above that datum. The potential energy per unit of time of a given discharge at any instant
with reference to any datum is proportional to the product of its weight per unit of time and the
elevation of its hydraulic grade line above that datum at that instant.
energy slope. See energy grade line.
energy, specific. The energy contained in a stream of water expressed in terms of head, referred to
the bed of a stream. It is equal to the mean depth of water plus the velocity head of the mean velocity.
The energy of a stream referred to its bed; namely, depth plus the velocity head based on the
mean velocity.
energy, total. See energy.
enhancement. See surface water enhancement.
entrance head. The head required to cause flow into a conduit or other structure; it includes both
entrance loss and velocity head; equivalent to headwater height; energy head at approach section to
culvert or bridge.
entrance loss. The head lost in eddies and friction at the inlet to a conduit or structure, expressed as a
coefficient (Ke) times velocity head = Ke (V2/2g).
entrenched stream. See stream, incised.
envelope curve. See flood envelope curve.
environmental effects. Pertaining to the effects of highway engineering works on their surroundings
and on nature.
EOC. Acronym for Emergency Operation Center.
eon. A geologic time interval containing two or more eras. Compare with era.
EPA. Acronym for the U.S. Environmental Protection Agency.
ephemeral stream. See stream, ephemeral.
epilimnion. The upper, circulating water layer of a stratified lake. See thermal stratification.
equalizer. An opening, such as a culvert or bridge, placed where it is desirable to equalize the water
level on both sides of an embankment.
Glossary
G-43
equivalent cross-slope. An imaginary, straight cross-slope that provides conveyance equal to that of
an actual or proposed compound cross-slope. Common usage in highway drainage is to facilitate
estimating the stormwater conveyance of a street gutter having a compound cross-slope.
era. A division of geological history of highest rank. There are five eras: Cenozoic, Mesozoic,
Paleozoic, Proterozoic, Archeozoic. Compare with eon.
erosion. Displacement of soil particles on the land surface due to such things as water or wind action.
The wearing away or eroding of material on the land surface or along channel banks by flowing water
or wave action on shores. Compare with abrasion, scour, mass wasting, and sloughing.
erosion control matting. Fibrous matting (e.g., jute, paper) placed or sprayed on a channel bank or
land surface for the purpose of preventing erosion or providing temporary stabilization until
vegetation is established.
erosion, lateral. Erosion in which the removal of material has a predominately lateral component (see
erosion), as contrasted with scour in which the component is predominately vertical (see scour).
estuary. Tidal reach at the mouth of a river. That portion of a river channel occupied at times or in
part by both sea and river flow in appreciable quantities. The water usually has brackish
characteristics.
ET. Acronym for evapotranspiration (67). See evapotranspiration.
eutrophic lake. See lake, eutrophic.
evaluation series. A list of floods or storms that produced floods during a representative period [of
time] and used in water project evaluation to obtain estimates of flood damages (67).
evaporation. The process by which water passes from the liquid to the vapor state (20).
The process by which water is changed from the liquid or the solid state into the vapor state. In
hydrology, evaporation is vaporization that takes place at a temperature below the boiling point.
evaporation opportunity. Relative evaportranspiration: The ratio of the rate of evaporation from a
land or water surface in contact with the atmosphere to the evaporativity under existing atmospheric
conditions. It is the ratio of actual to potential rate of evaporation, generally stated as a
percentage (46).
The opportunity for a given rate of evaporation to continue is determined by the available moisture
supply (48).
evaporation, total. The sum of water lost from a given land area [drainage area] during any specific
time by transpiration from vegetation and building of plant tissue; by evaporation from water
surfaces, moist soil, and snow; and by interception [questionable—Ed.]. It has been variously termed
“evaporation,” “evaporation from land areas,” “evapotranspiration,” “total loss” [questionable—Ed.],
“water losses,” and “fly off” (38).
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evaporativity. Potential rate of evaporation, i.e., the rate of evaporation under the existing
atmospheric conditions from a surface of water that is chemically pure and has the temperature of
the atmosphere (46).
evapotranspiration. Plant transpiration plus evaporation from the soil. Difficult to determine
separately, therefore used as a unit for study (see consumptive use) (67). The combined loss of water
from a given area by evaporation from the land and transpiration from plants (68). The sum of
evaporation plus transpiration (20).
Water withdrawn from a land area by evaporation from water surfaces and moist soil and plant
transpiration. It is a coined word; probably the first recorded use is on page 296 of the Transactions of
the American Geophysical Union, Part 2, 1934 (36).
evapotranspiration potential. See potential evapotranspiration.
event. As used in data collection, [event] represents a point at which a gage reaches a preset value and
records the occurrence or transmits it to a receiver (32). Compare with flood event.
event models. [Computer] Models that simulate the rainfall-runoff process for a single rainfall event,
where pre-event conditions must be specified. Event models usually incorporate short time intervals
in the simulation (1).
evergreen. Plants that retain their leaves at the end of the growing season and usually remain green
through the winter.
exceedance frequency. The percentage of values that exceed a specified magnitude, 100 times
exceedance probability (36).
exceedance interval. See flood frequency.
exceedance probability. Probability that a random event will exceed a specified magnitude in a
given time period, usually one year unless otherwise indicated (33).
excess rainfall. See rainfall excess.
excessive precipitation. See rainfall, excessive.
excessive rainfall. See rainfall, excessive.
exfiltration. The process by which stormwater leaks or flows to the surrounding soil through such
things as openings in a conduit, channel banks, or lake shores.
expanding reach. A reach of channel wherein flow is decelerating; where velocity head at the lower
cross section is less than the velocity head at the upper cross section.
exponential distribution. See probability distribution.
extremal distribution. See probability distribution.
Glossary
G-45
extreme value distributions. See probability distribution.
extreme values. The largest and the smallest value [from a sample of data] are commonly referred to
as extreme or [external] values, and are often associated with floods, droughts, surplus or deficit, and
similar economic and safety connotations (75). See probability distribution.
fabric filter. See synthetic filter.
fabriform. Grout-filled fabric mattress used for stream bank protection.
fall velocity. See velocity, fall.
falling limb. The declining portion of a hydrograph following a crest (32).
fallow. Cropland kept free of vegetation during the growing season. May be a normal part of the
cropping system for weed control, water conservation, soil conditioning, etc. (67).
fascine. See retard.
fauna. The animals of a particular region or time.
federal common law. See common law, federal.
Federal Register. A daily publication of the Federal Government making federal regulations, legal
notices, Presidential Proclamations, Executive Orders, etc., known to the public as they are proposed
and subsequently issued. See also Code of Federal Regulations.
FEMA. Acronym for Federal Emergency Management Agency.
fern allies. A group of nonflowering, vascular plants comprised of clubmosses, small clubmosses,
and quillworts.
fetch. The effective distance the wind blows over water in generating waves. The area in which
waves are generated by wind having a rather constant direction and speed; sometimes and incorrectly
used synonymously with “fetch length.” The horizontal distance (in the direction of the wind) over
which wind generates waves and wind setup.
fetch length. Redundant terminology, as the term “fetch” implies a length. See fetch.
FHWA. Acronym for Federal Highway Administration.
field capacity. See field-moisture capacity.
field data. All stormwater data collected in the field regardless of whether or not it was analyzed in
the field (1).
field ditch. An irrigation channel for: (1) delivering water to the rows in a field, or (2) flooding a
field. A field ditch commonly does not have a recorded water and flood right at its point of diversion.
These ditches deliver the water to the field from an irrigation canal having a recorded water or flood
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right at its point of diversion. The water right for a field ditch can sometimes be estimated based on
the area being irrigated by the ditch at a point in time. That portion of the water right delivered by a
field ditch can change when landowners modify their field ditch system.
field, jack. See jack field.
field-moisture capacity. The quantity of water that can be permanently retained in the soil in
opposition to the downward pull of gravity (24).
field-moisture deficiency. The quantity of water that would be required to restore the soil moisture to
field-moisture capacity (25).
fill slope. Side or end slope of an earth-fill embankment. Where a fill slope forms the streamward
face of a spillthrough abutment, it is regarded as part of the abutment. Compare with embankment end
slope and spillthrough abutment.
filter. Layer of synthetic fabric, sand, gravel, and/or graded rock placed (or developed naturally
where suitable in-place materials exist) between the bank revetment and soil for one or more of three
purposes: (1) to prevent the soil from moving through the revetment by piping, extrusion, or erosion
(exfiltrating); (2) to prevent the revetment from sinking into the soil; (3) to permit natural seepage
from the stream bank, thus preventing buildup of excessive hydrostatic pressure. Also may be a
device or structure for removing solid or colloidal material from stormwater and floodwater or
preventing the migration of fine-grained soil particles as water passes through soil, that is, the water is
passed through a filtering medium—usually a granular material or finely woven or non-woven
geotextile. Depending on context, may be used to remove material other than soils from a substance.
filter blanket. One or more layers of graded, intermediate-size gravel or a geotextile material laid
between fine-grained material and riprap to prevent the migration of the finer material (exfiltration).
Compare with synthetic filter.
filter cloth. See synthetic filter.
filtration. The process of passing water through a filtering medium consisting of either granular
material or filter geotextiles for the removal of suspended or colloidal matter.
fine sediment load. See wash load.
finger dike. See dike, finger.
first flush runoff. See runoff, first flush.
fish. An aquatic animal (pl. fish or fishes) according to Webster’s New Collegiate Dictionary.
fish, design. A hypothetical fish embodying predetermined size, swimming, and migration
characteristics that are used in the design of drainage facilities to minimize adverse effects to the
fishery. See total fish length.
fish ladder. A structure with pools and drops to facilitate the migration and movement of fish around
culverts, chutes, dams, or other obstructions in channels. Compare with fishway.
Glossary
G-47
fish length, total. See total fish length.
fish screen. A device intended to prevent the entrance of fish into a conduit or area where they are
not wanted.
fishway. A waterway, designed or natural, that facilitates the movement of fish. Compare with
fish ladder.
five-hundred-year flood. See flood, five-hundred-year.
fixed bed and banks (model). A channel whose bed and banks are considered to be unable to move
under the forces of moving water. Fixed bed and bank analyses and computer models ignore any
mobility (scour or erosion) of the channel bed or banks. Compare with mobile bed and banks (model).
flanking. Erosion resulting from streamflow between the bank and the landward end of a river
training or a grade-control (drop) structure.
flanking inlet. See inlet, flanking.
flare angle. See angle of flare.
flared inlet, outlet, and wingwalls. See inlet, flared.
flash flood. See flood, flash.
flash flood warning. The public notification issued by the National Weather Service (NWS) on a
county basis when flash flooding is imminent or has been reported (32). See flood, flash.
flash flood watch. The public notification issued by the National Weather Service (NWS) when
climatic, ground, and topographic conditions indicate that flash floods are likely to occur in a given
area (32). See flood, flash.
flashy stream. See stream, flashy.
float gage. See gage, float.
flocculation. The process to cause soil or other substances to form a lump or mass to facilitate
their removal.
flood. In common usage, an event that overflows the normal flow banks or runoff that has escaped
from a channel or other surface waters. See normal flow and bank. In frequency analysis it can also
mean an annual flood that may not overflow the normal flow banks. In technical usage, it refers to a
given discharge based, typically, on a statistical analysis of an annual series of events.
An overflow or inundation that comes from a river or other body of water and causes or threatens
damage. Any relatively high streamflow overtopping the natural or artificial banks in any reach of a
channel. A relatively high flow as measured by either gage height or discharge quantity.
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An overflow or other body of water that causes or threatens damage (6). Any relatively high
streamflow overtopping the natural or artificial banks in any reach of a stream (39). A relatively high
flow as measured by either gage height or discharge quantity (34). See floodwaters and flood, annual.
flood, annual. The maximum momentary peak discharge in each year of record. May be maximum
daily discharge or instantaneous discharge. The highest peak discharge in a water year. The maximum
flow in one year (33).
flood, base. A flood (or storm) or reservoir pool elevation having a one percent chance of being
exceeded in a one-year period; commonly termed a 100-year event. See flood, one-hundred-year
(100-year).
flood-control storage. Storage of water in reservoirs to abate flood damage as discussed in “General
Introduction and Hydrologic Definitions” (36). See also reservoir, retarding and basin, retention.
flood crest. See peak discharge.
flood, design. See design discharge.
flood envelope curve. A plot showing the upper and lower boundary limits of the maximum annual
floods for the range of drainage areas in a hydrologic region.
flood event. A flow of water in a stream constituting a distinct progressive rise, culminating in a
crest, together with the recession that follows the crest. Compare with event and peak discharge.
flood-exceedance probability. Probability that a random flood event will exceed a specified
magnitude in a given time period, usually one year unless otherwise indicated.
flood, five-hundred-year. The flood due to a storm and/or tide having a 0.2 percent chance of being
exceeded in any given year. Compare with superflood.
flood, flash. A flood that occurs in a short time (minutes to hours) after the causative event (32).
flood frequency. The average time interval between occurrences of a hydrologic event of a given or
greater magnitude, usually expressed in years. May also be called recurrence interval. (3) (50).
The average time interval, in years, in which a given storm or amount of water in a stream will be
exceeded. Also, referred to as exceedance interval, recurrence interval, or return period. May be
stated as: (1) the average time interval between actual occurrences of a hydrologic event of a given or
greater magnitude; (2) the percent chance of occurrence in any one-year period, e.g., a two percent
chance of flood. The chances that a specific flood magnitude (discharge) will be exceeded each year
expressed as a percent, i.e., a 100-year flood has a flood probability of one percent of being exceeded
each year. In the analysis of hydrologic data the flood frequency is simply called frequency and has
years as a unit of measure. Note that flood frequency is not hyphenated when referring to a specific
flood’s frequency, but is when referring to such things as a “flood-frequency” curve.
An expression or measure of how often a hydrologic event of given size or magnitude should, on an
average, be . . . exceeded. For example a 50-year frequency flood should be . . . exceeded in size, on
Glossary
G-49
the average, only once in 50 years. In drought or deficiency studies, it usually defines how many
years will, on the average, be . . . less than a given size or magnitude (36). Note: This reference
incorrectly stated “equalled or exceeded,” and “equal to or less than” where the ellipses (. . .)
appear. [Ed.]
flood-frequency curve. “General Introduction and Hydrologic Definitions” (36) offers two
definitions: (1) a graph showing the number of times per year on the average, plotted as abscissa, that
floods of magnitude, indicated by the ordinate, are . . . exceeded; (2) a similar graph but with
recurrence intervals [frequency] of floods plotted as the abscissa. A graph indicating the probability
that the annual flood discharge will exceed a given magnitude, or the recurrence interval
corresponding to a given magnitude. Compare with frequency curve and flood frequency.
According to Dalrymple (18): (1) a graph showing the number of times per year on the average,
plotted as abscissa, that floods of magnitude, indicated by the ordinate, . . . or exceeded; (2) a similar
graph but with recurrence intervals of floods plotted as abscissa.
Note: Flood-frequency is hyphenated when referring to a flood-frequency (flood versus frequency)
curve or relationship, and not hyphenated when referring to a specific flood’s frequency.
flood, greatest opening. The greatest flood expected to flow through a drainage structure where
roadway overtopping cannot occur from the probable maximum flood.
flood hazard. Potential consequences, hazards, and inconveniences encountered by the traveling
public, imposed on adjacent property owners, and incurred by the environment from a flood or a
highway action in areas subject to flooding; included are such things as potential property loss or
damage, loss of life, temporary or long-term loss of a transportation facility, permanent or
long-lasting environmental damage, circuitous or interrupted highway travel, hydroplaning and other
roadway overtopping related hazards. Compare with flow hazard.
flood hydrograph. See hydrograph.
flood index. The amount of rainfall that will produce a flood stage in six hours; it is calculated by
measuring soil moisture (32).
flood loss potential. The potential loss of life and property from flooding (31).
flood, maximum possible. Not an accepted term. See flood, greatest opening.
flood, mean annual. Maximum annual flood peak having a 2.33 year frequency interval (recurrence
interval). Flood where, if the total population of floods were known, half would be larger and the
remaining half would be smaller. Compare with flow, mean annual; mean daily discharge; and mean
monthly flow.
flood measurement. See indirect method (of flow measurement).
flood of record. The maximum estimated or measured discharge that has occurred at a site.
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flood, one-hundred-year. Magnitude of the flood that has a 0.01 probability of being . . . exceeded
[see following note—Ed.] in any given year and has approximately a 63 percent chance of being
exceeded during a 100 year [year period]. It is now in vogue to call the 100-year flood the one percent
probability (chance) flood (16). In highway drainage design the 100-year flood is sometimes termed
the base flood. See flood, base. Note: It is more proper to say “probability of being exceeded in any
. . .,” rather than “. . . exceeded.” [Ed.] Compare with flood, one-thousand-year. See X-percent chance
flood and flood frequency.
flood, one-thousand-year. Magnitude of the flood that has a 0.001 probability of being . . . exceeded
in any given year. Over an infinitely long period of time it would occur on an average once every
thousand years. It should be noted that over 1,000 years [year period] there is approximately a 63
percent chance of at least one occurrence and there is a significant chance of two, three, or more
floods of this magnitude occurring during the thousand years (16). Again, “probability of being
exceeded in any” is more proper. [Ed.] See flood, one-hundred-year and X-percent chance flood.
flood (or storm), catastrophic. Any flood (or storm) well in excess of the base flood (or storm) and
review flood (or storm) but not exceeding the probable maximum flood (or storm). A flood (or storm)
capable of causing a large amount of unavoidable damage and destruction. Compare with superflood;
flood (or storm), review; and design discharge.
flood (or storm), check. See flood (or storm), review.
flood (or storm), review. A flood (or storm) used to review (check) a drainage facility designed to
accommodate a lesser design flood (or storm) so as to judge whether a significant flood hazard due to
a flood larger than the proposed design discharge has been overlooked. The review flood (or storm)
for all but bridge scour analyses is the greater of either the base flood, or the greatest flood when
overtopping does not occur from the probable maximum flood. Bridge scour analyses shall use the
greatest flood or the 500-year flood, whichever is smaller as the review (check) flood.
A flood (or storm), larger or smaller than the selected design flood, which is used to assess the
performance of a drainage facility under other than design conditions.
Review (check) flood for scour is the flood resulting from a storm and/or tide having a flow rate in
excess of the design flood for scour. It is used in the design of a bridge foundation to determine
whether the foundations can withstand the flow and its associated scour without failing. Generally a
500-year flood. Compare with superflood; and flood (or storm), catastrophic.
flood, overtopping. Incipient discharge escaping via such things as over a highway, at a watershed
divide, or through emergency relief facilities. Stated another way, the flood that, if exceeded, results
in flow over a highway, bridge, or culvert; over a watershed divide or dike; or through structures
provided for emergency relief. The worse case scour condition may occur with the overtopping flood.
The total flood magnitude cannot exceed the probable maximum flood and its frequency accuracy is
limited by the state of the art capability to estimate a recurrence interval.
flood peak. See peak discharge.
floodplain. Any plain that borders a stream and is covered by its waters in time of flood. Topographic
area adjoining a channel that is covered by flood flows and those areas where the path of the next
Glossary
G-51
flood flow is unpredictable, such as a debris cone, alluvial fan, or braided channel. A nearly flat,
alluvial lowland bordering a stream and commonly formed by stream processes, that is subject to
inundation by floods.
Kirk Bryan, in “Erosion and Sedimentation in the Papago Country, Arizona” (11), defines a
floodplain as a strip of relatively smooth land bordering a stream, built of sediment carried by the
stream and dropped in the slack water beyond the influence of the swiftest current. It is called a living
floodplain if it is overflowed in times of highwater, but a fossil floodplain if it is beyond the reach of
the highest flood.
The lowland that borders a river, usually dry, but subject to flooding (27). That land outside of a
stream channel described by the perimeter of the maximum probable flood (72). Compare with flood
plane, flood zone, and backwater area.
floodplain, developed. A floodplain that contains a significant amount of development. See
development and floodplain development.
floodplain development. A floodplain containing, zoned to contain, or reasonably foreseen to
contain development that may incur a significant flood hazard or cause significant conveyance
changes in a stream or river reach. See development and floodplain. Compare with floodplain,
developed; and floodplain, undeveloped.
floodplain, undeveloped. A floodplain expected to either remain in a native or rural condition or
incur no development during the service life of the highway that will not result in a significant flood
hazard from a highway action. See development and direct and indirect support (of floodplain
development).
flood plane. The position occupied by the water surface of a stream during a particular flood. Also,
loosely, the elevation of the water surface at various points along the stream during a particular flood
(36). Compare with floodplain.
flood pool. Floodwater storage in a reservoir. In a floodwater retarding reservoir, the temporary
storage between the crests of the principal and emergency spillways (67).
flood probability. See flood frequency.
flood, probable maximum (PMF). The currently accepted term for the most severe flood that is
considered reasonably possible at a site as a result of hydrologic and meteorologic conditions. This
flood would result from the greatest depth of precipitation that is physically possible [“physically
possible” is not a good term—Ed.] at a particular site. In practice many assumptions/calculations have
to be made about the most severe combinations of meteorologic conditions, such as barometric
pressure, wind speed, temperature, etc., and such variables as antecedent moisture, average basin
infiltration rates, etc. It should be noted that floods greater than the computed PMF have
occurred (16).
A PMF is developed from the probable maximum precipitation (PMP) in much the same way as a
standard project flood (SPF) is developed. However, assumptions concerning rainfall losses,
snowmelt runoff, channel efficiency, etc., are adjusted to produce the largest flood reasonably
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Highway Drainage Guidelines
possible. The PMF is used to design high hazard structures (top of dam and spillway capacity) where
failure cannot be tolerated (29).
The largest flood for which there is any reasonable expectancy in this climatic era (39). The probable
maximum flood is the greatest flood that may reasonably be expected, taking into collective account
the most adverse flood-related conditions based on geographic location, meteorology, and terrain. A
very rare flood discharge value computed by hydrometeorological methods, usually in connection
with major hydraulic structures.
A catastrophic flood that, in highway design, may be defined by the upper limits of the flood
envelope curve for maximum floods that have occurred in a hydrologic area and physiographic
region. With highway design the PMF is sometimes arbitrarily considered to be a 10,000-year event
for computational purposes.
flood profile. A graph of elevation of the water surface of a river in flood, plotted as ordinate, against
distance, measured in the downstream direction, plotted as abscissa. A flood profile may be drawn to
show elevation at a given time, crests during a particular flood, or to show stages of concordant flows
(36). See flow, concordant.
flood, project. A flood discharge value adopted for the design of projects such as dams and flood
control works. The term “design flood” is more common to highway drainage design. See design
discharge. Compare with flood, project design.
flood, project design. A term common to the design of major dams and flood control works, but not
routine highway drainage design.
When used in connection with levees and floodwalls, the project design flood (PDF) is the flood
(discharge and elevation) that, when freeboard is added, establishes the top of levee or floodwall
grade. (Other, larger floods are used to design for overtopping of these kinds of structures, and to
establish overtopping impacts.) For rapid flow channels the PDF is generally defined in much the
same way. For reservoirs, the PDF is the flood that, with controlled releases, would fill the designated
flood control storage. Tranquil flow channels do not have a single PDF; instead, they have differing
design objectives over a range of flood magnitudes. They are generally formulated and designed to
continue to provide stage reductions in floods exceeding channel capacity (29).
See flood, project. Compare with level of protection as it is not necessarily synonymous with this
term. Compare with design discharge.
floodproof. Limiting or preventing the intrusion of floodwaters into a proposed or existing facility to
protect their contents and structural integrity against flooding.
flood, regulatory. See regulatory flood.
flood right. Where a State has jurisdiction over its waters, the flood right is an adjudication by that
State of any excess runoff waters to a specified user for beneficial use. Runoff waters in excess of
those required to satisfy all water rights on a stream or river may be adjudicated as a flood right. See
beneficial use. Compare with water right.
Glossary
G-53
flood routing. The process of determining progressively the timing and shape of a flood wave at
successive points along a river (12).
Determining the changes in a flood wave as it moves downstream through a valley or through a
reservoir (then sometimes called reservoir routing). Graphic or numerical methods are used (67).
floods above a base. See partial-duration flood series.
flood stage. The gage elevation of the lowest bank of the reach in which the gage is situated. The
term “lowest bank” is, however, not to be taken to mean an unusually low place or break in the
natural bank through which the water inundates unimportant and small areas (42). The [elevation or]
stage at which overflow of the natural banks of a stream begins to cause damage in the reach in which
the elevation is measured according to the U.S. Weather Bureau. Compare with bankfull stage.
floodwater retarding structure. A dam, usually with an earth fill, having a flood pool where
incoming floodwater is temporarily stored and slowly released downstream through a principal
spillway. The reservoir contains a sediment pool and sometimes storage for irrigation or other
purposes (67). Compare with reservoir, retarding; basin, retention; and basin, detention.
floodwaters. Waters that escape from a natural watercourse in great volume and flow over adjoining
lands in no regular channel. The fact that such errant waters make for themselves a temporary channel
or follow some natural channel, gully, or depression, does not affect their character as “floodwaters”
or give to the course that they follow the character of a natural “watercourse.” Compare with flood
and riparian water.
flood wave. A distinct rise in stage culminating in a crest and followed by recession to lower stages
(36). The rise and fall in streamflow during and after a storm (67).
floodway. “General Introduction and Hydrologic Definitions” (36) offers two definitions: (1) a part
of the floodplain, otherwise leveed, reserved for emergency diversion of water during floods; (2) part
of the floodplain that, to facilitate the passage of floodwater, is kept clear of encumbrances.
The channel of a river or stream and those parts of the floodplain adjoining the channel that are
reasonably required to carry and discharge the floodwater or floodflow of any river or stream (19).
Compare with regulatory floodway.
floodway, regulatory. See regulatory floodway.
flood zone. The land bordering a stream that is subject to floods of approximately equal frequency;
for example, a strip of the floodplain subject to flooding more often than once but not as frequently as
twice in a century (36). Compare with backwater area and floodplain.
flora. The aggregate of plants growing in and usually peculiar to a particular biotic or physiographic
region or period.
flow. A stream of water; movement of such things as water, silt and/or sand; discharge; total quantity
carried by a stream. Characterized by the haphazard movement of small elements of a fluid
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undergoing translation. In terms of the Reynold’s number, turbulent flow corresponds to high values
of that number.
flow, bypass. Flow that bypasses an inlet on grade and is carried in the street or channel to the next
inlet downstream. Sometimes termed carryover.
flow, choking. See choking (of flow).
flow concentration. A preponderance of the streamflow. Compare with flow distribution.
flow, concordant. Flows at different points in a river system that have the same recurrence interval or
the same frequency of occurrence. It is most often applied to flood flows (36).
flow-control structure. A structure within and/or outside a channel that controls the direction, depth,
or velocity of flowing water. May act as a countermeasure.
flow, critical. See critical flow.
flow distribution. The estimated or measured spatial distribution of the total streamflow from the
landward edge of one floodplain or stream bank to the landward edge of the other floodplain or
stream bank. Usually shown as a percent of accumulated flow from one edge (0 percent) to the other
edge (100 percent). Same as the cumulative conveyance only in terms of discharge rather than
conveyance. Compare with cumulative conveyance.
flow-duration chart. A graph indicating the percentage of time during which a given discharge
is exceeded.
flow-duration curve. A cumulative frequency curve that shows the percentage of time that specified
discharges are equaled or exceeded (56).
flow, frontal. The portion of flow that passes over the upstream side of a grate and is subsequently
captured. Compare with splash-over.
flow, gradually varied. Flow in which changes in depth and velocity take place slowly over large
distances, resistance to flow dominates, and acceleration forces are neglected.
flow hazard. Flow characteristics (discharge, stage, velocity, or duration) that are associated with a
hydraulic problem or that can reasonably be considered of sufficient magnitude to cause a hydraulic
problem or to test the effectiveness of a countermeasure. Compare with flood hazard.
flow, laminar. That type of flow in which each particle moves in a direction parallel to every other
particle and in which the head loss is approximately proportional to the first power of the velocity. It
is sometimes designated “streamline flow” or “viscous flow.” Laminar flow is characterized by the
steady, translatory motion of adjacent small elements of the fluid. Tendencies toward turbulence or
instability in truly laminar flow are damped out by viscous shear forces. In terms of the Reynold’s
number, laminar flow corresponds to low values of that number.
Flow in which the head loss is proportional to the first power of the velocity (68).
Glossary
G-55
That type of flow in which the fluid particles follow paths that are smooth, straight, and parallel to the
channel walls. In laminar flow the viscosity of the fluid damps out turbulent motion (20).
Laminar flow changes to turbulent flow in a pipe between the critical value of 2,000 and
approximately 50,000 (there is no definite upper limit). Compare with flow, turbulent.
flow line. Four definitions are offered: (1) the hydraulic grade line; (2) a conduit, as a pipe, laid on
the hydraulic gradient; (3) bottom invert of a conduit; (4) flowage line.
flow, mean annual. The annual mean flow, Qa, for the year based on the 12 monthly means.
Compare with flood, mean annual; mean daily discharge; and mean monthly flow.
flow measurement. See indirect method (of flow measurement).
flow, non-uniform. A flow, the velocity of which is undergoing a positive or negative acceleration. If
the flow is constant, it is referred to as “steady non-uniform flow.” A flow in which the velocities
vary from point to point along the stream or conduit, due to variations in cross section, slope, etc. See
flow, steady. Compare with flow, uniform.
flow, normal. See normal flow; and depth, normal.
flow, one-dimensional. See one-dimensional water surface profile and two-dimensional water
surface profile.
flow, orifice. Flow similar to that through an orifice. For highway drainage design, in culvert flow it
corresponds to flow-type V, i.e., a culvert flowing part-full under high head.
flow, overbank. Water movement over the top of a bank either due to a rising stream stage or to
inland surface water runoff. Compare with flood; and flow, overland.
flow, overland. The flow of rainwater [melting hail] or snowmelt over the land surface toward stream
channels. After it enters a stream, it becomes runoff (36).
The flow of water over a land surface due to direct precipitation. Overland flow generally occurs
when the precipitation rate exceeds the infiltration capacity of the soil and depression storage is full.
Also called Horton’s Overland Flow (20).
Runoff that makes its way over the land surface prior to concentrating in gullies and streams;
sometimes termed sheet flow. Compare with flow, overbank.
flow, pressure. Where flows passing through a bridge type opening are contracted vertically by the
superstructure to the extent that the flow has a pressure head and flow streamlines analogous to that
occurring at a sluice gate.
flow, rapid. See flow, supercritical.
flow, rapidly varied. Flow in which changes in depth and velocity take place over short distances,
acceleration forces dominate and energy loss due to friction is minor.
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flow regime. The system or order characteristic of streamflow with respect to velocity, depth, and
specific energy. See critical flow; flow, subcritical; and flow, supercritical.
flow, return. That part of irrigation water that is not consumed by evapotranspiration and that returns
to its source or another body of water. The term is also applied to the water that is discharged from
industrial plants. Also called return water (36).
flow, sheet. See flow, overland.
flow, sinuous. See flow, turbulent.
flow slide. Saturation of a bank to the point where the soil material behaves more like a liquid than a
solid; the soil/water mixture may then move downslope resulting in a bank failure. See sloughing.
flow, sluice. Flow through a culvert under high head whereby the entrance contracts the flow and
causes a part-time flow condition through the barrel; also referred to as orifice-type flow and similar
to flow from under a sluice gate.
flow, steady. A flow in which the flow rate or quantity of fluid passing a given point per unit of time
remains constant. A constant discharge with respect to time.
flow, steady non-uniform. See flow, non-uniform; and flow, steady.
flow stream. Flow from a catchment in an open or closed conduit (1).
flow, streaming. See flow, subcritical.
flow, subcritical. In this state, gravity forces are dominant so that the flow has a relatively low
velocity and is often described as tranquil or streaming. Also, that flow that has a Froude number
less than unity. Flow at velocities less than critical velocity; flow at depths greater than critical
depth. Flow at velocities less than one of the recognized critical values; specifically, turbulent
flow with a mean velocity less than Belanger’s critical velocity; streaming flow. Compare with
flow, supercritical.
flow, supercritical. In this state, inertia forces are dominant so that flow has a high velocity and is
usually described as rapid or shooting. Also, that flow that has a Froude number greater than unity.
Flow at velocities greater than critical velocity; flow at depths less than critical depth. Flow at
velocities greater than one of the recognized critical values; specifically turbulent flow with a mean
velocity equal to or greater than Belanger’s critical velocity; shooting flow; rapid flow. Compare with
flow, subcritical.
flow, tranquil. See flow, subcritical.
flow, turbulent. The flow condition in which inertial forces predominate over viscous forces and in
which head loss is not linearly related to velocity (68). That type of flow in which the fluid particles
move along very irregular paths. Momentum can be exchanged between one portion of the fluid and
another. That type of flow in which any particle may move in any direction with respect to any other
Glossary
G-57
particle and in which the head loss is approximately proportional to the second power of the velocity.
See turbulence. Compare with flow, laminar.
flow, two-dimensional. See two-dimensional water surface profile.
flow type, bridge. As applied to flow through bridges there are four principal flow types, enumerated
as follows: (1) Type I—Subcritical approach flow with subcritical flow in the most contracted section
of the bridge opening (backwater due primarily to opening geometry); (2) Type IIA—Subcritical
approach flow with flow passing through the normal flood depth (but not critical depth) in the most
contracted section of the bridge opening. This is a transition range where the opening geometry is still
influential, but the backwater starts to become influenced primarily by the amount of contraction;
(3) Type IIB—Subcritical approach flow with flow theoretically passing through the bridge opening
at critical depth in the most contracted section of the bridge opening. Backwater is due primarily to
the amount of contraction; (4) Type III—Supercritical approach flow with supercritical flow through
the bridge opening at a greater depth than the approach or tailwater flow (no backwater possible).
flow type, culvert. As applied to flow through culverts, there are six principal flow types,
enumerated as follows: (1) Type I flow—Part-full flow (low upstream head) with control at inlet;
(2) Type II flow—Part-full flow (low upstream head) with control at outlet; (3) Type III flow—
Part-full flow (low upstream head) under backwater conditions (tailwater control); (4) Type IV
flow—Full-flow with both inlet and outlet of culvert submerged; (5) Type V flow—Part full,
sluice-type flow under high upstream head; (6) Type VI flow—Full-flow under high upstream head.
flow, uniform. Flow of constant cross section and average velocity through a reach of channel during
an interval of time. A constant flow of discharge, the mean velocity of which is also constant.
Uniform flow is also referred to as “steady uniform flow.” It is an ideal condition that can, in reality,
only be approximated. If the velocity of the constant discharge varies, the flow is defined as “steady
non-uniform.” When the average velocities at successive points or sections in the direction of steady
flow are the same, the flow is described as uniform. Truly uniform flow, although frequently assumed
for computational convenience, seldom occurs in natural open channels. Constant depth flow through
a straight reach of a uniform artificial canal is an example of reasonably uniform flow. See flow,
steady. Compare with flow, non-uniform.
flow, unsteady. Flow of variable cross section and average velocity through a reach of channel
during an interval of time. A changing discharge with respect to time; opposite of steady flow;
frequently labeled varied flow. Compare with flow, steady.
flume. An open or closed channel used to convey water. An open conduit of such things as wood,
concrete, or metal on a prepared grade, trestle, or bridge. A flume holds water as a complete structure.
A concrete lined canal would still be a canal without the lining, but the lining supported
independently would be a flume. A large flume is also termed an aqueduct. Compare with
bench-flume.
flume, control. An open conduit or artificial channel arranged for measuring the flow of water that
generally includes a constricted section wherein critical depth exists. See flume, Parshall measuring;
and flume, Venturi.
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flume, Parshall measuring (formerly termed improved Venturi flume). A calibrated device
developed by engineers of the U.S. Department of Agriculture, of whom Ralph I. Parshall, Assoc. M.
Am. Soc. C. E., has been the principal experimenter. Its purpose is to measure the flow of water in
open conduits. It consists essentially of a contracting length, a throat (flume control), an expanding
length. At the throat is a sill over which the water is intended to flow at critical depth. The upper head
is measured a definite distance upstream and the lower head a definite distance downstream from the
sill. The lower head need not be observed except where the sill is submerged more than
approximately 67 percent. A special form of control flume. Compare with flume, Venturi.
flume, Venturi. A type of open flume with a contracted throat that causes a drop in the hydraulic
grade line; used for measuring flow. Compare with flume, Parshall measuring.
flush entrance. See inlet, flush.
fluvial geomorphology. A study of the structure and formation of the earth’s features that result from
the forces of water. Sometimes river engineers abbreviate fluvial geomorphology in discussions to a
simpler, but incorrect, term “morphology.” Compare with geomorphology. See morphology and
morphology problems.
fluvial sediment. See sediment, fluvial.
ford. A location where a highway crosses a channel by allowing high annual or larger flows to pass
over the highway and lower flows to pass through a culvert(s). Often used with cutoff walls, roadway
lane markers, and paved roadway embankments and traveled way (and shoulders). Warning signs
may also be included.
forest influences. Effects resulting from the presence of forest or brush upon climate, soil, water,
runoff, streamflow, floods, erosion, and soil productivity (36).
fork length. Fork length of a fish is a standard measure of length advocated by some fishery experts;
it is commonly measured from anterior most extremity (nose tip) to the notch in the tail fin of
fork-tailed fishes (or to the center of the fin when the tail is not forked).
frazil ice. A French-Canadian term for fine spicular ice, derived from the French for cinders that this
variety of ice most resembles. When formed in salt water, it is known as lolly ice. It is composed of
fine particles that, when first formed, are colloidal and not seen in the water in which they are
floating (5) (55).
freeboard. Vertical clearance between the lowest structural member of the bridge superstructure, the
top culvert invert, or the point of escape in a canal or channel to the water surface elevation of a
flood. Freeboard may also be the vertical distance above a design stage that is allowed for waves,
surges, drift, and other contingencies. The vertical distance between the level of the water surface,
usually corresponding to the design discharge (or wave runup) selected for freeboard considerations
and a point of interest, such as a low chord of a bridge beam; specific location on the roadway grade;
or top of a channel bank. The distance between such things as the normal operating level and the top
of the sides of an open conduit or channel, or the crest of a dam that is left to allow for wave action,
floating debris, or any other condition or emergency, without overtopping the structure. For irrigation
Glossary
G-59
flows intercepting runoff, freeboard is based on the expected water surface elevation determined for
the sum of the water right, flood right, and design discharge.
The marginal height provided above [the] design [discharge] lines, [stage] on levees, and in certain
channels to ensure, as fully as practicable, against overtopping due to uncertainties in [such things as]
the state of project maintenance or flood flow characteristics. In appropriate circumstances, special
increments of levee freeboard may be provided to achieve design objectives (e.g., to control, in such
an extremity, the location where initial overtopping of a levee would take place; to reduce wave
overtopping; to extend the interval between major maintenance efforts for removal of tree growth,
sediment deposition, etc., from the channel the levee bounds). Added height to earth levees is
sometimes provided to allow [freeboard] for settlement. In project evaluation, one-half of the
inundation reduction benefits creditable to the levee freeboard zone may be included (29).
freeboard allowance. See freeboard.
free flow. A condition of flow through or over a structure not affected by submergence.
free outlet. Those outlets whose tailwater is equal to, or lower than, critical depth at the outlet. For
culvert type structures having free outlets, lowering of the tailwater has no effect on the discharge or
the backwater profile upstream of the tailwater.
free water surface. The water surface of flow in an open channel or in a closed conduit not
flowing full.
free weir. A weir that is not submerged, i.e., tailwater is below the crest, or the flow is not affected by
the elevation of the tailwater.
frequency. See flood frequency.
frequency analysis. [The interpretation and analysis of] a past record of hydrologic events in terms
of future probabilities of occurrence. [The estimate] of frequencies [of occurrence] of floods,
droughts, storages, rainfalls, water qualities, waves, etc; the procedure involved is known as
frequency analysis (14).
frequency curve. A graphical representation of the frequency of occurrence of specific events. In
flood studies, frequency is expressed as the recurrence interval (RI) that is the average number of
years within which a given peak discharge or rainfall intensity will be exceeded. See frequency
analysis. Compare with flood-frequency curve.
frequency curve, low flow. See low-flow frequency curve.
frequency distribution. See probability distribution.
frequency line. The line on probability paper that represents a series of events and their frequencies.
frequency series. A sequence or array of actual events (floods, etc.) suitable for use in frequency
analysis, or a sequence or array of hypothetical events obtained from a frequency analysis (36). See
frequency analysis and probability distribution.
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friction loss (or head). The head or energy loss as the result of disturbances set up by the contact
between a moving stream of water and its containing conduit. For convenience, friction losses are
best distinguished from losses due to such things as bends, expansions, obstruction and impacts, but
there is no recognized line of demarcation between them, and all such losses are often included in the
term friction loss.
friction slope. The friction loss (or head) per unit length of conduit. For most conditions of flow, the
friction slope coincides with the energy grade line, but where a distinction is made between energy
losses due to such things as bends, expansions, and impacts, a distinction must also be made between
the friction slope and the energy grade line. Friction slope is equal to the bed or surface slope only for
relatively uniform flow in nearly uniform channels. Compare with energy grade line and hydraulic
grade line.
frontal flow. See flow, frontal.
Froude number. A dimensionless number (expressed as F = V/(gy)1/2) that represents the ratio of
inertial to gravitational forces, i.e., at a Froude number of unity the flow velocity and wave celerity
are equal (see celerity, wave). High Froude numbers can be indicative of a high velocity associated
with supercritical flow and thus the potential for scour and high momentum forces. Stated another
way, a number that varies in magnitude inversely with the relative influence of gravity on the flow
pattern: F > 1.0 indicates rapid (supercritical) flow; F < 1.0 indicates tranquil (subcritical) flow.
full height (or retaining) abutment. See vertical abutment.
functional highway classification. A grouping of highways into classes or systems according to the
quality of service they are expected to provide.
functions of surface waters. The functional environmental characteristics of surface waters include
such things as riparian and floodplain habitat for terrestrial and aquatic wildlife, flood control,
groundwater recharge, aesthetics, shore and bank line geometry, scenic and wild rivers, endangered
species habitat, and water pollution abatement ability. May also be termed natural and beneficial
values. See surface waters. Compare with surface water value.
fungi (fungus). Any of numerous plants of the division of subkingdom Thallophyta, lacking
chlorophyll, ranging in form from a single cell to a body mass of branched filamentous hyphae that
often produce specialized fruiting bodies and including the yeasts, molds, smuts, and mushrooms.
FWS. Acronym for U.S. Fish and Wildlife Service.
G (or g). The acceleration of gravity, m/s2 (ft/s2).
gabion. A rectangular basket made of steel wire fabric or mesh that is filled with rock or similar
material of suitable size and gradation. Used to construct such things as flow-control structures, bank
protection, groins, jetties, permeable dikes, and riparian spur dikes. When filled with cobbles,
masonry remnants, or other rock of suitable size and gradation, the gabion becomes a flexible and
permeable block with which the foregoing structures and devices can be built. Compare with riprap,
wire-enclosed.
Glossary
G-61
Galton distribution. Sometimes called the “law of Galton.” See probability distribution.
gage (gauge). Two definitions are provided: (1) a staff graduated to indicate the elevation of a water
surface; (2) a device for registering water levels, flow, velocity, and pressure. Compare with gaging
station, recorder, register, and indicator.
gage, bubble. An automated device that measures water pressure near the bottom of the stream; this
pressure equates with water depth above the gage (32).
gage, chain or tape. A device consisting of a tagged or indexed chain tape or other line attached to a
weight that is lowered to touch the water surface, whereupon the gage height is read on a graduated
staff or opposite an index. Especially suited to bridges with the graduated staff generally placed
horizontally with the line running over a pulley. Bridges and similar structures with significant
contractions incur rapidly varied flow conditions that may preclude any meaningful discharge
measurements when such devices are placed within the contraction.
gage, crest-stage. A simple and economical device used to record crest stages of floods or flows. It is
usually constructed of 50-mm (2-in.) pipe, with suitable intake holes, contains a wooden gage stick
and has finely ground cork placed in the bottom of the pipe to float up and temporarily mark the stage
on this gage stick. The cork adheres to the gage stick up to the crest elevation of a flood. Given this
elevation, the discharge is estimated based on a rating curve developed for the reach using the slopearea method. See slope-area method. Compare with gage, staff; and water-level (stage) recorder.
gage datum. The elevation level that corresponds to stage 0.0 at a stream gage; it is often set at the
stream bottom or the elevation of a very low flow (32).
gage, float. A chain or tape gage in which a float is substituted for the weight. Measurement of the
discharge of water by floats to determine velocities.
gage height. Height of the water surface above the zero reference mark on a gage. The water surface
elevation [is] referred to some arbitrary gage datum. Gage height is often used interchangeably with
the more general term stage although gage height is more appropriate when used with a reading on a
gage (36) (50).
gage, hook. A pointed hook attached to a graduated staff or vernier scale for accurately measuring the
elevation of the surface of still water. The hook is submerged and then raised until the point makes a
pimple on the water surface.
gage, inclined. A staff gage placed on a slope (incline) and graduated to read (or indicate) vertical
heights above the datum.
gage, point. A sharp, pointed rod attached to a graduated staff or vernier scale for measuring the
elevation of the surface of flowing water. The point is lowered until the tip barely touches the water,
forming a streak.
gage, pressure. See manometer.
gage, recording. See water-level (stage) recorder.
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Highway Drainage Guidelines
gage, self-reporting. Instruments that automatically transmit rainfall or stream stage data from a
remote gage to the base station (32).
gage, slope. See gage, inclined.
gage, staff. A vertical board or structure with a graduated scale for measuring the depth of a river in
millimeters [inches] (32). A graduated scale on such things as a staff, plank, metal-plate pier, or wall,
by which the elevation of the water surface may be read. Compare with gage, crest-stage.
gage, standard rain. Also standard gage. The USWB [now the NWS] nonrecording rain gage,
having an opening gage size 200 mm (8 in.) in diameter and holding capacity of 600 mm (24 in.) of
rainfall. The gage is usually examined once daily at a regular time and the rainfall catch (if any)
measured by depth in millimeters [inches].
gage, stream. Instruments that measure the depth of the water in a stream (32).
gage, tipping bucket. An electrical-mechanical device that accumulates a small, precise precipitation
gage amount of rainfall before tipping to spill the water. The spill triggers an electrical signal that is
counted and/or transmitted to a base station. Each count represents a preset rainfall accumulation
amount (1 mm (0.04 in.)) and is tagged with the time of occurrence (32).
gaging. The act of measuring the flow of streams.
gaging, chemical. See chemical gaging.
gaging/sampling. Refers to the measurement of precipitation, flow, and/or water quality
parameters (1).
gaging station. A particular site on a stream, canal, lake, or reservoir where systematic observations
of gage height or discharge are obtained (50). Used synonymously with gage (36). See stream-gaging
station.
A location on a stream where measurements of stage or discharge are customarily made. The location
includes a reach of channel through which the flow is nearly uniform, a control downstream from this
reach, and usually a small building to house the recording instruments. A plane on a stream, including
suitable gaging appurtenances, where systematic records of stream-flow are collected.
gaining stream. See stream, gaining (36).
Gaussian distribution. Normal distribution (33). See probability distribution.
Gauss-Laplace distribution. Normal distribution (33). See probability distribution.
generalized skew coefficient. A skew coefficient derived by a procedure that integrates values
obtained at many locations (33).
general scour. See scour, general.
Glossary
G-63
geomorphology. A study of the structure and formation of the earth’s features. That branch of both
physiography and geology that deals with the form of the earth, the general configuration of its
surface and the changes that take place due to erosion of the primary elements and in the buildup of
erosional debris. See morphology and morphology problems. Compare with fluvial geomorphology.
geotextile filter. See synthetic filter.
glacier. Bodies of land ice that consist of recrystallized snow accumulated on the surface of the
ground and move slowly downslope (45).
grade. Three definitions are suggested: (1) the longitudinal slope of a road, channel, or natural
ground; (2) the finished surface of a canal bed, road bed, top of embankment, or bottom of
excavation; (3) any surface prepared for the support of such things as conduit paving, ties, or rails.
grade control structure. Structure placed bank to bank across a stream channel (usually with its
central axis perpendicular to flow) for the purpose of controlling bed slope and preventing scour or
headcutting. Compare with drop and sill.
graded filter. An aggregate filter that is proportioned by particle size to allow water to pass through
at a specified rate while preventing the migration of fine-grained soil particles without clogging.
graded stream. See stream, poised.
gradient. Change of elevation, velocity, pressure, or other characteristics per unit length; slope.
Compare with energy grade line.
gradually varied flow. See flow, gradually varied.
grate inlet. See inlet, grate.
gravel. Particles, usually of rock, whose diameter is between 2 mm and 64 mm (0.08 in. and 2.5 in.).
The term gravel is also applied to a mixture of sizes (gravel with sand or gravel with cobbles) in
which the dominant or modal fraction is the gravel size range (FHWA Highways in the River
Environment Manual).
gravity dam. A dam depending solely on its weight to resist water pressure and any momentum
forces.
greatest opening flood. See flood, greatest opening.
greenbelt. See buffer zone.
Gringorten distribution. See probability distribution.
groin. See dike, riparian spur.
groove, drip. See drip groove.
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groundwater. Subsurface water occupying the saturation zone, from which wells and springs are fed.
A source of base flow in streams. In a strict sense the term applies only to water below the water
table. Water at and below, the water table; basal or bottom water; phraetic water. Used also in a broad
sense to mean all water below the ground surface.
Water in the ground that is in the zone of saturation, from which wells, springs, and groundwater
runoff are supplied (46). The Groundwater Subcommittee offers numerous definitions of
groundwater: (1) that part of the subsurface water that is in the saturated zone; (2) loosely, all
subsurface water as distinct from surface water; (3) all water that occurs below the land surface [and]
it includes both water within the unsaturated and saturated zones; (4) water below the land surface in
a zone of saturation; (5) the water contained within an aquifer; (6) all water that occurs below the land
surface; (7) all subsurface water as distinct from surface water; (8) subsurface water that fills
available openings in rock or soil materials to the extent that they are considered water-saturated;
(9) water below the land surface in a zone of saturation; (10) water in a saturated zone or stratum
beneath the surface of land or water.
The water contained in interconnected pores located below the water in an unconfined aquifer or
located in a confined aquifer (20).
The water in the saturated zone beneath the water table. A source of base flow in streams (67).
groundwater discharge. That part of the discharge from a drainage basin that occurs through the
groundwater. The term underflow is often used to describe the groundwater outflow that takes place
in valley alluvium (instead of the surface channel) and thus is not measured at a gaging station (36).
See underflow.
groundwater outflow. See groundwater discharge.
groundwater, perched. The Groundwater Subcommittee offers this definition (68): [Unconfined]
Groundwater separated from an underlying body of groundwater by an unsaturated zone. Its water
table is a perched water table. Perched groundwater is held up by a perching bed whose permeability
is so low that water percolating downward through it is not able to bring water in the underlying
unsaturated zone above atmospheric pressure.
groundwater runoff. That part of the runoff that has passed into the ground, has become
groundwater, and has been discharged into a stream channel as spring or seepage water. See also
runoff, base and runoff, direct (36).
grout. A fluid mixture of cement and water or of cement, sand, and water used to fill joints and voids.
guide. A book that explains, outlines, or gives practical instruction in some subject according to the
Reader’s Digest Great Encyclopedic Dictionary. Something that provides a person with guiding
information according to Webster’s New Collegiate Dictionary. A manual containing guidelines on
some subject. Compare with guideline.
guide bank. Formerly termed spur dike. Relatively short embankments generally in the shape of a
quarter of an ellipse and constructed at the upstream side (and sometimes the downstream side) of
either or both bridge ends as an extension of the abutment spillslope. The purpose is to align the flow
Glossary
G-65
with the bridge opening so as to decrease scour at the bridge abutment by spreading the flow and any
resultant scour throughout the bridge opening. May also be a training dike (usually when constructed
downstream). See dike, training. Sometimes referred to using the outdated term spur dike. See
dike, spur.
guideline. Any suggestion, rule, etc., that guides, directs, or sets a standard: [such as] government
guidelines designed to regulate wage or price increases according to Reader’s Digest Great
Encyclopedic Dictionary. Compare with guide.
Gumbel distribution. See probability distribution.
gutter. That portion of the roadway section adjacent to the curb that is utilized to convey
stormwater runoff.
habitat. The area or type of environment in which an organism or biological population normally
lives or occurs.
hard point. A channel bank protection technique whereby “soft” or erodible materials are removed
from a bank and replaced by stone or compacted clay. Some hard points protrude a short distance into
the channel to direct erosive currents away from the bank. See dike, riparian spur. Hard points also
occur naturally along channel banks as passing currents remove erodible materials leaving
nonerodible materials exposed. Natural hard points also result from clay plugs deposited near the
cutoff points when a meander is cutoff, and from such things as rock outcrops.
Hazen distribution. See probability distribution.
HE. Entrance head loss, m.
head. The height of water above any point, plane, or datum of reference. Used also in various
computations, such as energy head, entrance head, friction head, static head, pressure head, lost head,
etc. The height of the free surface of a body of water above a given point.
headcut. The relatively fast drop (as compared to the average channel bed profile slope through a
channel reach) in a channel bed profile that is, or has been, headcutting. See headcutting.
headcutting. Channel degradation associated with abrupt changes in the bed elevation (headcut) that
migrates in an upstream direction. Channel bed erosion moving upstream through a basin indicating
that a readjustment of the basin’s profile slope, channel discharge, and sediment load characteristics is
taking place. Headcutting may be evidenced by the presence of waterfalls or rapidly moving water
through an otherwise placid stream or river, provided there is flow present. In dry channels the
presence of a relatively steep drop in the channel bed in an erodible channel is evidence of a headcut.
Headcuts may range from 0.3 m (1 ft) or less to 3 m (10 ft) or more. Headcutting often leaves channel
banks in an unstable condition as it progresses through a reach as evidenced by large amounts of mass
wasting. See mass wasting. Compare with nick point.
head, elevation. The elevation of a given point in a column of liquid above a datum (68).
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Highway Drainage Guidelines
head, energy. The elevation of the hydraulic gradeline at any section plus the velocity head of the
mean velocity of the flow in that section. The energy head may be referred to any datum or to an
inclined plane, such as the bed of a conduit. Also the total head above a datum at any cross section.
Compare with energy grade line.
head, friction. See friction loss.
head, high. A general term applied to culvert flow denoting that culvert entrance is submerged with
“free getaway” (no adverse tailwater conditions) downstream from culvert. Compare with head, low.
headloss. A loss of energy in a hydraulics system.
head, lost. The energy of a given flow that is lost (converted into heat and, therefore useless) as a
result of friction, eddies, and impact expressed as a head; that is, as the height through which that
flow would have to fall to produce an equivalent amount of energy.
head, low. A general term applied to culvert flow, denoting that the culvert entrance is not
submerged. Compare with head, high.
head, piezometric. Elevation plus pressure head, total head at any cross section minus the velocity
head at that cross section; equivalent to water surface elevation in open-channel flow; equivalent to
elevation of hydraulic grade line at any point.
head, pressure. Hydrostatic pressure expressed as the height of a column of water that pressure can
support at the point of measurement (68). Compare with head, static; and head, total. The head at any
point in a conduit represented by the height of the hydraulic grade line above that point.
head, static. The height above a standard datum of the surface of a column of water (or other liquid)
that can be supported by the static pressure at a given point. The static head is the sum of the
elevation head and the pressure head (68). Compare with head, total; and head, pressure.
The total head without deduction for velocity head or losses; for example, the difference in elevation
of headwater and tailwater of a power plant. Compare with head, piezometric.
head, total. The total head of a liquid at a given point is the sum of three components: (1) the
elevation head, which is equal to the elevation of the point above a datum; (2) the pressure head,
which is the height of a column of static water that can be supported by the static pressure at the
point; and (3) the velocity head, which is the height at which the kinetic energy of the liquid is
capable of lifting the liquid (68).
head, velocity. The distance a body must fall freely under the force of gravity to acquire the velocity
it possesses; the kinetic energy, in meters [feet] of head, possessed by a given velocity. In flowing
water, the velocity squared divided by twice gravity (V2/2g).
headwall. The structural appurtenance usually applied to the end of a culvert inlet and outlet or storm
drain outlet to retain an adjacent highway embankment and protect the culvert ends or storm drain
outlet and highway embankment or storm drain outfall from bank erosion and channel bed scour.
Glossary
G-67
headwater. See headwater depth.
headwater depth. Depth of water above the inlet flow line at the entrance of a culvert or similar
structure. Depth of water upstream of a contraction such as occurs at a bridge or similar structure.
Natural flow depth plus backwater caused by a drainage structure.
That depth of water impounded upstream of a culvert, bridge, or similar contracting structure due to
the influence of the structures constriction, friction and configuration. The water depth upstream from
a structure. Compare with allowable headwater depth.
headwater elevation. Water surface elevation of the headwater.
headwater height. See headwater depth.
headwaters. The uppermost reaches for the source of water flowing in a stream. The geographic
regions near the divide of a watershed.
hectare-meter. The quantity of water required to cover an area of 1 ha to a depth of 1 m and is equal
to 9996 m3 or approximately 10 x 106 L. Common abbreviation is ha•m.
helical flow. Three-dimensional movement of water particles along a spiral path in the general
direction of flow. These secondary currents are of most significance as flow passes through a bend;
their net effect is to remove soil particles from the cut bank in a bendway and deposit this material on
a point bar, alternate bar, or middle bar.
herbaceous vegetation. Vegetation that has a fleshy stem as distinguished from the woody tissue of
shrubs and trees and that generally dies back at the end of each growing season.
herbicide. A substance used to destroy plants, especially weeds.
heterogeneity. A characteristic of a medium in which material properties vary from point to point
(68). Pertaining to a substance having different characteristics in different locations (20). (A synonym
is non-uniform.)
Hf. The friction headloss, m (ft).
high head. See head, high.
highwater elevation. The water surface elevation that results from the passage of flow. It may be an
“observed highwater mark elevation” as a result of someone actually viewing and recording a runoff
event or a “calculated highwater elevation” as part of a design process. See indirect method (of flow
measurement).
highwater mark. A mark left as evidence of the height to which a flood reached, usually in the form
of such things as deposited sediment, debris, and detritus. See drift line.
highway. See road and street.
highway action. See action (highway).
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Highway Drainage Guidelines
historical flood. A past flood event of known or estimated magnitude. A known flood event
predating systematic flow measurements at a given site.
historical series. A list of all actual storms (or floods) that caused flood damage in a watershed, in a
given period of years, with the data of each storm of flood being known (67).
historic wetland losses. See wetland losses, historic.
HL. Total energy head loss, m (ft). See head, energy; and energy grade line.
ho. The height of the hydraulic grade line above the outlet invert of a drainage structure, m (ft).
homogeneity. Records from the same populations (33). A characteristic of a medium in which
material properties are identical everywhere (68). Pertaining to a substance having identical
characteristics everywhere (20). (A synonym is uniform.)
homogeneous. See homogeneity.
hook gage. See gage, hook.
HW. Acronym for headwater depth. Compare with AHW.
hydraulic. Moved, operated, or effected by means of water; relating to hydraulics as in engineer
[note that Webster’s uses the plural form when referring to an engineer]; relating to water or other
liquid in motion as in erosion; operated by the resistance offered or the pressure transmitted when
a quantity of water, oil, or other liquid is forced through a comparatively small orifice or through
a tube. These definitions have been taken from Webster’s New Collegiate Dictionary. Compare
with hydraulics.
hydraulic bore. See bore, hydraulic.
hydraulic elements. The depth, area, perimeter, mean depth, hydraulic radius, velocity, energy, and
other flow-related quantities pertaining to a particular stage of flowing water.
hydraulic-fill dam. A dam composed of such materials as earth, sand, and gravel, sluiced into place;
generally the fines are washed toward the center for greater imperviousness.
hydraulic friction. A force-resisting flow that is exerted on contact surface between a stream and its
containing channel. It usually includes the normal eddies and cross-currents attendant upon turbulent
flow occasioned by the roughness characteristic of the boundary surface, moderate curvature, and
normal channel variations. Wherever possible, the effects of excessive curvature, eddies, and impact,
obstructions and pronounced channel changes are segregated from the effects of hydraulic friction.
hydraulic grade line. In a closed conduit, a line joining the elevation to which water could stand in
risers. In an open conduit, the hydraulic grade line is the water surface; piezometric head line. See
head, piezometric. Compare with energy grade line.
A profile of the piezometric level to which the water would rise in piezometer tubes along a pipe run.
In open-channel flow, it is the water surface.
Glossary
G-69
hydraulic gradient. The slope of the hydraulic grade line; the slope of the water surface in uniform,
open-channel flow.
The change in total head with a change in distance in a given direction. The direction is that which
yields a maximum rate of decrease in head (20). The slope of the hydraulic grade line through a
channel reach or drainage structure. Compare with energy grade line and friction slope.
hydraulic head. See head.
hydraulic jump. The sudden and usually turbulent passage of water from a stage below critical depth
(supercritical flow) to a stage above critical depth (subcritical flow) during which the velocity passes
from supercritical to subcritical. It represents the limiting conditions of the water surface curve (or
profile) wherein it tends to become perpendicular to the streambed.
A hydraulic phenomenon, in open-channel flow, whereby supercritical flow is converted to
subcritical flow. This can result in a relatively abrupt and turbulent rise in the water surface. See
depth, conjugate; depth, alternate; and critical depth.
hydraulic model. A small-scale physical representation of a flow situation.
hydraulic performance curve. Computed estimates of how a drainage facility will perform over a
wide range of discharges. Commonly these may include discharge and recurrence interval versus
headwater, velocity, scour, and/or stage (depth of flow).
hydraulic problem. The effect from such things as channel flow, tidal flow, or wave action on a
crossing such that traffic is immediately or potentially disrupted or some other detrimental effect is
expected or caused.
hydraulic radius. In simplest terms, the cross section area of a stream divided by its wetted
perimeter. The cross section area of a stream of water (normal to flow) divided by the length of that
part of its periphery in contact with its containing conduit; the ratio of area to wetted perimeter. A
measure of the boundary resistance to flow, computed as the quotient of cross section area of flow
divided by the wetted perimeter. For wide shallow flow, the hydraulic radius can be approximated by
the average flow depth. Compare with wetted perimeter.
hydraulic roughness. A composite of the physical characteristics that influence the flow (or
conveyance) of water across the earth’s surface, whether natural, channelized, or in a conduit. It
affects both the time response of a watershed and drainage channel or conduit, and the channel or
conduit storage characteristics. Compare with Manning’s n.
hydraulics. The applied science concerned with the behavior and flow of liquids, especially in pipes,
channels, structures, and the ground. In highway drainage, the science addressing the characteristics
of fluid mechanics involved with the flow of water in or through drainage facilities.
A branch of science that deals with practical applications (as the transmission of energy or the effects
of flow) of water or other liquid in motion according to Webster’s New Collegiate Dictionary.
Compare with hydraulic.
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Highway Drainage Guidelines
hydraulics designer. A hydraulics engineer or in some cases a technician who designs hydraulics
structures under the supervision of a more experienced hydraulics engineer.
hydraulics engineer. An engineer whose practice is limited primarily to hydraulics and river
mechanics. See hydraulic, hydraulics, and river mechanics.
hydraulic structure. A facility used for such things as to impound, accommodate, convey, or control
the flow of water, such as a dam, weir, intake, culvert, channel, or bridge.
hydric soil. Soil that, in its undrained condition, is saturated, flooded, or ponded long enough during
a growing season to develop an anaerobic condition that supports the growth and regeneration of
hydrophytic vegetation.
Some wetland biologists and/or regulators may prefer a soil that is saturated, flooded, or ponded long
enough during the growing season to develop anaerobic conditions in the upper part, which
influences plant growth.
hydrograph. The graph of stage or discharge versus time. A graph showing, for a given point on a
stream or for a given point in any drainage system, the discharge, stage, velocity, or other property of
water with respect to time.
A graph showing [relating] stage, flow, velocity, or other property [characteristics] of water with
respect to time (36) [Groundwater Subcommittee (68)].
A graph that shows some property of groundwater or surface water as a function of time (20). A
graph showing, for a given point on a stream or for a given point in any drainage system, the
discharge, stage, velocity, or other property of water with respect to time (67). A chart that shows the
relationship between streamflow or water elevation to time at a certain location (32). Compare with
hydrograph, storm and hyetograph.
hydrograph, composite. A plot of mean daily discharges for a number of years of record on a single
year time base for the purpose of showing the average occurrence of high and low flows.
hydrograph, distribution. A unit hydrograph of direct runoff modified to show the portions of the
volume of runoff that occurs during successive equal units of time (28).
hydrograph, storm. A graph of the discharge of a stream over the time period when, in addition to
direct precipitation, overland flow, interflow, and return flow are adding to the flow of the stream.
The storm hydrograph will peak due to the addition of these flow elements (20). Compare with
hydrograph.
hydrograph, synthetic. A hydrograph determined from empirical rules. Usually a hydrograph based
on the physical characteristics of the basin. A graph developed for an ungaged drainage area, based
on known physical characteristics of the watershed basin.
hydrograph, unit. The hydrograph of direct runoff from a storm uniformly distributed over the
drainage basin during a specified unit of time; the hydrograph is reduced in vertical scale to
correspond to a volume of runoff of 1 mm (1 in.) from the drainage basin (2).
Glossary
G-71
The hydrograph of surface runoff (not including groundwater runoff) on a given basin due to an
active rain falling for a unit of time (59). A discharge hydrograph coming from 1 mm (1 in.) of direct
runoff distributed uniformly over the watershed, with the direct runoff generated at a uniform rate
during the given storm duration. A watershed may have 1-hour, 2-hour, etc., unit hydrographs (67).
A typical streamflow hydrograph of a river basin produced by 1 mm (1 in.) of surface runoff
uniformly distributed over the watershed during a specified period of time (32).
A hydrograph of a direct runoff resulting from 1 mm (1 in.) of effective rainfall generated uniformly
over the watershed area during a specified period of time or duration. The discharge hydrograph
resulting from 1 mm (1 in.) of direct runoff generated uniformly over the tributary area at a uniform
rate during a specified period of time. Compare with hyetograph.
hydrologic area. A geographic area having homogeneous topographical, soil, vegetation and
meteorological properties as they relate to its flood-frequency relationship; i.e., subareas within such
an area have very similar flood-frequency relationships.
hydrologic budget. An accounting of the inflow to, outflow from, and storage in, a hydrologic unit,
such as a drainage basin, aquifer, soil zone, lake, reservoir, or irrigation project (36).
hydrologic characteristics of watershed. The parameters that control the runoff [and
flood-frequency relationship] in a watershed. These include [such things as] the basin size, ground
cover conditions, slope of the land, stream lengths [stream slopes, soil infiltration characteristics],
topographic and geologic features, and physical features constructed by man that alter runoff (32).
hydrologic cycle. A convenient term to denote the circulation of water from the sea, through the
atmosphere, to the land, and thence, with many delays, back to the sea by overland and subterranean
routes and in part by way of the atmosphere; also, the many short circuits of the water that are
returned to the atmosphere without reaching the sea (47).
hydrologic equation. The equation balancing the hydrologic budget (36). An expression of the law
of mass conservation for purposes of water budgets that may be stated as inflow equals outflow plus
or minus changes in storage.
hydrologic models. Mathematical equations, algorithms, and/or logic that represents the rainfall
runoff process in a watershed (32).
hydrologic soil-cover complex. A combination of a hydrologic soil group and a type of cover (67).
Compare with hydrologic soil group.
hydrologic soil group. A group of soils having the same runoff potential under similar storm and
cover conditions (67). Compare with hydrologic soil-cover complex.
hydrologic studies. Studies to determine the runoff and flood characteristics to be expected at a
highway drainage site. A most important step prior to the hydraulic design of a highway drainage
structure. Such studies are necessary for determining the rate of flow, runoff, or discharge that the
drainage facility will be required to accommodate.
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Highway Drainage Guidelines
hydrologic unit. A geographic area representing part or all of a surface drainage basin or distinct
hydrologic feature as delineated by the Office of Water Data Coordination on the State Hydrologic
Unit Maps; each hydrologic unit is identified by an eight-digit number (50).
hydrologist. A person who studies water on or through the earth’s surface (32).
hydrology. The science and study concerned with the occurrence, circulation, distribution, and
properties of the waters of the earth and its atmosphere, including precipitation, runoff, and
groundwater. The science dealing with the waters of the earth in their various forms: precipitation,
evaporation, runoff, and groundwater. In highway drainage, the science dealing with the runoff and
flood-producing process. In practice the study of the water of the oceans and the atmosphere is, in
some cases, considered part of the sciences of oceanography and meteorology. hydrologic (adj.);
hydrologically (adv.).
The science encompassing the behavior of water as it occurs in the atmosphere, on the surface of the
ground, and underground (2). The science that relates to the water of the earth (46). The science
treating [dealing with] the waters of the earth, their occurrence, distribution, and movements (34).
The science that deals with the occurrence and behavior of water in the atmosphere, on the ground,
and underground. Rainfall intensities, rainfall interception by trees, effects of crop rotations on
runoff, floods, droughts, the flow of springs and wells, are some of the topics studied by a
hydrologist (67).
hydrology, deterministic. [The] deterministic process of hydrology is the result of physical,
chemical, and biological deterministic laws, but primarily the result of fluid mechanics and
thermodynamics laws and regularities (75). Compare with hydrology, stochastic.
hydrology loss. In hydrology, a loss for one purpose is usually a gain for another, so that the net
effect may be more important than the loss. At various times, evapotranspiration, initial abstraction,
infiltration, surface storage, direct runoff, seepage, etc., have been called losses according to the aims
of a water user. See water loss (67).
hydrology, stochastic. [The] laws of chance and the sequence of various variables that describe
[the] . . . random phenomena [of such things as] precipitation, evaporation, runoff, groundwater
levels, sediment transport, lake levels, snow and ice accumulation and melt, water quality properties
and properties of porous environments, river basin geomorphic forms, etc., . . . are stochastic
processes (75).
hydrometeorology. The branch of hydrology concerned with the relationship of preciptation and
runoff to climate and weather.
hydrometry, chemical. See chemical gaging.
hydrophyte. Plant life growing in water or on a substrate that is periodically flooded, causing
deficiency of oxygen. These plants are typically found in wetlands and other aquatic habitats.
Compare with hydrophytic vegetation.
Glossary
G-73
hydrophytic vegetation. A plant growing in water or a substrate that is at least periodically deficient
in oxygen during a growing season as a result of excessive water content. Compare with hydrophyte.
hydrostatic pressure. See pressure, hydrostatic.
hyetograph. Graphical representation of rainfall intensity against time (36). A graph plotting rainfall
amounts or intensities during various time increments versus time (1). A graphical representation of
average rainfall, rainfall-excess rates or volumes over specified areas during successive units of time
during a storm. Compare with hydrograph.
hypolimnion. The lower layer, noncirculating water, of a stratified lake. Compare with thermal
stratification.
ice, frazil or lolly. See frazil ice.
icing. Masses or sheets of ice formed on the frozen surface of a river or floodplain. When shoals in
the river are frozen to the bottom or otherwise dammed, water under hydrostatic pressure is forced to
the surface where it freezes.
IFLOWS. [Acronym] Abbreviation for Integrated Flood Observing and Warning System. A network
of [100 county] automated local flood warning systems. The county systems consist of automatic
radio reporting rain gages, radio relays or repeaters (if required), a radio receiver, and system
software. Each county is usually capable of collecting and displaying real-time precipitation data. All
of the counties are linked to a designated State Emergency Operation Center and offices so data can
be retrieved by any county, State, or NWS office (32).
impact. The striking together of two masses. When particles or streams of water suffer impact,
energy losses result. According to Webster’s New Collegiate Dictionary, the force of impression or
operation of one thing on another; effect.
Impact can also refer to the short- and long-range changes and their significance to surface waters and
related social and environmental relationships resulting from an effect(s) brought about by a highway
drainage facility. Compare with effect.
impact loss. The head lost as a result of the impact of particles of water; included in and scarcely
distinguishable from eddy loss.
impermeable strata. A strata in which texture is such that water cannot move perceptibly through it
under pressures ordinarily found in subsurface water.
impervious. Impermeable to the movement of water.
imperviousness. That quality or condition of a material that minimizes percolation.
improved inlet. See inlet, improved.
incised channel. Those channels that have been cut relatively deep into underlying formations by
natural processes. Characteristics include relatively straight alignment and high, steep banks such that
overflow rarely occurs, if ever. See stream, incised.
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incised reach. The stretch of river with an incised channel that only rarely overflows its banks. See
incised channel.
incised stream. See stream, incised.
inclined gage. See gage, inclined.
incomplete record. A streamflow record in which some peak flows are missing because they were
too low or high to record or the gage was out of operation for a short period because of flooding (33).
independent variable. See variable.
index of loss. See wetland loss index.
indicator. A device that shows by such things as an index, pointer and/or dial, the instantaneous
value of such quantities as depth, pressure, velocity, or the movements or positions of
water-controlling devices; a gage. Compare with recorder, register, and gage.
indirect method (of flow measurement). A method of determining peak discharge, other than by
current meter such as with the slope-area, contraction, culvert, dam, or critical-depth methods;
methods are usually based on survey of highwater marks following the flood.
inert. Unable to move or react.
infiltration. The flow of a fluid into a substance through pores or small openings. It connotes flow
into a substance in contradistinction to the word percolation, which connotes flow through a porous
substance (25).
The downward entry of water into the soil or rock (68).
Rainfall minus interception, evaporation, and surface runoff. The part of rainfall that enters the
soil (67).
That part of rainfall that enters the soil. The passage of water through the soil surface into the ground.
Compare with percolation.
infiltration capacity. The maximum rate at which a soil or rock is capable of absorbing water or
limiting infiltration (68). The maximum rate at which the soil, when in a given condition, can absorb
falling rain or melting snow (24).
The maximum rate at which infiltration can occur under specific conditions of soil moisture. For a
given soil, the infiltration capacity is a function of the water content (20).
infiltration index. An average rate of infiltration, in millimeters [inches] per hour, equal to the
average rate of rainfall such that the volume of rainfall at greater rates equals the total direct
runoff (37).
infiltration rate. The rate at which water enters the soil under a given condition. The rate is usually
expressed in millimeters [inches] per hour, meters [feet] per day, or cubic meters [feet] per second.
Glossary
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inflow. The rate of discharge arriving at a point (in a stream, structure, or reservoir).
inflow design flood. An inflow design flood is the flood hydrograph (in combination with a starting
lake level, spillway and outlet works operation, and freeboard [where applicable]) that establishes the
top of dam elevation [or highway drainage facility geometry and size] required to meet hydrologic
requirements. In some older documents [of the USACE] this may be referred to as a spillway design
flood (29).
influent stream. See stream, losing (20).
initial abstraction. When considering surface runoff [the initial abstraction] Ia is all the rainfall
before runoff begins. When considering direct runoff, Ia consists of interception, evaporation and the
soil-water storage that must be exhausted before direct runoff may begin. Sometimes called initial
loss (67). See hydrology loss.
initial loss. See initial abstraction.
inlet. Consider four definitions: (1) a surface connection to a closed drain; (2) a structure at the
diversion end of a conduit; (3) the upstream end of any structure through which water may flow;
(4) an inlet structure for capturing concentrated surface flow. Inlets may be located in such places as
along the roadway, a gutter, the highway median, or a field.
inlet, combination. Drainage inlet usually composed of two or more inlet types, e.g., such
combinations as curb opening and grate inlet, grate, and slotted drain inlet (21).
inlet control. A condition where the relation between headwater elevation and discharge is controlled
by the upstream end of any structure through which water may flow. For example, a culvert on steep
slope and flowing part full as in inlet control. Compare with outlet control.
inlet, curb opening. Drainage inlet consisting of an opening in a curb (21).
inlet, drop. Drainage inlet with a horizontal or nearly horizontal opening that is generally flush with
the street or land surface.
inlet efficiency. The ratio of flow intercepted by an inlet to the total flow.
inlet, flanking. Inlets placed upstream and on either side of a storm drain inlet that is located at the
low point in a sag vertical curve. The purpose of these inlets is to intercept debris as the longitudinal
gutter slope decreases and to act as an emergency relief for the sump inlet at the low point of the
vertical curve.
inlet, flared. A specially fabricated culvert end appurtenance at the inlet and outlet, or a special end
feature of box culverts where the walls flare outward from the culvert sides at the culvert inlet and
outlet. This type of inlet is effective in reducing the calculated headwater caused by less efficient inlet
types where inlet control prevails. It also serves to retain the roadway embankment. The walls form
an angle to the centerline of the culvert. A type of culvert design having an inlet or outlet larger than
the main barrel. Compare with inlet, improved and end section.
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inlet, flush. Culvert barrel whose entrance (or outlet) does not project beyond the plane of the slope
or headwall.
inlet grate. Drainage inlet composed of a grate in the roadway section or at the roadside, in a low
point, swale, or ditch (21).
inlet, improved. Flared, depressed, or tapered culvert inlets that decrease the amount of energy
needed to pass the flow through the inlet and thus increase the capacity of culverts with inlet control
or supercritical flow.
inlet, mitered. A flush-entrance culvert where the barrel is mitered to the slope of the embankment.
inlet, parallel wing wall. A culvert with wing walls parallel to the culvert centerline.
inlet, projecting. Culvert barrel projects beyond the plane of the slope or headwall; sometimes
referred to as a “re-entrant” entrance.
inlet, slotted drain. Drainage inlet composed of a continuous slot built into the top of a pipe that
serves to intercept, collect, and transport the flow. Often used in conjunction with a single grate inlet
for clean-out access.
inlet, square-edged. An approximately 90-degree corner formed by the inside of the barrel and the
upstream end of the culvert. Small chamfers ordinarily used in concrete construction are considered
as producing a square-edged entrance, i.e., no hydraulic improvement.
inlet, submerged. See submerged inlet.
inlet, sump. Inlet located at the low point in a sag vertical curve.
inlet, tapered. A type of culvert design having an entrance face area larger than the main barrel.
inlet time. The time required for stormwater to flow from the most distant point in a drainage area to
the point at which it enters a storm drain.
instantaneous discharge. A discharge at a given moment. The discharge at a particular instant of
time (50).
insulated stream. See stream, insulated (36).
integration method. A means of determining the mean velocity at a vertical depth of a stream by
noting the total number of revolutions of a current meter vane and the time consumed, while the
meter is slowly lowered from the surface to the bed and returned one or more times.
intensity. The rate of rainfall upon a watershed, usually expressed in millimeters [inches] per hour.
intercepting channel. A channel excavated at the top of earth cuts, or at the foot of slopes, or at other
critical places to intercept surface flow; sometimes termed a catch-drain.
Glossary
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interception. The process and the amount of rain or snow stored on leaves and branches and
eventually evaporated back to the air. Interception equals the precipitation on the vegetation minus
stemflow and throughfall (23). See stemflow and throughfall.
Precipitation retained on plant or plant residue surfaces and finally absorbed, evaporated, or
sublimated. That which flows down the plant to the ground is called stemflow and not counted as
true interception (67).
interflow. The lateral movement of water in the unsaturated zone during and immediately after a
precipitation event. The water moving as interflow discharges directly into a stream or lake (20).
intermittent stream. See stream, intermittent.
interrupted stream. See stream, interrupted.
interstice. A narrow or small space between things or parts, such as riprap; crevice.
inundate. To cover or fill as with a flood.
invert. The flow line in a channel cross section, pipe, or culvert. The lowest point in the channel cross
section or at flow-control devices such as weirs or dams. The floor, bottom, or lowest part of the
internal cross section of a conduit. Compare with soffit.
invertebrates. Animals that have no backbone or spinal column.
inverted siphon. See culvert, sag.
irrigated area. The gross farm area upon which water is artificially applied for the production of
crops, with no reduction for access roads, canals, or farm buildings (60).
irrigation. The controlled application of water to arable lands to supply water requirements not
satisfied by rainfall (26).
irrigation efficiency. The percentage of water applied that can be accounted for in soil-moisture
increase (52).
irrigation pool. Reservoir storage used to store water for release as needed for irrigation.
irrigation requirement. The quantity of water, exclusive of precipitation, that is required for crop
production. It includes surface evaporation and other economically unavoidable wastes (9).
irrigation, supplemental. Commonly, irrigation as carried on in . . . humid areas. The term means
that the irrigation water is supplementary to the natural rainfall source of moisture as in the arid and
semi-arid West. Supplemental irrigation is used generally to prevent retardation of growth during
periods of drought (29).
irrigation, supplemental sources. When irrigation water supplies are obtained from more than one
source, the source furnishing the principal supply is commonly designated the primary source and the
sources furnishing the additional supplies, the supplemental sources (26).
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island. A permanently vegetated area, emergent at normal stage, that divides the flow of a stream.
Some islands originate by establishment of vegetation on a bar and others originate by channel
avulsion or at the junction of minor tributaries with a stream.
isohyet. See isohyetal line.
isohyetal line. A line drawn on a map or chart joining points that receive the same amount of
precipitation (36). A line on a map, connecting points of equal rainfall amounts (67).
jack. Devices cabled together in near parallel rows for flow control and protection of banks against
lateral erosion. A Kellner jack has six mutually perpendicular arms formed by three steel struts
rigidly fixed at the center. Steel jacks are strung with wire. Concrete jacks are made of three
reinforced concrete beams bolted together at the midpoints, or sometimes cast as a monolithic unit.
See jack field.
jack field. Multiple rows of jacks tied together with cables. Generally one or more rows are parallel
with the low-flow bank or along a line where re-establishment of the low-flow bank is desired. The
remaining jack rows are placed perpendicular or at an angle to this row of low bank jacks. This
combination of rows is termed a jack field. Jack fields may be placed outside a channel on the
floodplain and/or within a channel where it is desired to re-establish a low-flow channel bank and
attendant floodplain. See jack.
Jackson turbidity unit. See turbidity.
jetty. See dike, riparian spur.
JTU. Acronym for Jackson Turbidity Unit. See turbidity.
jump. See hydraulic jump.
jurisdictional surface waters. See navigable waters.
karst topography. Irregular topography characterized by sinkholes, streamless valleys, and streams
that disappear into the underground, all developed by the action of surface and undergroundwater in
soluble rock such as limestone.
kinetic energy. See energy, kinetic.
kinematic viscosity. Dynamic viscosity µ (mu) divided by the mass density ρ (rho) of the liquid.
Kutters formula. An empirical formula expressing the value of the coefficient C, in the Chezy
formula, in terms of the friction slope, hydraulic radius, and a coefficient of roughness.
lacustrine. Of or pertaining to a lake. See lake. Compare with limnology.
lag. See lag time.
lag time. [Lag time, TL, is] Variously defined as time from beginning (or center of mass) of rainfall to
peak (or center of mass) of runoff (2).
Glossary
G-79
The difference in time between the centroid of the excess rainfall (that rainfall producing runoff) and
the peak of the runoff hydrograph. Often estimated as 60 percent of the time of concentration
(TL = 0.6Tc) (67).
lake. An area of open, relatively deep water sufficiently large to produce somewhere on its periphery
a barren, wave-swept shore. See lacustrine. Compare with pond.
lake area. See lake surface area.
lake, dimictic. Lakes with a directly stratified epilimnion in summer and an inversely stratified
epilimnion in winter. See epilimnion.
lake, eutrophic. Lakes that have large supplies of nutrients and heavy layers of organic bottom
sediment.
lake, meromictic. Lakes in which some water remains partly or wholly unmixed with the main water
mass at [during] circulation periods is said to be meromictic. The process leading to a meromictic
state is termed meromixis. The perennially stagnant deep layer of a meromictic lake is called the
monimolimnion. The part of a meromictic lake in which free circulation can occur is called the
mixolimnion. The boundary between the monimolimnion and the mixolimnion is called the
chemocline (31).
Lakes that have perennially stagnant water below a steep salinity gradient.
lake, mesotrophic. Lakes at an in-between stage nutritionally, with ecosystems functioning in a
stable fashion, supporting a diverse community of aquatic plant and animal life.
lake, monomictic. Lakes that have a temperature minimum of 4°C (40°F). Water circulates freely in
the wintertime and stratifies during the summer.
lake, oligitrophic. Lakes where plant growth is limited by a low chemical concentration of nutrients.
lake, oxbow. Lakes formed by an oxbow. See oxbow.
lake, playa. Lakes having a nearly level area at the bottom of a basin in an arid or semi-arid region;
the beach or bank of a river. An intermittent pond or lake with no outlet. An undrained basin that, at
times, becomes a temporary, shallow lake. The term playa is more common. See playa lake.
lake shore. See coastal zone and shore.
lake surface area. That area of a lake outlined on the latest U.S. Geological Survey topographic map
as the boundary of the lake and measured by a planimeter in hectares [acres]. In localities not covered
by topographic maps, the areas are computed from the best maps available at the time planimetered.
All areas shown are those for the stage when the planimetered map was made (50).
lake turnover. A seasonal change in heat distribution occurring in most North American lakes. When
air temperatures drop in the autumn, epilimnion, and hypolimnion temperatures equalize and achieve
the same densities. As surface waters become cooler and heavier, they begin to mix with the water
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below. This mixing, combined with the movement due to winds and currents, results in a total
turnover or reversing of the epilimnion and hypolimnion. Another turnover occurs in the spring in
northern regions when ice melts and water temperatures become uniform throughout the lake.
laminar flow. See flow, laminar.
laminar velocity. That velocity below which, in a particular conduit, laminar flow will always exist
and above which the flow may be either laminar or turbulent depending on circumstances. See flow,
laminar; and flow, turbulent.
land line. Relates to communications: typically telephone service but includes any communication
line [land line].
land management agency. A State or Federal government agency responsible for the process of
planning, organizing, programming, coordinating, directing, and controlling land use actions for its
government on lands for which it is responsible under the statutes, regulations, rules, and mandates
that provide its authority.
land-surface datum. Is a datum plane that is approximately at the land surface at each groundwater
observation well (50).
land treatment measure. A tillage practice, a pattern of tillage or land use, or any land improvement
with a substantial effect of reducing runoff and sediment production or of improving use of drainage
and irrigation facilities. Examples are contouring, improved crop rotations, controlled grazing, land
leveling, [and] field drainage. In hydrologic computations, nonbeneficial measures (such as
straight-row, poor-rotation corn) are included for convenience in evaluation. In general conservation
work “land treatment measure” has a broader meaning that includes measures to improve the soil,
control sheet erosion, [and] increase soil fertility (67). Compare with development and land use.
land use. A term that relates to both the physical characteristics of the land surface and the human
activities associated with the land surface (1). A highway facility to accommodate land uses is termed
a land use structure or facility. See land use facilities.
A land classification. Cover, such as row crops or pasture, indicates a kind of land use. Roads may
also be classified as a separate land use (67). Compare with development and land treatment measure.
land use facilities. With highways, stock passes and machinery passes under a road are often termed
land use facilities. See land use.
Laplace-Gauss distribution or Laplacean distribution. See probability distribution.
lateral. A conduit, ditch, canal, or channel conveying water diverted from a main conduit, canal, or
channel for delivery to distributaries; sometimes considered a secondary ditch. See channel.
lateral erosion. See erosion, lateral.
lateral-flow spillway. See spillway, side-channel.
Glossary
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launching. Release of undercut material (stone riprap, rubble, slag, etc.) downslope; if sufficient
material accumulates on the stream bank face, the slope can become effectively armored.
law, water. See water law.
left bank of a channel. The left-hand bank of a channel when the observer is looking downstream.
Compare with right bank of a channel.
legal liability. Liability between litigants recognized and enforced by the courts. Compare
with liable.
levee. An embankment, generally landward of a top bank, that confines flow during highwater
periods, thus preventing overflow into lowlands. A linear embankment outside a channel for
containment of flow. Longer than a dike. Compare with dike.
level of protection. This is the flood level at which flood damages and/or other adverse effects not
eliminated by a project are considered relatively minor. Level of protection is a convenient term to
express the flood control effectiveness of a project and may be based on flood discharge, stage,
volume, duration, or any combination of factors that express project functional characteristics. Most
commonly, it is expressed as the frequency (statistically estimated) of occurrence of the flood
discharge and attendant levels thereof that the project will accommodate with acceptably minor
residual flood damages in such an occurrence (e.g., . . . the one percent chance or 100-year flood).
However, to fully define how a project is expected to function requires describing project impacts at
several flood levels and locations. The terms “level of protection” and “project design flood” are not
necessarily synonymous; separate reaches of a project may have different levels of protection
(variance in the scope of project response to the flood threat) and level of protection may change over
time (because of new upstream development or other changed conditions). There is no minimum level
of protection required for Corps [USACE] projects. However, in urban areas . . . it may well be
desirable to consider alternatives providing a higher level of protection if . . . significant portions of
the urban area [are] within the residual 100-year floodplain; or if, with overtopping or failure . . .
there would be attendant risk to the lives of many, which could not be reasonably guarded against
without a higher level of protection (29).
Not a term commonly used in highway drainage design. See flood, project. Compare with flood,
project design because it is not necessarily synonymous with this term. Also compare with economic
analysis and economic assessment.
level of significance. The [statistically computed] probability of rejecting a hypothesis when it is in
fact true. At a “ten percent” level of significance the probability is 1/10 (33).
liable. Subject to civil action against, or for redress from infringement of private rights. Compare
with legal liability.
liability. See liable.
limnetic zone. The deeper open water zones of lakes or lake size ponds. A relatively large expanse of
open water above the profundal zone. See pond. Compare with littoral zone and profundal zone.
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limnology. That branch of hydrology pertaining to the study of lakes. Compare with lacustrine.
linear system. A system in which the principle of superposition applies and system response is only a
function of the system itself (e.g., unit hydrograph model) (1).
line of sight. An unobstructed straight line from a radio transmitter to a receiver site (32).
lining. See channel lining. Compare with blanket and apron.
littoral drift. The transport of material along a shoreline. Sometimes termed “long-shore sediment
transport.” The movement of sediments in the near shore zone by waves and currents. The movement
can be parallel to the shore (long-shore transport) or perpendicular to the shore (onshore-offshore
transport).
littoral transport. See littoral drift.
littoral zone. A shore or coastal zone. Also the shallower depths of a lake where sunlight penetrates
so that rooted plants can grow. Compare with limnetic zone and profundal zone.
livestock. Domestic animals, such as cattle, horses, hogs, and chickens that are raised for home use or
for profit. Compare with wildlife.
load (or sediment load). Amount of sediment being moved by a stream.
local scour. See scour, local.
logarithmically transformed distribution. See probability distribution.
log-log paper. See log paper.
log-normal. Short for “logarithmic-normal probability distribution” (67).
log-normal paper. Graph paper used in estimating frequencies of floods, etc. Has a logarithmic scale
for the flood (or other) amounts and a cumulative distribution scale (also called frequency or percent
chance scale) for the probability plotting positions (67). Compare with log paper and semilog paper.
log paper. Short for “full-logarithmic graph paper,” which is a graph paper (available commercially)
that has logarithmic scales on both horizontal and vertical axes. Sometimes called “log-log paper.”
The scales may be any number of cycles, but usually in [cycle] combinations like 1 × 1, 2 × 2, 3 × 3,
3 × 5, 4 × 7, etc. (67). Compare with semilog paper and log-normal paper.
log Pearson distribution. See probability distribution.
lolly ice. See frazil ice.
longitudinal profile. The profile of a stream or channel drawn along the length of its centerline. In
drawing the profile, elevations of the water surface or the thalweg are plotted against distance as
measured from the mouth or from an arbitrary initial point.
Glossary
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long-period variations. Secular when a cycle or a change in trend is completed within a century;
climatic when the period of change runs through centuries or a few millennia; geologic when the
period runs into geological time (73). See trend.
long-shore sediment transport. See littoral drift.
long-term degradation. See short-term degradation.
losing stream. See stream, losing.
loss, hydrology. See hydrology loss.
lost head. See head, lost.
lower bank. See bank, lower.
low-flow channel. See channel, low-flow.
low-flow frequency curve. A graph showing the magnitude and frequency of minimum flows for a
period of given length. Frequency is usually expressed as the average interval, in years, between
recurrences of an annual minimum flow equal to or less than that shown by the magnitude scale.
Compare with flood-frequency curve.
low head. See head, low.
lumped system. A [statistical] system in that the variations in space either do not exist or have been
ignored (opposite of distributed system) (1).
lysimeter. Structure containing a mass of soil and designed to permit the measurement of water
draining through the soil (22).
macrophile. Rooted aquatic plants.
main stem. Main branch of the watershed (drainage area) stream system. Compare with tributaries.
major irrigation facilities. Major irrigation facility is an agency-specific term that might be defined
by using arbitrary definitions such as where: (1) a water right is recorded for a conveyance facility,
such as a canal and any appurtenant structures; (2) complex system and/or structure geometries are
required; (3) significant sediment and/or erosion problems occur or are expected; or (4) complex
hydraulic analysis practices are needed.
major storm drain. A major storm drain system is an agency-specific term that might be defined by
using arbitrary definitions such as where: (1) either three or more inlets enter a common trunkline and
outfall; or (2) judgment indicates the need for a storm drain system to avoid a significant flood
hazard. See drain, storm. Compare with design flood (or storm) system.
manhole. Considered to be a gender-neutral term for a structure by which one may access a drainage
system. Also referred to as access hole.
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Manning’s Equation. An empirical formula devised by Manning, based upon original work by
Ganguillet and Kutter, for computing flow in open channels and pipes. In its present form it has been
modified to: V = (1/n)R2/3S1/2 where V = velocity, R = hydraulic radius or A/WP where A = cross
section area and WP = wetted perimeter, and S = hydraulic gradient. See Manning’s n.
Manning’s n. A coefficient of roughness, used in a Manning’s equation for estimating the capacity of
a channel to convey water. Generally, n values are determined by inspection of the channel (67). The
roughness coefficient, n, in the Manning equation for determination of a discharge. See Manning’s
Equation. Compare with hydraulic roughness.
manometer. A tube containing a liquid, the surface of which moves proportionally to changes of
pressures; a tube type of differential pressure indicator; a pressure gage.
mass curve. A graph of the cumulative values of a hydrologic quantity (such as precipitation or
runoff), generally as ordinate, plotted against time or date as abscissa (33). See also double-mass
curve and residual-mass curve.
mass inflow curve. A graph showing the total cumulative volume of stormwater runoff plotted
against time for a given drainage area.
mass wasting. The collapse of a bank by undercutting due to wearing away of the toe or an erodible
soil layer above the toe. The ongoing undercutting of stream banks by erosion and scour followed by
the slumping and subsequent erosion of upper bank material. Sudden collapse of a bank due to an
instability condition such as removal of a portion of the bank by scour. Compare with erosion,
abrasion, scour, and sloughing.
master drainage plan. Planning model for drainage in a particular regional or local geographic area.
mathematical model. A symbolic representation of a flow situation using mathematical equations.
mattress. A covering of concrete, wood, stone, or other material used to protect a stream bank against
erosion. See lining and blanket.
maximum possible flood. See flood, probable maximum.
maximum probable flood. See flood, probable maximum.
mean annual flood, Q2.33. See flood, mean annual.
mean annual flow, Qa. See flow, mean annual.
mean daily discharge. The average of mean discharge of a stream for one day. Usually given in m3/s
(ft3/s) (67). Compare with flood, mean annual; flow, mean annual; and mean monthly flow.
mean depth. Cross section area of a channel divided by its surface width.
meander. The winding of a stream channel (36). The changes in direction and winding of flow,
usually in an alluvial channel that is sinuous in character. Any reverse or letter-S channel pattern
fashioned in alluvial materials by erosion of the concave bank, which is free to shift its location and
Glossary
G-85
adjust its shape as part of a stage in the migratory movement of the channel as a whole down an
erodible, alluvial valley. A meander is characterized by curved flow patterns and alternating shoals
and bank erosion.
meander amplitude. Distance between points of maximum curvature of successive meanders of
opposite phase in a direction normal to the general course of the meander belt, measured between
centerlines of channels (36).
meander belt. Area between lines drawn tangential to the extreme limits of fully developed
meanders (36).
meander breadth. The distance between the lines used to define the meander belt (36).
meandering channel. A channel exhibiting a characteristic process of bank erosion, crossover and
point bar deposition associated with systematically shifting meanders. See meander and meander belt.
A channel having a sinuosity greater than some arbitrary value, herein placed at 1.25. The term also
implies a moderate degree of pattern symmetry, imparted by regularity of size and repetition of
meander loops.
meandering stream. See meandering channel.
meander length. Twice the distance between successive points of inflection of the meander wave
(41). Distance, following the general, sinuous course of the meanders, between corresponding points
of successive meanders of the same amplitude.
meander loop. An individual loop of a meandering or sinuous channel lying between inflection
points with adjoining loops.
meander phase. See meander amplitude.
meander plugs. Deposits of cohesive materials in old channel bendways due to a cutoff. These plugs,
sometimes termed “clay plugs,” are sufficiently resistant to erosion to serve as essentially
semipermanent geological controls to advancing channel migrations. See clay plugs.
meander scroll. Topographical markings on old floodplains resembling a cross section of the edge
pattern of a partly unrolled sheet of paper or having a spiral or coiled form, which have been left on
a floodplain as a result of the historic migratory movement of the channel. Stated another way, low
concentric ridges and swales on a floodplain, marking the successive positions of former
meander loops.
mean discharge. The arithmetic mean of individual daily mean discharges during a specified
period (50).
mean monthly flow. Average monthly flows, expressed in percent of annual flow, are determined for
each of nearby gaged basins in the same hydrologic region. The overall percentage for each month is
computed for the gaged sites, and these averages are multiplied by the estimate of mean annual flow,
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Qa, to determine the mean monthly streamflows at the ungaged site. Compare with flow, mean annual
(Qa); mean daily discharge; and flood, mean annual.
mean sea level. Mean sea level is the plane about which the tide oscillates. It is determined from tidal
observations by averaging the recorded hourly heights of the tide over a period of several years. Mean
sea level varies with locality of observation and method of computation, and hence it has no absolute
constant value. To simplify the computation, mean tide level is sometimes used as a substitute for
mean sea level (14). Compare with mean tide level.
The mean of all hourly ocean elevations over a 19-year period (32).
mean-square error. Sum of the squared differences between the true and estimated values of a
quantity divided by the number of observations. It can also be defined as the bias squared plus the
variance of the quantity (33).
mean tide level. Mean Tide Level is simply equal to the average of the observed high and low
waters, and it is considered a poor substitute [for the mean sea level] in many cases because of the
appreciable influence of short-period tides (14). Compare with mean sea level.
mean velocity. Two definitions are provided: (1) the velocity at a given section of a stream obtained
by dividing the discharge of the stream by the cross section area at that section; (2) mean velocity
may also apply to a reach of a stream by dividing the discharge by the average area of the reach.
measurable. Computing or measuring a change that is more than the inherent error encountered in
accepted hydraulics practices but exclusive of such errors found in flood predicting methods.
measured scour. See scour, measured.
measuring point. An arbitrary, permanent reference point from which the distance to the water
surface in a well is measured to obtain the water level (50).
measuring weir. See weir, measuring.
median diameter. See D50.
meromictic lake. See lake, meromictic.
mesotrophic lake. See lake, mesotrophic.
meteor burst. Radio transmission uses the phenomenon of meteors entering the upper atmosphere to
bounce signals back to earth (32).
meteorological sensor data. Collected weather data such as rainfall, humidity, temperature, and
wind (32).
meter, current. See current meter.
meters per second, m/s (ft/s). Velocity of flow.
Glossary
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method of moments. A standard statistical computation for estimating the moment of a distribution
from the data of a sample (33).
mg/L. Abbreviation for milligrams per liter (mg/L). See parts per million.
microprocessor. A small computer that is usually more powerful in processing ability and more
flexible in using other equipment than a typical home computer (32).
microwave radio. A radio whose transmission [frequency] requires line-of-sight. The frequency has
high resistance to atmospheric interruption (telephone tower relay stations are an example) (32).
mid-channel bar. See bar, middle.
middle bank. See bank, middle.
middle bar. See bar, middle.
migration, channel. See channel migration.
mild slope. See slope, mild.
minimize. Reducing to the smallest practicable amount.
minimum energy line. See energy line, minimum.
mitered entrance. See inlet, mitered.
mitigate. The act of lessening, offsetting, or compensating an impact on surface waters. To moderate
(a qualifying or condition) in force or intensity. To decrease or rectify an adverse condition or action.
See mitigation alternatives, mitigation measures, and mitigation methods.
mitigation alternatives. Environmental mitigation alternatives for surface waters in order of priority
are currently (1992) defined as: (1) avoidance; (2) on-site mitigation; (3) off-site mitigation within the
same drainage area; (4) off-site mitigation within the same drainage and biotic region; (5) no
mitigation. See biotic region.
mitigation measures. Mitigation measures for surface waters are defined as the site-specific action or
construction necessary to accomplish the mitigation to the extent practicable. See practicable.
mitigation methods. Mitigation methods for surface waters are defined as either the on-site or
offsite: (1) construction of new surface waters; (2) enhancement of existing surface waters;
(3) acquisition in perpetuity and enhancement of existing surface waters; (4) combinations thereof.
mobile bed and banks (model). A channel whose bed and banks are free to move under the forces of
flowing water. Mobile bed and bank analyses and computer models estimate channel hydraulics
taking into account this mobility. See fixed bed and banks (model).
model. Reference (68) provides two definitions: (1) a conceptual, mathematical, or physical system
obeying certain specified conditions, whose behavior is used to understand the physical system to
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which it is analogous in some way; (2) a conceptual description and the associated mathematical
representation of a system, subsystem, components, or condition that is used to predict changes from
a baseline state as a function of internal and/or external stimuli and as a function of time and space.
Relates output to input by attempting to describe the characteristics and processes of the system in the
form of mathematical algorithms usually attempting to reflect real cause-effect relationships (1).
model, computer. The representation of a drainage system with computer software. Compare with
model, physical.
model parameter. A constant whose value varies with the circumstances of its application (e.g.,
Manning n-value) (1).
model, physical. The representation of a drainage system with a hydraulically scaled laboratory
model. Compare with model, computer.
model variable. A term [that] has no fixed value (e.g., rainfall) (1).
moisture. Water diffused in the atmosphere or the ground.
moisture equivalent. The ratio of (1) the weight of water that the soil, after saturation, will retain
against a centrifugal force 1000 times the force of gravity to (2) the weight of the soil when dry. The
ratio is stated as a percentage (46).
momentum. The impetus of a moving body; the quantity of motion in a body as measured by the
product of its mass by its velocity.
momentum coefficient (β). A correction factor (beta) applied to the flow momentum, to correct for
non-uniformity of momentum in a cross section. For a fairly straight, nearly uniform channel β varies
from 1.01 to 1.12 and may be as high as 1.33 for overflooded river valleys (13). Compare with
velocity head coefficient (α).
monomictic lake. See lake, monomictic.
morphology. The biological study of the form and structure of living organisms. May also be
shortened term hydraulics engineers, for convenience, often used (or misused) when referring to
fluvial geomorphology (technically this is incorrect but commonly used).
morphology problems. These are fluvial geomorphology problems related to such things as channel
aggradation or degradation, bendway migration, bank erosion, bed scour, and bendway cutoffs. See
fluvial geomorphology, geomorphology, and morphology.
mud. A soft, saturated mixture mainly of silt and clay.
mudflow. A well-mixed mass of water and alluvium that, because of its high viscosity and low
fluidity as compared with water, moves at a much slower rate, usually piling up and spreading over
the [alluvial] fan like a sheet of wet mortar or concrete (74).
Glossary
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mussel. Any of several marine bivalve mollusks, especially the edible Mytilus edulis, having a
blue-black shell.
n (Manning’s), or n value. See Manning’s n.
nappe. A sheet or curtain of water overflowing such things as a weir or drop structure. A stable nappe
has an upper and a lower water surface exposed to the atmosphere. See nappe, flapping.
nappe, flapping. A nappe whose lower water surface periodically loses contact with the atmosphere
that results in a negative pressure thereby causing the nappe to oscillate up and down in a flapping
motion. Hydraulic structures with overflows should be designed to ensure the nappe remains aerated
throughout the design range of flows. See nappe.
national geodetic vertical. Datum of 1929. A geodetic datum derived from a general adjustment of
the first order level nets of both the United States and Canada. It was formerly called Sea level Datum
of 1929 or mean sea level [in the annual WRD data reports]. Although the datum was derived from
the average sea level over a period of many years at 26 tide stations along the Atlantic, Gulf of
Mexico, and Pacific Coasts, it does not necessarily represent local mean sea level at any particular
place (50).
natural and beneficial floodplain values. Such things as wildlife, plants, open space, natural beauty,
outdoor recreation, agriculture, aquaculture, forestry and artifacts, and natural flood control and water
quality control features such as wetlands and groundwater recharge areas. Floodplains also provide
these values and supporting food chains for aquatic life and provide other aquatic-related needs.
Natural Drainage Doctrine or Rule. The precept in civil law dealing with the management and use
of naturally occurring waters, based on preservation and continuance of the natural drainage system
and runoff conditions. Compare with Common Enemy Doctrine or Rule, Civil Law Doctrine or Rule,
and Reasonable Use Doctrine or Rule.
natural levee. A low ridge along a stream channel, formed by deposition as floods abate, that slopes
gently away from the channel.
natural scour. See scour, natural.
natural watercourse. See channel, natural.
natural water surface. See normal water surface.
navigable waters. Primarily a regulatory term as it applies to highway drainage planning, design and
construction in jurisdictional surface waters. The term navigable waters refers to jurisdictional
surface waters as used in PL 92-500 and defined in Section 502(7) as “waters of the United States
including the territorial seas.” The territorial seas; coastal and inland waters, lakes, rivers, and streams
that are navigable waters of the United States, including adjacent wetlands; tributaries to navigable
waters of the United States, including adjacent wetlands; interstate waters and their tributaries,
including adjacent wetlands; and all other waters not identified above, the degradation or destruction
of which could affect interstate commerce (33 CFR 323.3, 42 FR 37144, 1977); also see 80 Stat. 941,
Volume 80 of the U.S. Statutes at Large, page 941; 49 U.S.C. 1651 et seq.: Title 49, United States
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Code, Section 1651 and that which follows (or “and following”); PL 92-500: Public Law number 500
enacted by the 92nd Congress; 40 FR 55810: Volume 40 of the Federal Register, page 55810; 40
CFR 126: Title 40 of the Code of Federal Regulations, part 126. Those waters of the United States
that are subject to the ebb and flow of the tide and/or those waters of the United States that are
presently used, or have been used in the past, or may be susceptible to use in the future to transport
interstate or foreign commerce (33 CFR 322.2, 42 FR 37139, 1977).
Further, navigable waters are considered to be territorial seas of the United States and internal waters
of the United States that are subject to tidal influence and internal waters of the United States not
subject to tidal influence that: (1) are or have been used, or are or have been susceptible for use by
themselves or in connection with other waters, as highways for substantial interstate or foreign
commerce, not withstanding natural or man-made obstructions that require portage, or (2) a
governmental or nongovernmental body, having expertise in waterway improvement, determines to
be capable of improvement at a reasonable cost (a favorable balance between cost and need) to
provide, by themselves or in connection with other waters, highways for substantial interstate or
foreign commerce (40 FR 49327; 33 CFR 2.05-25).
neck cutoff. See cutoff.
nephelometric turbidity unit. Used in the direct reading measurement of the suspended particles.
See turbidity.
network. An assembly of gaging/sampling points located and operated in such a manner as to
provide a representative and transferable “sample” of the spatial and temporal variability of processes
occurring within a geographic region (1).
NFIP. Acronym for National Flood Insurance Program.
nibble effect. A minor or insignificant (see significant) amount of detrimental change in the functions
and values of surface waters that may not be immediately detrimental, but the accumulation of many
such small changes over a period of time will eventually cause detrimental problems.
nick point. A relatively small, localized point of scour in the thalweg of a vegetated swale or channel
characterized by a small or shallow vertical area of channel degradation. If nick points enlarge
upstream and downstream so as to join together, a headcut forms and headcutting begins. Nick
points occur when the vegetation is unable, for various reasons, to resist the eroding action of the
flows occurring in the swale.
nominal sediment. Equivalent spherical diameter of a hypothetical sphere of the same volume as a
given stone or sand particle.
nonlinear system. A system in that the system response depends both upon the system itself and the
input intensity (e.g., equations of gradually varied, open-channel flow) (1).
nonparametric. Statistically, the same as distribution-free.
nonstructural measures. These include: (1) flood warning and preparedness; (2) temporary or
permanent evacuation and relocation; (3) emergency flood fighting and financial relief; (4) land use
Glossary
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regulations including floodway delineation, floodplain zoning, subdivision regulations, and building
codes; (5) flood proofing with or without land use regulations; (6) area renewal and conversion to
open space; (7) flood insuring. The fundamental goal is to develop, define and recommend a robust
solution that has public and institutional support (having appropriately determined how well an
economical plan can be made to function, how capable are the responsible interests to operate and
maintain it and how safe will be the people who will depend on it). Methods of reducing damage from
floods include local flood warning and response systems, temporary or permanent evacuation and
relocation of people or property, emergency flood fighting and financial relief, land use regulations
and building codes, flood proofing, area renewal and conversion to open space, and flood insurance
(29). Compare with structural measures.
non-uniform channel. See channel, non-uniform.
non-uniform flow. See flow, non-uniform.
normal. A central value (such as arithmetic average or median) of annual quantities for a 30-year
period ending with [on] an even 10-year, thus 1921–50; 1931–60 and so forth. This definition accords
with that recommended by the Subcommittee on Hydrology of the Federal Interagency Advisory
Committee on Water Data (the Subcommittee on Hydrology was formerly under the Federal
InterAgency Committee on Water Resources) (36).
A mean or average value established from a series of observations, for purposes of comparison of
some meteorological or hydrologic event (67).
normal depth. See depth, normal.
normal distribution. See probability distribution.
normal flow. Flow at normal depth. Average flow prevailing during the greater part of the year. See
normal stage; and depth, normal.
normal stage. The average water stage prevailing during the greater part of the year. The water
surface elevation corresponding to the normal flow. See normal flow; and depth, normal.
normal velocity. Mean velocity of flow at normal depth. See depth, normal.
normal water surface. The free surface associated with flow in natural streams. The natural water
surface. See depth, normal.
NTU. Acronym for nephelometric turbidity units.
nutrient. Something that nourishes, especially, a nourishing ingredient in a food. Something found in
surface waters that nourishes aquatic life.
NWS. Acronym for the National Weather Service. More specifically, the United States Department
of Commerce, National Oceanic and Atmospheric Administration, National Weather Service;
formerly the U.S. Weather Bureau.
oligitrophic lake. See lake, oligitrophic.
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one-dimensional water surface profile. An estimated water surface profile that recognizes flow only
in the upstream-downstream direction; vertical and transverse velocity vector components are
ignored. Compare with two-dimensional water surface profile.
one-hundred-year flood. See flood, one-hundred-year.
one-thousand-year flood. See flood, one-thousand-year.
on-shore, off-shore transport. See littoral drift.
open channel. See channel, open.
open-channel constriction. In application the term is used to refer to width constrictions, such as a
highway bridge with long approach fills across a floodplain. In a general sense it applies to any type
of constriction in an open channel, whether natural or constructed such as a dam, bridge, or culvert.
open-channel flow. Flow in any open or closed conduit where the water surface is free, that is, where
the water surface is at atmospheric pressure. Compare with channel, open.
ordinary highwater (OHW). A term for defining a regulatory-related water surface for a natural
channel or the shore of standing waters. This intersection reflects the highest level water reaches in an
average runoff year as indicated by such things as erosion, shelving, change in the character of soil,
destruction of terrestrial vegetation or its inability to grow, the presence of litter and debris; or in the
absence of such evidence, an arbitrarily estimated water surface might be used such as that associated
with the mean annual flood. For the purposes of this glossary, in no instance will the ordinary
highwater (OHW) be considered as exceeding the estimated water surface level of the mean annual
flood unless so mandated by the cognizant regulatory agency(ies). The sum of the water right, flood
right, and mean annual flood may be used to arbitrarily determine the maximum OHW for irrigation
channels intercepting runoff.
organic mixtures and mulches. Any of a number of agents (e.g., petrochemicals or vegetative
matter) used to stabilize a stream bank against erosion by providing protection and nutrients while
vegetation becomes established. These agents, which may be in the form of liquids, emulsions or
slurries, are normally applied by mechanical means.
orifice. Two definitions are pertinent: (1) a hole or opening, usually in a plate, wall, or partition,
through which water flows, generally for the purpose of control or measurement; (2) the end of a
small tube, such as the orifice of a Pitot tube or piezometer.
orifice flow. See flow, orifice.
orifice plate. A plate containing an orifice. In pipes, the plate is usually inserted between a pair of
flanges. The orifice is smaller than the pipe and the drop in the hydraulic grade line caused thereby is
an index of the discharge.
orifice, submerged. See submerged orifice.
Glossary
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others. A generic term sometimes used for such things as another governmental agency or quasi
governmental agency (such as a flood control district, or irrigation district), and party(ies) or
individual(s) from the private (non-governmental) sector of society.
outfall. The point where: (1) water flows from a conduit; (2) the mouth (outlet) of a drain or sewer;
(3) drainage discharges from a channel or storm drain.
outlet control. A condition where the relation between headwater elevation and discharge is
controlled by the conduit, outlet, or downstream conditions of any structure through which water
may flow. In culvert flow, outlet control exists for Flow Types II, III, IV, and VI. Compare with
inlet control.
outlet, submerged. See submerged outlet.
outlier. Outliers (extreme events) are data points that depart from the trend of the rest of data (33).
See probability distribution.
overbank flow. See flow, overbank.
overland flow. See flow, overland.
overtopping flood. See flood, overtopping.
oxbow. The abandoned bow-shaped or horseshoe-shaped reach of a former meander loop, that is left
when the stream cuts a new shorter channel across the narrow neck (see cutoff) between closely
approaching bendways of the meander. See lake, oxbow.
paliolimnologists. Those whose study the history of lakes.
parallel wing wall inlet. See inlet, parallel wing wall.
parameter. A characteristic descriptor, such as a mean or standard deviation (33). Sometimes
considered as a variable comprised of the product of two or more variables.
The Reader’s Digest Great Encyclopedic Dictionary offers three mathematical definitions: (1) a
constant whose value determines the operation or characteristics of a system. In y = ax2 + bx + c (a, b
and c are the parameters of a family of parables); (2) a variable t, such that each of a related system of
variables may be expressed as a function of t; (3) a fixed limit or guideline.
[Statistically], a number that describes the universe is called a parameter. [As an example] the value
of 66 mm (2.6 in.), which we found as the standard deviation of the heights of the Harvard students is
a statistic. But if we were to say that the average birth weight of all newborn male babies is 3.4 kg
(1.5 lb), our number would be a parameter (70). Compare with variable and statistic.
Parshall measuring flume. See flume, Parshall measuring.
partial-duration flood series. A list of all flood peak discharges that exceed a chosen base stage or
discharge, regardless of the number of peaks occurring in a year (also called basic-stage flood series
or floods above a base) (36).
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A list of all events, such as floods, occurring above a selected base, without regard to the number,
within a given period. In the case of floods, the selected base is usually equal to the smallest annual
flood of a series, to include at least one flood in each year (67).
partial-duration series. See partial-duration flood series.
partially bounded distribution. See probability distribution.
partial-record station. A particular site where limited streamflow and/or water-quality data are
collected systematically over a period of years for use in hydrologic analyses (50).
particle size. See D16, D50, and D85.
parts per million. The ratio used in expressing the concentration of suspended sediment . . . given in
either parts per million (ppm) or percent. Methods of determination: (1) weight of dried sediment
divided by weight of sample; (2) weight of the dried sediment divided by weight of distilled water
with a volume equal to that of the sampler; (3) weight of the dried sediment divided by the weight of
the water in the sample, including the dissolved material. Either method (1) or (2) is used when the
concentration is less than one percent. Current practice is to use milligrams per liter (mg/l) rather than
parts per million.
pathogens. Any agent that causes diseases, especially a microorganism such as a bacterium
or fungus.
paucity. Smallest of number, fewness, lack of.
pavement. See blanket and mattress. Compare with apron.
paving. See channel lining.
PDF. Acronym for project design flood. See flood, project design.
peak discharge. The highest value of the stage or discharge attained by a flood; thus, peak stage or
peak discharge. Flood crest has nearly the same meaning, but because it connotes the top of the flood
wave, it is properly used only in referring to stage—thus, crest stage, but not crest discharge (36).
Maximum discharge rate on a runoff hydrograph for a given flood event. The instantaneous,
maximum discharge of a particular flood at a given point along a stream.
In a frequency study of annual floods, it is the maximum instantaneous discharge rate reached during
the year.
peaked stone dike. Riprap placed parallel to the toe of a stream bank (at the natural angle of repose
of the stone) to prevent erosion of the toe and induce sediment deposition behind the dike. Compare
with reinforced revetment.
peak flood. See peak discharge.
peak stage. See flood stage.
Glossary
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Pearson distribution. See probability distribution.
percent chance. A probability multiplied by 100 (33).
A name often given to the probability scale on log-normal paper. A two percent chance flood is a
50-year frequency flood (see frequency) because 100 (percent chance = frequency in years) (67). See
log-normal paper.
perched groundwater. See groundwater, perched.
perched stream. See stream, perched.
perched water table. See groundwater, perched; and water table.
percolating waters. Those waters that pass through the ground beneath the surface of the earth
without any definite channel and do not form a part of the body or flow of any surface or
subterranean water course. They may be either infiltrating rain waters or snowmelt, or waters that
have seeped through the banks or bed of a stream to a distance where they lose their character as
streamflow.
percolation. The flow of a fluid through a substance via pores or small openings. Two definitions are
offered by the Groundwater Subcommittee: (1) the downward movement of water through the
unsaturated zone; (2) the downward flow of water in saturated or nearly saturated porous medium at
hydraulic gradients of the order of 1.0 or less.
Movement of water through the interstices of a substance, as through soils. The movement or flow of
water through the interstices or the pores of a soil or other porous medium.
The movement, under hydrostatic pressure, of water through the interstices of a rock or soil, except
the movement through large openings such as caves (46). Compare with infiltration.
perennial flow or stream. See stream, perennial.
permafrost. Perennially frozen ground, occurring wherever the temperature remains at or below 0°C
(32°F) for two or more years in a row (20).
permeability. The property of a material that permits appreciable movement of water through it when
it is saturated and movement is actuated by hydrostatic pressure of the magnitude normally
encountered in natural subsurface water.
permissible velocity. See velocity, permissible.
pervious soil. Soil containing voids through which water will move under hydrostatic pressure,
percolate, or infiltrate.
pH. The reciprocal of the logarithm of the hydrogen ion concentration. The concentration is the
weight of hydrogen ions, in grams, per liter of solution. Neutral water (or soil) has a pH value of 7.
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photosynthesis. The process by which chlorophyll-containing cells in green plants convert incident
light to chemical energy and synthesize organic compounds from inorganic compounds, especially
carbohydrates from carbon dioxide and water with the simultaneous release of oxygen.
phreatic line. The upper boundary of the seepage water surface landward of a stream bank.
physical model. See hydraulic model.
physiographic region. In highway hydrology considerations, a geographic area whose pattern of
landforms and other runoff-producing features for its contiguous subregions are homogeneous but
differ significantly from such feature(s) of an adjacent physiographic region(s).
pier shaft. The main part of a pier above the footing or foundation.
piezometer. An instrument for measuring pressure head, usually consisting of a small pipe tapped
into the side of a closed or open conduit and flush with the inside, connected with a pressure gage,
mercury, water column, or other device for indicating pressure head.
piezometric head. See head, piezometric.
pile. An elongated member, usually made of such things as timber, concrete, or steel, that serves as a
structural component of a river-training structure or bridge foundation. Compare with pile bin.
pile bin. A pier composed of piles capped or decked with a timber grillage or with a
reinforced-concrete slab forming the bridge foundation. See pile.
pile pier. See pile bin.
piping. The action of water passing through or under an embankment and carrying some of the finer
material with it to the surface at the downstream face. Removal of soil material through subsurface
flow or seepage water that develops channels or “pipes” within the soil bank.
Pitot tube. A device for measuring and observing the velocity head of flowing water, consisting
essentially of an orifice held so as to point upstream in flowing water and connected with a tube by
which the rise of water in the tube above the water surface may be observed. It may be constructed
with an upstream and a downstream orifice and two water columns, the difference of water levels
being an index of the velocity head.
plankton. Plant and animal organisms, generally microscopic, that float or drift in great number in
fresh or salt water.
planning models. Models used in large-scale applications such as for metropolitan master plans (1).
playa. See lake, playa.
playa lake. Playa means playa lake thereby making this term redundant. See lake, playa.
plotting position. The point computed by an equation and used to locate given data on probability
[graph] paper (67). See probability plotting position.
Glossary
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PMF. Acronym for probable maximum flood. See flood, probable maximum.
PMP. Acronym for probable maximum precipitation. See precipitation, probable maximum.
point bar. See bar, point.
point gage. See gage, point.
point rainfall. See rainfall, point.
poised stream. See stream, poised.
policy. A definite course of action or method of action, selected to guide and determine present and
future decisions whereas criteria are the standards by which a policy is carried out or placed in action;
design criteria are needed for design, policy statements are not. Compare with design criteria.
pollutant. Anything that pollutes, such as grease, oil, and toxic substances.
pond. Very small, very shallow bodies of standing water in which quiescent water and extensive
occupancy by higher aquatic plants are common characteristics. Regional usage may refer to a lake as
a pond. Compare with lake.
pondage. Small-scale storage at a water power plant to equalize daily or weekly fluctuations in
riverflow or to permit irregular hourly use of the water for power generation to accord with
fluctuations in load (36).
pool. A small, rather deep body of quiescent water, as a pool in a stream.
A deep reach of a stream. The reach of a stream between two riffles. Natural streams often consist of
a succession of pools and riffles (36).
population. The entire (usually infinite) number of data from which a sample is taken or collected.
The total number of past, present, and future floods at a location on a river is the population of floods
for that location even if the floods are not measured or recorded (33). Compare with sample.
pore water pressure. See pressure, pore water.
porosity. Reference (68) offers two definitions: (1) the ratio, usually expressed as a percentage, of the
total volume of voids of a given porous medium to the total volume of the porous medium; (2) the
volume percentage of the total bulk not occupied by solid particles.
The ratio of the volume of void spaces in a rock or sediment to the total volume of the rock or
sediment (20).
Stated two other ways: (1) an index of the void characteristics of a soil or stratum as pertaining to
percolation; degree of perviousness; (2) ratio of void volume to total volume of a soil, or rock,
generally expressed in percentages.
possible maximum flood. See flood, maximum possible.
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potential energy. See energy, potential.
potential evapotranspiration. Water loss that will occur if at no time there is a deficiency of water
in the soil for use of vegetation (63).
potential natural water loss. The water loss during years when the annual precipitation greatly
exceeds the average water loss. It represents the approximate upper limit to water loss under the type
and density of vegetation native to a basin, actual conditions of moisture supply, and other basin
characteristics, whereas potential evapotranspiration represents the hypothetical condition of no
deficiency of water in the soil at any time for use of the type and density of vegetation that would
develop (64).
potential rate of evaporation. See evaporativity.
potomology. The science of surface streams (14). Compare with river mechanics.
ppm. Acronym for parts per million.
practicable. Capable of being accomplished within prudent natural, social, or economic constraints
using readily available resources and reasonably reliable technology and practices that are available
and can be economically applied.
precipitation. The process by which water in liquid or solid state falls from the atmosphere. The total
measurable supply of water received directly from clouds, as rain, snow, and hail; usually expressed
as depth in a day, month, or year and designated as daily, monthly, or annual precipitation. Not
synonymous with rainfall. Compare with rainfall.
As used in hydrology, precipitation is the discharge of water, in liquid or solid state, out of the
atmosphere, generally upon a land or water surface. It is the common process by which atmospheric
water becomes surface or subsurface water. The term “precipitation” is also commonly used to
designate the quantity of water that is precipitated (46). Precipitation includes rainfall, snow, hail, and
sleet and is therefore a more general term than rainfall (36).
precipitation, effective. Two definitions from “General Introduction and Hydrologic Definitions”
(36) are: (1) that part of the precipitation that produces runoff; (2) a weighted average of current and
antecedent precipitation that is “effective” in correlating with runoff. As described by U.S. Bureau of
Reclamation (65), that part of the precipitation falling on an irrigated area that is effective in meeting
the consumptive use requirements. Compare with runoff, direct; and rainfall, effective.
precipitation, probable maximum. Probable maximum precipitation (PMP) is an estimate that
approaches the theoretically largest storm physically possible. Development of the PMP considers all
storms of record and the observed precipitation is increased by maximizing the moisture inflows to
the storm system. Generalized depth, area, duration, and season relationships for the continental
United States are published by the National Weather Service in a series of hydrometeorological
reports (HMR).
predator. An animal that lives by preying upon others.
Glossary
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prescription. Acquirement of the title or right to something through its open and continued use or
actual possession from time immemorial or over a legally recognized or prescribed period. Also the
diversion of water by a person at a point upstream from the land of a riparian owner, under the above
conditions would give such person an appropriate right, perfected by prescription, to the use of the
water, as against the owner.
prescriptive drainage easement. A prescription (prescriptive right) that has been established through
long, uninterrupted and undisputed use of a drainage facility or channel and thus precludes increasing
the existing flood hazard, or changing the drainage pattern or amount at such a drainage facility or
channel for any recurrence interval. The free or unencumbered use of some drainage facility for
drainage or other purposes.
prescriptive right. See prescription.
preserve. To perpetuate an existing condition. In highway drainage design, maintaining the natural
environmental functions of surface waters as close as practicable to their existing state. See
practicable.
pressure. Total load or force acting upon a surface; also appropriately used to indicate intensity of
pressure or force per unit area.
pressure and momentum force. The force due to the sum of the pressure and momentum forces
caused by moving (flowing) water. See pressure and momentum.
pressure head. See head, pressure. See subsurface-water flow and solute transport in the Federal
Glossary of Selected Terms (68).
pressure, hydrostatic. The pressure exerted by the weight of water at any given point in a body of
water at rest (68).
pressure, pore water. Unit stress (pressure) carried by the water in the pores of a soil mass. Excess
negative pressure will displace soil particles. Can be measured with piezometers.
pressure ridges. Ridges on an ice-sheet over a body of water caused by expansion and consequent
upheaval of the ice.
principal spillway. See spillway, principal.
probability. [The science that] deals with the measure of chance or likelihood based on the sampled
data (14). Compare with statistic and statistics.
The ratio of observed or expected events to all possible events; it is expressed as a decimal less than
or equal to one (32). See X-percent chance flood.
probability distribution. Function describing the relative frequency with which events of various
magnitudes occur (33).
Probability distributions of interest to a hydraulics engineer are: normal distribution; Pearson
distributions; extremal [extreme] distributions; logarithmically transformed distributions.
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Normal distribution. A probability distribution that is symmetrical about the mean, median, and
mode (bell-shaped). It is the most studied distribution in statistics (even though most data are not
exactly normally distributed) because of its value in theoretical work and because many other
distributions can be transformed into normal. It is also known as Gaussian, the Laplacean, the
Gauss-Laplace, the Laplace-Gauss distribution, or the Second Law of Laplace.
This is a symmetrical, bell-shaped, continuous distribution, theoretically representing the distribution
of accidental errors about the mean, or so-called Gaussian law of errors [the area under the probability
curve is between the variates of –∞ and x].
Pearson distributions. Karl Pearson . . . derived a series of probability functions to fit virtually any
distribution. Although these functions have only slight theoretical basis, they have been used widely
in practical statistical works to define the shape of many distribution curves, [under certain
conditions] the resulting Pearson distribution is identical with the normal distribution. Types I and III
distributions are often used in hydrologic frequency analysis (14).
Type I. This is a skew [probability] distribution with limited range in both directions, usually
bell-shaped but may be J-shaped or V-shaped.
Type III. This is a skew [probability] distribution with limited range in the left direction, usually
bell-shaped but may be J-shaped.
Extremal distributions. The probability distribution of extreme values. See extreme values. Extreme
value distributions of interest are Types I, II, and III (14).
Type I. This [extreme value probability] distribution results from any initial distribution of
exponential type that converges to an exponential function as x increases. [–∞ < x < ∞]. Examples
of such distributions are normal, the chi-square and the log normal distributions. The Type I
distribution is sometimes known as Gumbel distribution because Gumbel first applied it to flood
frequency analysis.
Type II. This [extreme value probability] distribution results from an initial distribution of [the]
Cauchy type that has no moments from a certain order and higher [0 ≤ x < ∞].
Type III. This [extreme value probability] distribution results from a type of initial distribution in
which x is limited by x ≤ ε [–∞ < x ≤ ε]. Type III is known as Weibull distribution because
Weibull first applied it to the description of the strength of brittle materials although Gumbel also
applied it later to drought frequency analysis.
Logarithmically transformed distributions. Many probability distributions can be transformed by
replacing the variate with its logarithmic value. Three transformed distributions commonly used in
hydrologic studies are [log normal; log extremal; truncated log normal] (14).
Log normal. This is a transformed normal [probability] distribution in which the variate is
replaced by its logarithmic value. This distribution represents the so-called Law of Galton
because it was first studied by Galton as early as 1875.
Log extremal. [This is a probability distribution where the] variate x in [the] Type I [Pearson]
distribution is replaced by a linear function of the logarithm of x and (x – ε) [so that] the resulting
logarithmically transformed distributions become [Pearson] Type II and Type III distributions
respectively.
Glossary
G-101
Truncated log normal. [These are] two truncated and shifted logarithmically transformed normal
distributions for hydrologic frequency analysis. One is called the partly bounded distribution that
has the positive direction of the variate. The other is called the totally bounded distribution that
has the maximum and minimum limits of fluctuations from the mean.
Student’s t distribution. A distribution used in [the] evaluation of variables that involve sample
standard deviation rather than population standard deviation (33). Sometimes referred to as t
distribution.
probability of exceedance. See exceedance probability.
probability paper. Any graph paper prepared especially for plotting magnitudes of events versus
their frequencies or probabilities. See log-normal paper (67).
The cumulative probability of a distribution may be represented graphically on a probability paper
that is designed for the distribution. On such paper the ordinate usually represents the value of x
[flood magnitude] in [a] certain scale and the abscissa represents the probability P(X ≥ x) or P(X ≤ x)
[actually P(X > x) or P(X < x)—Ed.] or the recurrence interval T [or frequency T]. The ordinate and
abscissa scales are so designed the distribution plots as a straight line and the data to be fitted appear
close to a straight line. The objective of using the probability paper is to linearize the distribution so
that the plotted data can be easily analyzed for extrapolation or comparison purposes. In the case of
extrapolation, however, the effect of sampling errors is often magnified (14). See probability
distribution and probability plotting position.
probability plotting position. Probability paper is used to plot the observed data. (Gumbel paper is
one of various types of probability paper.) The various probability distributions can then be plotted on
the probability paper to visually determine which probability distribution best fits the data. The
observed data is plotted on the probability paper using various plotting position formulas. Nine
probability plotting positions formulas are of interest to hydrologists. With N = total number of items,
m = order of items (rank) arranged in descending magnitude (i.e., m = 1 for largest item); P(X • xm):
(1) California, m/N;
(2) Modified California, (m í 1)/N;
(3) Hazen, (m í 0.5)/N;
(4) Weibull, m/(N + 1);
(5) Beard, (1/(1 í 0.5)1/N);
(6) Chegodayev, (m í 0.3)/(N + 0.4);
3
(7) Blom, (m í /8)/(N + 0.25);
(8) Tukey, (m í 0.333)/(N + 0.333);
(9) Gringorten, (m í a)/(N + 1 í 2a).
This formula is valid for all plotting positions where a = 0 < a < 1. The value of a is dependent on the
probability distribution. A value of a = 0.44 is commonly used for the Gumbel distribution.
Paraphrased from Introduction to Hydrology (69).
probable maximum flood. See flood, probable maximum.
probable maximum precipitation. See precipitation, probable maximum.
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profile. A graphical representation of elevation plotted against distance. In open-channel hydraulics a
water surface profile is a plot of water surface elevation against channel distance. See hydraulic grade
line and water surface profile.
profile grade. The trace of a vertical plane intersecting any given roadway surface as shown on the
plans. Profile grade means either elevation or gradient of such trace according to the context.
profundal zone. The underlying deep, dark region of the body of water where light penetration is
insufficient to support the production of green plants. Compare with littoral zone and limnetic zone.
project. The specific section of highway, together with all appurtenances and construction to be
performed under a contract.
project design flood. See flood, project design.
project flood. See flood, project.
project hydrology. The designated design flood predicting practice(s) to be used for the design of
drainage facilities on a particular highway project.
projecting entrance. See inlet, projecting.
promontory. A high ridge of land or rock extending out into a sea or other expanse of water.
protozoa. Any of the single-celled, usually microscopic organisms of the phylum or subkingdom.
Puls method. See storage-indication method.
quarry-run stone. Natural material, often used for stream bank protection, as received from a quarry
without regard to gradation requirements.
rack. A screen composed of parallel bars to catch floating debris.
radio relays. Repeating or relaying devices that transfer data from a self-reporting gage to a base
stations (32).
radio repeaters. See radio relays.
railbank protection. A type of countermeasure composed of rock-filled wire fabric and supported by
steel rails or posts driven into the channel bed and used for such things as retards, riparian spur dikes,
and training dikes.
rain. Liquid precipitation (36). Also, precipitation in the form of water. Usage includes snow and hail
in the term.
rainfall. The quantity of water that falls as rain only. Not synonymous with precipitation (36).
Compare with precipitation.
Glossary
G-103
rainfall, effective. [Sometimes used as] another term for direct runoff. Usually not the same quantity
on upland streams as on downstream rivers because of variability of seepage flows (67). Compare
with runoff, direct; and precipitation, effective.
rainfall, excess. The volume of rainfall available for direct runoff. It is equal to the total rainfall
minus interception, depression storage, and absorption (2). Not the same as excessive rainfall. Direct
runoff at the place where it originates (67). Compare with rainfall, excessive; and rainfall, effective.
rainfall, excessive. Rainfall in which the rate of fall is greater than certain adopted limits, chosen
with regard to the normal precipitation (excluding snow) of a given place or area. In the National
Weather Service, it is defined, for States along the southern Atlantic coast and the Gulf coast, as
rainfall in which the depth of precipitation is 22 mm (0.85 in.) at the end of 30 minutes and 38 mm
(1.5 in.) at the end of an hour and for the rest of the country as rainfall in which the depth of
precipitation at the end or each of the same periods is 13 mm (0.5 in.) and 20 mm (0.8 in.),
respectively (36).
Standard U.S. NWS term for “rainfall in which the rate of fall is greater than certain adopted limits,
chosen with regard to the normal precipitation (excluding snow) of a given place or area.” Not the
same as excess rainfall (67). Compare with rainfall, excess.
rainfall intensity. Amount of rainfall occurring in a unit of time, converted to its equivalent in
millimeters [inches] per hour at the same rate.
rainfall, point. Rainfall at a single rain gage (67).
rapid drawdown. See drawdown, rapid.
rapidly varied flow. See flow, rapidly varied.
rapids. One of two definitions may apply: (1) a term used by some for “chute”; (2) swift and
turbulent flow, without pronounced falls.
raptor. A bird of prey.
rare surface waters. See surface waters, rare.
rate of rise. Indicates how quickly a stream is rising, typically expressed in meters [feet] per hour or
meters [feet] per day (32).
rating. Three definitions are provided: (1) the relation, usually determined experimentally, between
two mutually dependent quantities, such as stage and discharge of a stream; (2) a calibration of
current-meter vane revolutions versus water velocity, etc.; calibration; (3) the taking of measurements
or the making of observations to establish a rating or calibration.
rating curve. A graph of the discharge of a river at a particular point as a function of the elevation of
the water surface (32). A graphic (or tabular) representation of rating; a calibration; a curve (table)
relating stage to discharge. Compare with stage-discharge curve.
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rating flume. There are two types of rating flumes: (1) an open conduit built in a channel to maintain
a consistent regime for the purpose of measuring the flow and developing stage-discharge relation;
(2) a flume containing still water for rating current meters, Pitot tubes, etc.
rational formula. An emerical equation for estimating the flood discharge given as Q = CIA/360,
where Q = peak discharge, C = a runoff coefficient, I = rainfall intensity in millimeters [inches] per
hour for a duration equal to the concentration time of the basin, and A = area of basin in hectares
[acres]. The original formula is based on the approximation that 1 mm/hr/ha = 0.0278 m3/s
[1 in./hr/acre = 1 ft3/s].
reach. A segment of stream or valley, selected with arbitrary bounds for purposes of study. A
comparatively short length of a stream or channel.
“General Introduction and Hydrologic Definitions” (36) offers five definitions: (1) the length of
channel uniform with respect to discharge, depth, area, and slope; (2) the length of a channel for
which a single gage affords a satisfactory measure of the stage and discharge; (3) the length of a river
between two gaging stations; (4) more generally, any length of a river; (5) a length of stream or
valley, selected for convenience in a study. See damage reach and stream reach.
Reasonable Use Doctrine or Rule. A rule under which, in some jurisdictions, a riparian owner,
acting in good faith and for a legitimate purpose, may use and/or alter the natural flow of water from
his land without liability to other owners, so long as such use and action is reasonably necessary and
beneficial and reasonable care is taken to avoid unnecessary injury or hindrance to the use of the land
below. Compare with Common Enemy Doctrine or Rule, Civil Law Doctrine or Rule, and Natural
Drainage Doctrine or Rule.
receiving water. “Natural” [Any—Ed.] body of water that one or more catchments enter into (i.e.,
stream, tributary, river, estuary, bay, lake, etc.) (1).
recession curve. [That portion of] a hydrograph showing the decreasing rate of runoff following a
period of rain or snowmelt. Because direct runoff and base runoff recede at different rates, separate
curves, called direct runoff recession curves and base runoff recession curves, respectively are
generally drawn. The term “depletion curve” in the sense of base runoff recession is not
recommended (36).
The receding portion of a hydrograph, occurring after excess rainfall has stopped (67).
recharge. Addition of water to the zone of saturation from precipitation or infiltration (68). The
process of adding water to the saturated zone; also, the water added. For constructed recharge
facilities, see basin, recharge.
recharge area. An area in which water reaches the zone of saturation by surface infiltration (68).
See recharge.
An area in which there are downward components of hydraulic head in the aquifer. Infiltration moves
downward into the deeper parts of an aquifer in a recharge area (20).
recharge basin. See basin, recharge.
Glossary
G-105
recorder. A device that makes a graph of the stage, pressure, depth, velocity, or the movement or
position of water-controlling devices. See indicator and register.
recording gage. See water-level (stage) recorder.
recovery zone. A clear zone that includes the total roadside border area, starting at the edge of the
traveled way, available for safe use by errant vehicles. This area may consist of a shoulder, a
recoverable slope, a non-recoverable slope and/or a clear run-out area. The desired width is dependent
upon the traffic volumes and speeds and on the roadside geometry.
rectangular weir. See weir, rectangular.
recurrence interval. See flood frequency.
refusal. Erosion-resistant material placed in a trench (excavated landward) at the upstream end of a
revetment to prevent flanking.
regime. The condition of a stream and its channel as regards to their stability. A river or canal is “in
regime” if its channel has reached a stable form as a result of its flow characteristics.
General pattern of variation around a mean condition, as in such things as flow regime, tidal regime,
channel regime, and sediment regime; used also to mean a set of physical characteristics of a river.
The system or order characteristic of a stream; its behavior with respect to such things as velocity and
volume, form of and changes in channel, capacity to transport sediment, and amount of material
supplied for transportation. Compare with regime channel; channel, stable; and stream, poised.
regime change. A change in channel characteristics resulting from such things as changes in imposed
flows, sediment loads or slope.
regime channel. Alluvial channel that has attained more or less a state of equilibrium with respect to
erosion and deposition. Compare with stream, poised; and channel, stable.
regime formula. A formula relating stable alluvial channel dimensions or slope to discharge and
sediment characteristics.
regimen. In unspecialized use “regime” and “regimen” are synonyms (36). See regime, regime
theory, and regime of a stream.
regime of a stream. The system or order characteristic of a stream; in other words, its habits with
respect to velocity and volume, form of and changes in channel, capacity to transport sediment, and
amount of material supplied for transportation. The term is also applied to a stream that has reached
an equilibrium between erosion and deposition or, in other words, to a graded stream (11). See
stream, poised and regime channel.
regime theory. “Regime theory” is a theory of the forming of channels in material carried by the
streams. As used in this sense, the word “regime” applies only to streams that make at least part of
their boundaries from their transported load and part of their transported load from their boundaries,
carrying out the process at different places and times in any one stream in a balanced or alternating
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manner that prevents unlimited growth or removal of boundaries. A stream, river, or canal of this type
is called a “regime stream, river, or canal.” A regime channel is said to be “in regime” when it has
achieved average equilibrium; that is, the average values of the quantities that constitute regime do
not show a definite trend over a considerable period (generally of the order of a decade). In
unspecialized use, “regime” and “regimen” are synonyms (10). See stream, poised; regime channel;
and channel, stable.
regional analysis. Flood-frequency [relationships] lines for gaged watersheds in a similar
[homogeneous physiographic] area or region are used to develop a flood-frequency line for an
ungaged watershed in that [same] region. Also used with other types of hydrologic data. Method is a
simple (usually graphical and freehand) form of “regression analysis” used by statisticians (67).
A statistically based regional study of gaged stream data from a homogeneous physiographic region
that produces regression equations relating various watershed and climatological parameters to such
things as discharge frequency for application on ungaged streams. Used to formulate methods of
predicting flood-frequency relationships for the hydraulic design of drainage facilities in
hydrologically similar ungaged watersheds having characteristics similar to those used in the
regression analysis.
regional flood of record. Maximum flood known or recorded in a drainage area. Peak flood flows
from thousands of sites (Crippin and Bue, WSP-1987) in the conterminous United States were plotted
against drainage area. An enveloping curve for each hydrologic area provides an estimate of
maximum floods within drainage basins (16).
register. One of two definitions may apply: (1) a device that notes quantities; it may make a graph or
a printed or stamped record by figures or symbols on a dial or on an assembly of dials, indicate by
pointer, index, or otherwise note such quantities as stage, pressure, velocity, depth, or quality; it may
note the movement or position of water-controlling devices as gates or valves; a gage, indicator, or
recorder; (2) to note such quantities. See indicator, recorder, and gage.
regulated river or stream. See river, regulated.
regulation. The artificial manipulation of the flow of a stream (36).
regulations. Formal instructions or rules governing the application and administration of specified
legislative acts, which have the force and effect of law. As used in the CFR, rule and regulation have
the same meaning.
regulatory flood. The 100-year flood, which was adopted by FEMA, as the base flood for most
floodplain management purposes. See flood, base.
regulatory floodway. The floodplain area that is reserved in an open manner by Federal, State, or
local requirements, i.e., unconfined or unobstructed either horizontally or vertically, to provide for the
discharge of the base flood so that the cumulative increase in water surface elevation is no more than
a designated amount. May also be considered as: (1) a channel and floodplain regulated by an agency
having jurisdiction over dredge and fill activities; or (2) a channel and floodplain requiring approval
for a channel change or flood storage. Compare with floodway.
Glossary
G-107
regulatory waters. See navigable waters.
reinforced-earth retaining wall. A retaining structure consisting of vertical panels and attached to
reinforcing elements embedded horizontally in compacted backfill for supporting a natural or
artificial channel bank (a specific type of bulkhead).
reinforced revetment. A channel bank protection method consisting of a continuous stone toe-fill
along the base of a bank slope with intermittent fillets of stone placed perpendicular to the toe and
extending back into the natural bank. Compare with peaked stone dike.
relief bridge. See bridge, relief.
re-regulating reservoir. See reservoir, re-regulating.
reservoir. A pond, lake, or basin, either natural or artificial, for the storage, regulation, and control of
water (36). Reservoirs regulate floods downstream from the dam by temporarily storing some part of
the flood volume and releasing it later. The impact downstream is to lower flood stages, increase the
duration of flooding and shift the flood to a later time. It is normal for dam and reservoir projects to
effect some control on and lower flood stages for, all magnitudes of floods (29).
reservoir, re-regulating. A reservoir for reducing diurnal fluctuations resulting from the operation of
an upstream reservoir for power production (36).
reservoir, retarding. Ungated reservoir for temporary storage of floodwaters. Sometimes called
detention reservoir (36). Compare with floodwater retarding structure; basin, retention; and
basin, detention.
reservoir routing. Flood routing through a reservoir (67). Flood routing of a hydrograph through a
reservoir taking into account reservoir storage, spillway, and outlet works discharge relationships.
residence time. The time that water stays in lake with no outlet, or with a very limited outlet. May
also refer to the time floodwaters for a given frequency flood are expected to be detained in a
retention basin, retarding reservoir, recharge basin, or flood control reservoir.
residual-mass curve. A graph of the cumulative departures from a given reference such as the
arithmetic average, generally as ordinate, plotted against time or date, as abscissa (36). See
mass curve.
response system. The planned protective reaction to flooding or the threat of flooding (32).
restorable surface waters. Surface waters and wetlands having functions and values diminished by
human impacts that can be restored through various management techniques.
restore. To re-establish to the extent practicable a geometry, setting, or environment in which the
essential elements of the natural surface waters such as floodplains, shores, riparian areas, and
channel and any previously existing constructed features can again function as they did prior to a
highway action. See practicable.
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retaining wall. With drainage design, a structure used to maintain an elevation differential between
the water surface and top bank while at the same time preventing bank erosion and instability.
retard. A channel bank protection technique consisting of such things as wire mesh, chain-link, steel
rails, or timber-framed fence attached to a series of posts, sometimes in double rows; the space
between the rows may be filled with rock, brush, or other suitable permeable materials. Fences may
be placed either parallel to the bank and/or extended into the channel; in either case these structures
decrease the stream velocity and encourage sediment deposition as the flow passes through the fence.
Explained another way, a frame structure, filled with earth or stone ballast, designed to absorb energy
and to keep erosive channel flows away from a bank. A retard is designed to decrease velocity and
induce sediment deposition or accretion. Retard type structures are permeable structures customarily
constructed at and parallel to the toe of a highway fill-slope and/or channel banks. A permeable or
impermeable linear structure in a channel, parallel with the bank and usually at the toe of the bank,
intended to reduce flow velocity, induce deposition, or deflect flow from the bank.
retarding reservoir. See reservoir, retarding.
retention basin. See basin, retention.
return flow or water. See flow, return.
return period. See flood frequency.
revetment. Rigid or flexible armor placed on a bank or embankment as protection against scour and
lateral erosion. Compare with armor, blanket, and mattress.
review flood (or storm). See flood (or storm), review.
review flood (or storm) system. Drainage facility (or storm drain system) that uses both natural
terrain and constructed features to provide for conveyance of runoff in excess of the design flood (or
storm system) so as to minimize an increase in the flood hazard from a catastrophic flood (or storm).
Flood control facility (or storm drain system) designed through use of prudent judgment and the
review flood (or storm runoff).
review storm system. See review flood (or storm) system.
Reynolds number. The effect of viscosity relative to inertia or R = (VL)/ν where V is the velocity of
flow, L is a characteristic length, and ν (nu) is the kinematic viscosity of the liquid.
RFR. Acronym for regional flood of record or review flood of record.
RI. Acronym for recurrence interval.
riffle. A rapid in a stream (36). Shallow rapids in an open channel, where the water surface is broken
into waves by obstructions wholly or partly submerged. A natural shallow-flow area extending across
a channel bed in which the surface of flowing water is broken by waves or ripples. Typically, riffles
alternate with pools along the length of a channel.
Glossary
G-109
riffle-pool ratio. The sum of the riffle lengths divided by the sum of the pool lengths expressed in
percent for a given reach. These lengths are usually measured at a relatively low stage.
right bank of a channel. The right-hand bank of a channel when the observer is looking
downstream. Compare with left bank of a channel.
right-of-way. A general term denoting land, property, or interest acquired for, or devoted to
highway purposes.
rill. Very small brook or channel made by a small stream, according to Webster’s New Collegiate
Dictionary. A small trickle flowing well within the low-flow channel or thalweg.
riparian. Pertaining to the banks of a stream (36). Of, on, or pertaining to the bank of a channel or
the shore of a pond or a lake. Pertaining to anything connected with or adjacent to the banks of a
channel or other body of water; a riparian owner is one who owns the banks.
Riparian Doctrine or Rule. A doctrine that holds that the property owner adjacent to a surface water
body has first right to withdraw and use the water (20). This doctrine may be set aside by a State’s
statutory law that holds that all surface waters are the property of the State. See water right, flood
right, and statutory law.
riparian owner. A riparian proprietor who owns land on the bank of a river, lake, channel, or other
body of water. An owner of land, in part bounded generally by a stream or river of water and having a
qualified property in the soil to the [thalweg] thread of the channel with the privileges annexed
thereto by law. Compare with Riparian Doctrine or Rule and water right.
riparian proprietor. See riparian owner.
riparian rights. The rights of the owners of lands along a watercourse, relating to such things as
water, its use, ownership of soil under the stream or river, and accretions. The legal right of a riparian
owner to use the water on his riparian land originated in the common law, which permitted him to
require that the waters of a stream or river reach his land undiminished in quantity and unaffected in
quality except for minor domestic uses. Compare with Riparian Doctrine or Rule and water right.
riparian spur dike. See dike, riparian spur.
riparian water. Water that is below the highest line of normal flow of a river or stream, as
distinguished from floodwaters. Compare with floodwaters.
ripple. Two definitions may apply: (1) the light fretting or ruffling of a water surface caused by a
breeze; (2) undulating ridges and furrows, or crests and troughs formed by action of the flow.
riprap. Stones, masonry, or similar constructed material such as broken concrete placed in a loose
assemblage along such things as the banks and bed of a channel or the shore of a lake, pond, gulf,
bay, or ocean to inhibit erosion and scour. Broken stone or concrete placed on earth surfaces for their
protection against the erosive action of water; also, sometimes applied to brush or pole mattresses, or
brush and stone, or other similar materials used for protection. In the restricted sense, layer or facing
of broken rock or concrete dumped or placed to protect a structure or embankment from erosion and
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scour; also, the broken rock or concrete suitable for such use. The term “riprap” has also been applied
to almost all kinds of armor, including such things as wire-enclosed riprap, grouted riprap, sacked
concrete, and concrete slabs. Compare with riprap, stone; riprap, wire-enclosed; and riprap, plated.
riprap, plated. A dense layer of riprap that results from the successive dropping of a heavy, steel
plate onto the riprap from a position above the riprap. The plate is raised into dropping position by a
crane or similar machine.
riprap, stone. Natural cobbles, boulders, or rock preferably of a specified gradation dumped or
placed on a channel bank and/or channel bed (commonly with an underlying filter) as protection
against erosion. Compare with riprap, blanket, and mattress.
riprap, wire-enclosed. Relatively flat mattresses, cylindrical wire cages, or baskets filled with stone
or other suitable material of a specified gradation and placed on a stream bank or bed, often with an
underlying filter and used as protection against erosion and scour. Sometimes staked in-place with
steel pipe or rail that passes through the mattresses, cages, or baskets into the underlying
embankment. Usually used as a mattress (see mattress) rather than in combinations to construction
stream and river structures using gabions. Compare with gabion.
rise. Terminology used with culverts (and similar type openings) as the vertical height dimension of a
box, pipe-arch, and arch structure, as in Span X rise. Compare with span. Can also mean the increase
in river stage; a flood; a peak.
rising limb. The rising portion of a hydrograph preceding a crest (32).
river. Natural stream of water of considerable volume. Depending on local usage, a larger form of a
stream. See channel.
river, controlled. A river and attendant floodplain that is regulated by such constructed devices as
dams, flood control measures, navigational locks, and diversions. Occasionally natural controls may
exist such as natural diversions, sinks, deep sand beds, porous geology strata, etc., that also exert
some measure of control or regulation of a river. Sometimes a controlled river is referred to as a
regulated river. Compare with river, regulated.
river forecast centers. The organizational units within the National Weather Service that forecast
floods (32).
river mechanics. Term for the practices that relate the physical laws governing channels, streams,
and rivers to practicable engineering applications. Compare with potomology.
river, regulated. A river and attendant floodplain that is subject to such things as governmental
regulations and/or interstate compacts. May also be a river that is controlled by constructed or natural
measures. Compare with river, controlled.
river training. The practice of employing structural measures to try and force a stream or river
channel to perform in a specified manner. See channelization and river training structure. Compare
with nonstructural measures.
Glossary
G-111
river training structure. Any configuration of structural measures constructed in a channel or placed
on, adjacent to, or in the vicinity of a channel bank that is intended to deflect currents, induce
sediment deposition, induce scour, or in some other way alter the flow and sediment regimes of a
stream, or river channel.
river training works. See river training structure.
road. A general term denoting a public way for purposes primarily of vehicular travel and includes
the entire area within the right-of-way; generally with emergency parking areas adjacent to the
traveled ways (shoulders), but without curb and gutters. The portion of a road within the limits of
construction. Compare with street.
roadbed. The graded portion of a highway within top and side slopes, prepared as a foundation for
the pavement structure and shoulder.
roadside. A general term denoting the area adjoining the outer edge of the roadway. Extensive
area between the roadways of a divided highway (not necessarily the median) may also be
considered roadside.
roadside development. Those items necessary to complete a highway that provide for:
(1) preservation of landscape materials and features; (2) the rehabilitation and protection against
erosion of all areas disturbed by construction, seeding, sodding, mulching, and the placing of other
ground covers; and (3) such suitable planting and other improvements as may increase the
effectiveness and enhance the appearance of the highway.
roadway cross slope. Transverse slope and/or superelevation described by the roadway section
geometry. Usually provided to facilitate drainage and/or resist the centrifugal force of a
moving vehicle.
rock-and-wire mattress. See riprap, wire-enclosed.
rod-float. A rod or staff designed to float in a practically vertical position for the purpose of
observing velocities.
root zone. The zone from the land surface to the depth penetrated by plant roots. The root zone may
contain part or all of the unsaturated zone, depending upon the depth of the roots and the thickness of
the unsaturated zone (20).
rough fish. A fish that is neither a sport fish nor important food for sport fishes.
roughness. See roughness coefficient.
roughness coefficient. The estimated measure of texture at the perimeters of channels and conduits.
Usually represented by the n-value coefficient used in Manning’s channel flow equation. Numerical
measure of the frictional resistance to flow in a channel, as in the Manning or Strickler equations. See
channel coefficient, Manning’s Equation, Manning’s n, and Chezy formula.
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rubble. Rough, irregular fragments of random size placed on a channel bank or shore to retard
erosion. The fragments may consist of broken concrete slabs, masonry, or other suitable refuse.
rule. An established guide for action. A rule may not be authoritatively enforced, but is generally
observed in the interest of such things as order and uniformity. A formal decree, as by court decision.
Such rules constitute a large portion of the body of common law. Compare with rule of law and
doctrine.
rule of law. A legal principle of general application, sanctioned by the recognition of authorities and
usually expressed as a maxim or a logical proposition. Called a “rule” because in doubtful or
unforeseen cases it is a guide or norm for making decisions. Compare with rule and doctrine.
run. Term of regional geographic use. See channel.
runoff. Surface water, stream water, and floodwater as defined in Chapter 5 of the Highway
Drainage Guidelines. That part of the precipitation that runs off the surface of a drainage area after
accounting for all abstractions. The portion of precipitation that appears as flow in streams; total
volume of flow of a stream during a specified time.
That part of the precipitation that appears in surface streams. It is the same as streamflow unaffected
by artificial diversions, storage, or other works of man in or on the stream channels (36).
The total amount of water flowing in a stream. It includes overland flow, return flow, interflow and
base flow (20).
Runoff may be classified as to speed of appearance after rainfall or snow melting:
direct runoff, and
base runoff;
and by source:
surface runoff (see flow, overland),
storm seepage, and
groundwater runoff.
Compare with discharge; runoff; and runoff, base.
runoff, annual. The total natural discharge of a stream for a year, usually expressed in millimeters
[inches] of depth.
The total amount of water obtained in a year from such things as a stream, spring, or artesian well.
Usually expressed in millimeters [inches] of depth, millions of liters [gallons], or cubic meters
[feet] (67).
Glossary
G-113
The runoff from the drainage basin, including groundwater outflow that appears in the stream plus
groundwater outflow that bypasses the gaging station and leaves the basin underground. Water yield
is the precipitation minus the evapotranspiration (36).
The actual streamflow, at a given place, from a watershed. This is natural annual runoff that may be
affected by irrigation uses, reservoir losses, diversions into or out of the watershed, etc. (67).
runoff, base. Sustained or fair weather runoff. In most streams, base runoff is composed largely on
groundwater effluent. [Other terms] are often used in the same sense as base runoff groundwater
runoff, direct runoff, base discharge, and base flow. However, the distinction is the same as that
between streamflow and runoff. When the concept in the terms base flow and base runoff is that of
the natural flow in a stream, base runoff is the logical term (36).
That part of the stream discharge that is not attributable to direct runoff from precipitation or melting
snow; it is usually sustained by groundwater discharge (68). That part of a stream discharge derived
from groundwater seeping into the stream (20). Contribution of groundwater to runoff (1).
Stream discharge derived from groundwater sources. Sometimes considered to include flows from
regulated lakes or reservoirs. Fluctuates much less than storm runoff (67). See runoff. Compare with
runoff, direct.
runoff coefficient. A factor representing that portion of runoff that results from a unit of rainfall.
Dependent on terrain and topography. The rate of runoff to precipitation.
runoff, direct. The runoff entering stream channels promptly after rainfall or snowmelt.
Superimposed on base runoff, it forms the bulk of the hydrograph of a flood. See runoff, surface. The
terms base runoff and direct runoff are time classifications of runoff. The term surface runoff is
classified according to source (36).
The water that enters the stream channels during a storm or soon after, forming a runoff hydrograph.
May consist of rainfall on the stream surface, surface runoff, and seepage of infiltrated water (rapid
subsurface flow) (67).
See runoff. Compare with runoff, base.
runoff, first flush. The condition, often occurring, in which a disproportionately high pollution load
is carried in the first portion of urban runoff (1).
runoff in millimeters [inches]. The depth to which the drainage area would be covered if all the
runoff for a given time period were uniformly distributed on it (50).
runoff, subsurface. Water that infiltrates the soil and reappears as seepage or spring flow and forms
part of the flood hydrograph for that storm. Difficult to determine in practice and seldom worked with
separately. See runoff, direct (67). See storm seepage.
runoff, surface. That part of the runoff that travels over the soil surface to the nearest stream channel.
It is also defined as that part of the runoff of a drainage basin that has not passed beneath the surface
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since precipitation. The term is misused when applied in the sense of direct runoff. See also runoff;
flow, overland; runoff, direct; groundwater runoff; and surface waters (36).
Total rainfall minus interception, evaporation, infiltration, and surface storage and that moves across
the ground surface to a stream or depression (67).
runoff, urban. Storm-generated surface runoff from an urban drainage area. The term may relate to
either the quantity or quality of the runoff or both, depending upon its application (1).
run-up, wave. See wave run-up.
sack revetment. Stream bank protection consisting of sacks (e.g., burlap, paper, or nylon) filled with
such things as mortar, concrete, sand, stone, or other available material placed on a bank to serve as
protection against erosion.
saltating. Bed load particles that skip along the bed while being transported by streamflows. See
also bed load.
saltation load. That portion of the total sediment load bounced along the streambed by energy and
turbulence of flow and by other moving particles. Compare with bed load and sediment load,
suspended.
sample. An element, part, or fragment of a population. Every hydrologic record is a sample of a much
longer record (33). Compare with population.
sampling. See gaging/sampling.
sand. Soil material that can pass the No. 4 (4.76 mm) U.S. Standard Sieve and be retained on the
No. 200 (0.074-mm) sieve. FHWA, HIRE, 1987. Granular material that is smaller than 2.0 mm
(0.08 in.) and coarser than 0.062 mm (0.0024 in.). FHWA, HIRE, 1990.
saturated soil. Soil that has its interstices, pores, or void spaces filled with water to the point at
which runoff occurs.
saturated zone. See zone of saturation.
scour. The displacement and removal of channel bed material due to flowing water; usually
considered as being localized as opposed to general bed degradation or headcutting. The result of the
erosive action of running water that excavates and carries away material from a channel bed.
Compare with erosion; abrasion; erosion, lateral; mass wasting; and sloughing.
scour, bendway. That component of natural scour consisting of the removal of material from the
channel bed or banks that occurs along the concave (outside) bank in a channel bendway located,
generally, across the channel from any point bar.
scour, contraction. The response of a river or drainage facility (such as bridge) to the change in its
bed load requirement as a result of a natural or constructed contraction of flow, i.e., the flow
contraction is due to an encroachment of either the main channel or the floodplain by a natural
constriction or the highway embankment. Compare with scour, general.
Glossary
G-115
scoured depth. Total depth of the water from water surface to a scoured channel bed. Compare with
depth of scour.
scour, general. Scour in a channel or on a floodplain that is not localized at a pier, abutment,
bendway, or other obstruction to flow. In a channel, general scour usually affects all or most of the
channel width; i.e., general scour involves the removal of material from the bed across all or most of
the width of a channel as a result of a natural flow contraction that causes increased velocities and bed
shear stress. Compare with scour, contraction.
scour, local. Removal of material from the channel bed or banks that is restricted to a relatively
minor part of the width of a channel. Scour in a channel or on a floodplain that is localized at a pier,
abutment, or other obstruction to flow. Local scour is caused by the acceleration of the flow and the
development of a vortex system induced by the obstruction to the flow. Does not include the
additional scour caused by any contraction, natural channel degradation, or bendway.
scour, measured. The measured depth to which a surface is lowered by scour below a
reference elevation.
scour, natural. Removal of material from the channel bed or banks that occurs in channels due to the
migration of bed forms, shifting of the thalweg, and at bendways and natural contractions. Scour that
occurs along a channel reach due to natural causes or an otherwise unstable stream, i.e., no external,
constructed causes.
scupper. A device and/or vertical hole through such things as bridge decks or roofs for the purpose of
deck or roof drainage. Sometimes a horizontal opening in the curb or barrier is called a scupper.
SD (s.d.). Acronym for standard deviation (67).
seasonal stream. See stream, intermittent.
Secchi disk. A disk-type device that, when lowered into surface waters, measures their clarity.
second-foot. Same as cubic foot per second. Generally, this term is no longer used in published
reports of Federal agencies (36).
See cfs in the National Engineering Handbook (67). A term, although incorrect, which is sometimes
used in verbal discussions in place of “cubic feet per second.” Compare with cubic feet per second.
second law of Laplace. See probability distribution.
sediment. Fragmental material that originates from weathering of rocks and is transported by,
suspended in, or deposited by water or air, or is accumulated in beds by other natural agencies (15).
Compare with sediment, fluvial.
sedimentation. The process involving the deposition of soil particles that have been carried by
floodwaters. Sometimes erroneously termed “silting” by those unfamiliar with sediment transport and
deposition as the deposited material does not necessarily contain much, if any, material classified as
silt. Compare with aggradation.
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sedimentation basin. See basin, sedimentation.
sediment concentration. Weight or volume of sediment relative to quantity of transporting or
suspending fluid or fluid-sediment mixture.
sediment discharge, suspended. The rate at which the dry weight of sediment passes a section of a
stream [or river] or is the quantity of sediment (as measured by dry weight, or by volume) that is
discharged in a given time (15). The quantity of sediment that is carried past any cross section of a
stream or river above the bed layer in a unit of time. Discharge may be limited to certain sizes of
sediment or to a specific part of the cross section. Compare with bed load discharge; sediment load,
suspended; sediment yield; and wash load.
sediment discharge, total. The sum of suspended sediment discharge and bed load discharge or the
sum of bed material discharge and wash load discharge of a stream or river. See wash load.
sediment, fluvial. Fragmental material transported, suspended, or deposited by water. Compare
with sediment.
sediment load, suspended. Sediment that is supported by the upward components of turbulent
currents in a stream and that stays in suspension for an appreciable length of time. Compare with
sediment discharge, suspended; and wash load.
sediment pool. [That portion of the total] reservoir storage [expressedly] provided for sediment, thus
prolonging the usefulness of floodwater or irrigation pools (67).
sediment yield. The total sediment outflow from a watershed or a drainage area at a point of
reference and in a specified time period. This outflow is equal to the sediment discharge from the
drainage area. Compare with sediment discharge, suspended.
seepage. The slow movement of water through small cracks and pores of the bank material.
seiche. The free oscillation of the bulk of water in a lake and the motion caused by it on the surface of
the lake (7).
Long-period oscillation of a lake or similar body of water. An oscillation of the water surface of a
lake or other large land-locked body of water due to unequal atmospheric pressure, wind, landslides,
earthquakes, or other causes, which sets the surface in vibration. Waves that oscillate in lakes, bays,
and gulfs from a few minutes to a few hours. Also associated with hurricanes on waters that are
not landlocked.
seismic wave. A gravity wave caused by an earthquake.
self-reporting gage. See gage, self-reporting.
semi-arid. Geographic areas characterized by light rainfall. More specifically, having from
approximately 250 mm (10 in.) to approximately 500 mm (20 in.) of annual precipitation. Compare
with arid and desert.
Glossary
G-117
semilog paper. Short for “semilogarithmic graph paper,” which is graph paper having an arithmetic
scale along one axis and a logarithmic scale along the other. Either scale is used for the independent
variable, as the data require. Commercially available paper has various divisions for the arithmetic
scale and various cycles for the logarithmic side [scale] (67). Compare with log paper and
log-normal paper.
sensitive surface waters. See surface waters, sensitive.
set-down. See wind set-down.
set-up. See wind set-up.
sewer. A conduit for conveying sanitary waste flows. Compare with sewer, storm; sewer, combined;
and drain, storm.
sewer, combined. A sewer that conveys stormwater and, at times, sanitary sewage. See sewer; and
drain, storm.
sewer, storm. Principally a drain for conveying stormwater, but at least part of the time, a drain that
also conveys raw sewage is termed a storm sewer. Compare with design flood (or storm) system;
sewer; and sewer, combined.
shallow water wave. See wave, shallow water.
sharp-crested weir. See weir, sharp-crested.
shear force. See tractive force.
sheet flow. See flow, overland.
shifting control. See control (36).
shoal. A place in any body of water where the water is especially shallow. A sandbank or sandbar.
Compare with shoaling.
shoaling. The process of deposition of alluvial material resulting in areas with relatively shallow
depth (shoals). Compare with shoal.
shooting flow. See flow, supercritical.
shore. Interface between the land and the waters of a lake or pond. See coast line.
shore line. See coast line and shore.
short-term degradation. A regulatory term for adverse surface water impacts or losses (generally
associated with surface water quality) that essentially disappear within some arbitrary period of time
(usually one year) following the disturbance from a highway action.
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shoulder. The portion of the roadway contiguous with the traveled way for accommodating stopped
vehicles, for emergency use, and for lateral support of the road’s base and surface courses.
side slopes. The slope of the sides of a canal, dam, or embankment. Currently sanctioned the naming
of the vertical distance first as 1 to 2 (or, frequently, 1:2) meaning a vertical distance of 1 m (1 ft) to
2 m (2 ft) horizontal. Another form, not as subject to misinterpretation by thoughtless transposition, is
1V on 2H.
For slopes less than 45 degrees, the vertical component should be unitary (for example, 1:20). For
slopes over 45 degrees, the horizontal component should be unitary (for example, 5:1)
side-channel spillway. See spillway, side-channel.
sidewalk width. Unobstructed space for exclusive pedestrian use between barriers or between a curb
and a barrier.
significance. See significant.
significance test. See test of significance.
significant. Determined by relating an effect to such things as risk (probability or frequency of
occurrence) and cost. Included in environmental determinations are such things as past or expected
future accumulative effects (often termed “nibble” effect). See effect.
significant encroachment. An encroachment or any direct support of likely base floodplain
development by a highway action that: (1) causes a potentially significant flood hazard by adversely
impacting natural and beneficial floodplain values; (2) interrupts or terminates the use of a
transportation facility needed for emergency vehicles or a community’s only evacuation route; (3) is
in environmentally sensitive surface waters; or (4) is in regulatory waters.
significant flood hazard. Where it can be shown with reasonable hydrologic certainty that the
(1) expected floodplain limits; or (2) roadway encroachment of the review flood (or storm) caused by
an existing or proposed drainage facility will measurably exceed existing or natural floodplain limits
or, in the case of guttered stormwaters, encroach on a traveled way (see spread) to where either
prudent judgment, regulatory criteria, an economic analysis, or political considerations indicate an
unacceptable flood hazard. The base flood (or storm) will be used to review the design flood (or
storm) system to judge if there might be an unacceptable flood hazard. The 500-year flood is used for
this review flood when evaluating bridge scour rather than the 100-year flood.
significant wave. See wave, significant.
sill. A grade control structure. See grade control structure. Also a device placed across a channel, or
generally transverse to flow in a steep culvert to provide a fishway.
silt. Material passing the No. 200 (0.074-mm) U.S. Standard Sieve that is nonplastic or very slightly
plastic and exhibits little or no strength when air-dried (Unified Soil Classification System) according
to the FHWA’s Highways in the River Environment Manual, January 1987. According to the
Glossary
G-119
FHWA’s Highways in the River Environment Manual (February 1990), material finer than 0.062 mm
(0.0025 in.) and coarser than 0.004 mm (0.00015 in.)
Two other definitions may have some application: (1) waterborne sediment that refers to material that
is generally confined to fine earth, sand, or mud, but is sometimes broadened to include all material
carried, including both suspended and bed load; (2) deposits of waterborne material, as in a reservoir,
on a delta, or on overflowed lands.
silting. An incorrect term for sedimentation, or sediment deposition. See sedimentation.
sink holes. See sinks.
sinks. Geological provinces [areas] characterized by carbonate rocks or evaporite rocks [that] are
susceptible to solution by groundwater, that results eventually in the formation of rock basins
called sinks (14).
sinuosity. The ratio of the length of the river thalweg to the length of the valley proper.
sinuous flow. See flow, turbulent.
siphon, or inverted siphon. See culvert, sag.
skew. The measure of the angle of intersection between a line normal to the roadway centerline and
the direction of the flow in a channel at flood stage in the lineal direction of the main channel.
skew coefficient. A numerical measure [statistical] or index of the lack of symmetry in a frequency
distribution. Function of the third moment of magnitudes about their mean, a measure of a symmetry.
Also called coefficient of skew (33).
skewness. When data plot in a curve on log-normal paper, the curvature is an indication of data
skewness (67). Compare with skew coefficient.
sliplaw. The initial and separate publication in brochure form of a statute upon enactment, prior to its
formal publication in the U.S. Statutes at Large.
slope, adverse. A conduit is on an adverse slope when its slope is positive (slopes upward) in the
downstream direction. Compare with slope, mild; and slope, steep.
slope-area method. A method of estimating unmeasured flood discharges in a uniform channel reach
using observed highwater levels. More explicitly a flow measurement method based on an indirect
measurement of peak discharge by field survey of a reach of channel and highwater marks, usually
after a flood has passed. Discharge is computed by the Manning equation, modified to account for
non-uniform flow. See indirect method (of flow measurement).
slope, channel. See channel slope.
slope, critical. See critical slope.
slope, fill. See fill slope.
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slope gage. See gage, inclined.
slope, mild. A conduit is on mild slope when its slope is less than the critical slope for a particular
value of discharge. Whether a conduit slope is steep or mild depends on the discharge under
consideration and the conduit slope—the conduit may be steep for one discharge and mild for
another. A conduit is on mild slope if, for a given discharge, the normal depth is greater than critical
depth. Compare with slope, steep and slope, adverse.
slope, steep. A conduit is on steep slope when its slope is greater than the critical slope for a
particular value of discharge. Whether a conduit slope is steep or mild depends on the discharge
under consideration and the conduit slope—the conduit may be steep for one discharge and mild for
another. A conduit is on steep slope if, for a given discharge, the normal depth is less than critical
depth. See flow, supercritical; flow, subcritical; and critical flow. Compare with slope, mild and
slope, adverse.
slope, surface. The inclination of the water surface expressed as change of elevation per unit of slope
length; the sine of the angle that the water surface makes with the horizontal. The tangent of that
angle is ordinarily used, with no appreciable error resulting except for the steeper slopes.
slotted drain inlet. See inlet, slotted drain.
sloughing. Shallow, transverse movement of a soil mass down a stream bank as the result of an
instability condition at or near the surface (also called slumping and sometimes, perhaps incorrectly,
mass wasting). Conditions leading to sloughing are bed degradation, attack at the bank toe, rapid
drawdown and slope erosion to an angle greater than the angle of repose of the material. Compare
with mass wasting, erosion, abrasion, and scour.
sluice flow. See flow, sluice.
slumping. See sloughing.
snow. A form or precipitation composed of ice crystals (36).
snow course. A line or series of connecting lines along which snow samples are taken at regularly
spaced points (36) (66).
snow density. Ratio between the volume of melt water derived from a sample of snow and the initial
volume of the sample; this is numerically equal to the specific gravity of the snow (36). The waste
content of snow expressed as a percentage by volume. In snow surveys it is the ratio of the scale
reading (millimeters [inches] of water) to the length of the snow sample in millimeters [inches].
snowline. The general altitude to which the continuous snow cover of high mountains retreats in
summer, chiefly controlled by the depth of the winter snowfall and by the temperature of the
summer (36).
snowline, temporary. A line sometimes drawn on a weather map during the winter showing the
southern limit of the snow cover (36).
Glossary
G-121
sodding. The act of covering the ground with a section of grass-covered soil held together by
matted roots.
soffit. The inside top of a conduit such as a culvert or storm drain pipe. Compare with invert.
soil. Finely divided material composed of disintegrated rock mixed with organic matter; the loose
surface material in which plants grow. Dirt, an incorrect term, is not soil as it contains humus and
other foreign material.
soil-cement. A designed mixture of soil and portland cement compacted at a proper water content to
form a veneer or structure that can prevent the erosion of such things as a channel bank, dam face, or
inlet and outlet of a drainage structure.
soil colloids. Suspended, finely divided particles in a continuous medium. The particles do not settle
out of the substance readily and are not readily filtered.
soil-cover complex. See hydrologic soil-cover complex.
soil evaporation. Evaporation of water from moist soils.
soil group. See hydrologic soil group.
soil moisture. Water diffused in the soil; the upper part of the zone of aeration from which water is
discharged by the transpiration of plants or by soil evaporation (36). The water contained in the
unsaturated zone (20). Subsurface liquid water in the unsaturated zone expressed as a fraction of the
total porous medium volume occupied by water; it is less than or equal to the porosity (68).
soil porosity. The percentage of the soil (or rock) volume that is not occupied by solid particles,
including all pore space filled with air and water. See porosity.
soil water. See soil moisture (68).
soil-water-storage. The amount of water the soils (including geologic formations) of a watershed
will store at a given time. Amounts vary from watershed to watershed. The amount for a given
watershed is continually varying as rainfall or evapotranspiration takes place (67).
span. Terminology used with culverts (and similar type openings) as the horizontal width dimension
of such things as a box, pipe-arch, or arch structure, as in Span X Rise. May also be the horizontal
distance between bridge piers or abutments. Compare with rise.
spatial concentration. The dry weight of sediment per unit volume of water-sediment mixture in
place or the ratio of dry weight of sediment or total weight of water-sediment mixture in a sample or
unit volume of the mixture.
spawning. The act of depositing eggs or producing spawn.
specific energy. See energy, specific.
SPF. Acronym for Standard Project Flood.
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spillthrough abutment. A bridge abutment having a fill slope on the channel side. The term
originally referred to the “spillthrough” of fill at an open abutment but is now applied to any
abutment having such a slope. See abutment. Compare with embankment end slope, fill slope, and
vertical abutment.
spillway. A passage for spilling surplus water; a wasteway. See spillway, principal; and spillway,
emergency (67).
spillway, controlled. A reservoir outlet works wherein the outflow is controlled by gates, valves, or
similar flow control devices.
spillway design flood. See inflow design flood (29).
spillway, emergency. A rock or vegetated earth waterway around a dam, built with its crest above the
normally used principal spillway. Used to assist [or supplement] the principal spillway in conveying
extreme amounts of runoff safely past the dam [so as to minimize damage and flood hazards] (67).
See spillway, uncontrolled. Compare with spillway, principal.
spillway, lateral-flow. See spillway, side channel.
spillway, principal. Conveys all ordinary discharges coming into a reservoir and that portion of an
extreme discharge that does not pass through the emergency spillway or outlet works. See spillway,
uncontrolled.
A concrete or metal pipe or conduit used with a drop inlet dam or floodwater retarding structure. It
conveys, in a safe and nonerosive manner, all ordinary discharges coming into a reservoir and all of
an extreme amount that does not pass through the emergency spillway (67). Compare with spillway,
emergency.
spillway, side-channel. A spillway in which the initial and final flow are approximately at right
angles to each other; a side-channel spillway.
spillway, uncontrolled. A spillway for a reservoir at which floodwater discharge is governed only by
the inflow and resulting head in the reservoir. Usually the emergency spillway is uncontrolled.
splash-over. That portion of frontal flow at a grate that splashes over the grate and is not intercepted.
Compare with flow, frontal.
spread. The accumulated flow in and next to the roadway gutter. The transverse encroachment of
stormwater onto a street. See street. This water often represents an interruption to traffic flow,
splash-related problems and a source of hydroplaning during rainstorms. The lateral distance, in
meters [feet], of roadway ponding extending out from the curb or edge of the traveled way. See
encroachment. Compare with significant encroachment.
spread footing. A pier or abutment footing that transfers load directly to the earth.
sprigging. The act of planting a woody vegetation such as bushes and trees by using sprigs (twigs cut
from live vegetation) stuck into the prepared ground. Sprigging must be “greenside up.”
Glossary
G-123
spring. A discrete place where groundwater flows naturally from a rock or the soil onto the land
surface or into a body of surface waters (68).
spring box. An enclosure constructed to protect or otherwise contain a flow of water emerging from
the ground.
SPS. Acronym for Standard Project Storm.
spur or spur dike. See dike; dike, spur; dike, riparian spur; dike, embankment spur; and guide bank.
square-edged entrance. See inlet, square-edged.
stable channel. See channel, stable.
staff gage. See gage, staff.
stage. Height of water surface above a specified datum. Water surface elevation of a channel with
respect to a reference elevation. The elevation of a water surface above its minimum; also above or
below an established “low-water” plane; hence, above or below any datum of reference; gage height.
The height of a water surface above an established datum plane (36). See also gage height. The depth
of water in a river or stream above the gage datum, or 0.0 level (32).
stage-capacity curve. A graph showing the relation between the surface elevation of the water in a
reservoir, usually plotted as ordinate, against: (1) the volume below that elevation, plotted as abscissa;
or (2) the amount of water flowing in a channel, expressed as volume per unit of time, plotted as
abscissa. Compare with stage-discharge curve and rating curve.
stage-damage graphs. [Charts or graphs that] show the relation between flood depth or elevation and
damages in the area (32).
stage-discharge curve. A graph showing the relation between the gage height, usually plotted as
ordinate, and the amount of water flowing in a channel, expressed as volume per unit of time, plotted
as abscissa (36).
The relation expressed by the stage-discharge curve (36). The relation between gage height (stage)
and the volume of water, per unit of time, flowing in a channel (50).
The relation between stage and the discharge of a stream or river at a particular site, usually presented
in the form of a graph or table. Sometimes referred to as the rating curve of a channel cross section. A
correlation between channel flow rates and corresponding water surface elevations. A rating curve
showing the relation between stage and discharge of a channel.
The relation expressed by the stage-discharge curve. Note that stage-discharge (stage versus
discharge) is hyphenated when referring to a curve or relationship; for a particular event, it is more
appropriate to refer to the discharge stage (not hyphenated). Compare with rating curve and
stage-capacity curve.
stage, flood. See flood stage.
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standard deviation(s). Statistician’s name for an important measure [statistic] of dispersion,
abbreviated s.d. Data grouped closely about their mean have a small s.d.; grouped less closely, they
have a larger s.d. (67).
The standard deviation measures the spread of a probability distribution [i.e., the spread of a sample
of data] (49).
A measure [statistic] of the dispersion or precision of a series of statistical values such as precipitation
or streamflow. It is the square root of the sum of squares of the deviations from the arithmetic mean
divided by the number of values or events in the series. It is now standard practice in statistics to
divide by the number of values minus one to get an unbiased estimate of the variance from the
sample data (33).
standard error. An estimate of the standard deviation of a statistic. Often calculated from a single set
of observations. Calculated like the standard deviation but differing from it in meaning (33).
Two definitions are paraphrased from Reference (49). (1) The standard error of estimate σ or Se is
a measure (statistic) of the vertical (Y axis) variance about a least squares line (a least squares line
describing the relationship between a dependent and independent variable/parameter); (2) The
standard error of the mean is a measure (statistic) of the reliability the mean (x) as measured
by S/(n)1/2.
standard plans. Supplementary drawings of standard construction details that are not unique to any
one project and are included with the project plans for construction.
Standard Project Flood. A totally theoretical (“deterministic” is better term) flood. The magnitude
of the flood is computed by taking the precipitation from the greatest storm in the hydrologic region
and transposing it to the stream basin and hydraulically routing it through the point of interest. It is [a
term] in common usage by USACE. It has no frequency associated with it, but studies indicate that it
is usually approximately half the probable maximum flood and usually has approximately a 200- to
500-year (USACE uses 500- to 1000-year) recurrence interval (16).
A Standard Project Flood (SPF) is a relationship of discharge and/or water surface elevation versus
time and is developed by applying Standard Project Storm (SPS) runoff (SPS rainfall minus observed
infiltration and other losses) to a watershed model. A base flow is usually added and snowmelt runoff
may be added to obtain the total flood hydrograph. Depending on the watershed and river channel
physical characteristics, available data and study needs, the watershed model may be a simple unit
hydrograph or a complex flood routing technique. Then, various techniques are used, ranging from
stage versus discharge curves to step backwater calculations to unsteady flood routing, to convert the
flood discharge to an SPF water surface elevation. The SPF is used as one convenient way to compare
levels of protection between projects, calibrate watershed models, and provide a deterministic check
of statistical flood frequency estimates (16) (29).
Standard Project Storm. The Standard Project Storm (SPS) is a relationship of precipitation versus
time that is intended to be reasonably characteristic of large storms that have occurred or could occur
in the locality of concern. It is developed by studying the major storm events in the region, excluding
the most extreme. For areas east of 105 longitude, the [findings] of SPS studies are published in [the
COE] EM 1110-2-1411 as generalized regional relationships for depth, duration, and area of
Glossary
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precipitation. For areas west of 105 longitude, special studies are made to develop the appropriate
SPS estimates. These special studies may involve transportation and adjustment of a large storm from
its observed location to the locality of concern (29).
standard rain gage. See gage, standard rain.
standing wave. A sudden rise in the water surface, generally fixed in position, such as a hydraulic
jump; a standing wave may exist, however, where the principles of the hydraulic jump are not
involved. More specifically, a term that when used to describe the upper flow regime in alluvial
channels, means a vertical oscillation of the water surface between fixed nodes without appreciable
progression in either an upstream or downstream direction. To maintain the fixed position, the wave
must have a celerity (velocity) equal to the approach velocity in the channel, but in the opposite
direction. See celerity, wave; and Froude number.
static head. See head, static.
station identifier. Information transmitted with data to identify the sending station (32).
statistic. [In computing] a number that describes a sample (such a number, for example, as an
arithmetic mean, a median, a standard deviation, or a coefficient of variation) we call the number a
statistic (70). Compare with parameter, variable, and statistics.
statistics. [The science that] deals with the computation of [a statistic from] sampled data (14).
Compare with statistic and probability.
statute. See statutory law.
statutory law. Law established by a legislative body and set forth in a formal document. In its
specific application, law implies prescription and enforcement by a ruling authority.
statutory liability. Liability by virtue of a legislative act as differentiated from liability based on the
contract of the parties.
steady flow. See flow, steady.
steep slope. See slope, steep.
stemflow. Rainfall or snowmelt led to the ground down the trunks or stems of plants (23). See
interception. Compare with throughfall.
stem, main. See main stem.
stilling basin. A device or structure placed at or near the outlet of a structure for the purpose of
inducing energy dissipation where flow velocities are expected to cause unacceptable channel bed
scour and bank erosion.
stilling well. A pipe, chamber, or some other type of compartment with closed sides and bottom
except for a comparatively small inlet or inlets communicating with a main body of water. Its purpose
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is to dampen waves or surges while permitting the water level within the well to rise and fall with the
major fluctuations of the main body.
stochastic. See hydrology, stochastic. Compare with hydrology, deterministic.
stone riprap. See riprap, stone.
stop-logs. Devices used for temporary closure of an opening in a hydraulic structure. A generic term
that is not intended to imply wood logs are used exclusive of other material, i.e., a log, wood plank,
cut timber, steel, or concrete slab, or beam of some synthetic material fitting into end guides between
walls or piers to close or limit an opening to the passage of water.
storage. “General Introduction and Hydrologic Definitions” (36) suggests two definitions: (1) water
artificially impounded in surface or underground reservoirs, for future use (the term regulation refers
to the action of this storage in modifying streamflow; see also storage, conservation; storage, total;
storage, dead; and storage, usable); (2) water naturally detained in a drainage basin, such as
groundwater, channel storage, and depression storage [where] the term “drainage basin storage” or
simply “basin storage” is sometimes used to refer collectively to the amount of water in natural
storage in a drainage basin.
storage, bank. The water absorbed into the banks of a stream channel when the stages rise above the
water table in the bank formations; then the water returns to the channel as effluent seepage when the
stages fall below the water table (26).
storage, basin. Term sometimes used to refer collectively to the amount of water in natural storage in
a drainage basin. See storage. Compare with surface storage.
storage, channel. The volume of water at a given time in the channel or over the floodplain of the
streams in a drainage basin or river reach. Channel storage is [occurs] during the progress of a flood
event [and thus is transient in nature] (36).
storage, conservation. Storage of water for later release for useful purposes such as municipal water
supply, power, or irrigation in contrast with storage capacity used for flood control (36).
storage, dead. The volume in a reservoir below the lowest controllable level (36) (62).
storage, depression. The volume of water contained in natural depressions in the land surface, such
as puddles (24). Water from precipitation that collects in puddles at the land surface (20). Rainfall
that is temporarily stored in land surface depressions within a watershed.
storage-indication method. Name often given to a flood-routing method. Also often called the Puls
method (after Lows G. Puls), though it is actually a variation of the method devised by Puls (67).
storage ratio. The net available storage divided by the mean flow for one year (62).
storage-required frequency curve. A graph showing the frequency with which storage equal to or
greater than selected amounts will be required to maintain selected rates of regulated flow.
Glossary
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storage, total. The volume of a reservoir below the maximum controllable level, including dead
storage (62).
storage, usable. The volume normally available for release from a reservoir below the stage of the
maximum controllable level (62).
storm. A disturbance of the ordinary average conditions of the atmosphere that, unless specifically
qualified, may include any or all meteorological disturbances, such as wind, rain, snow, hail, or
thunder (36).
storm, base. See flood, base.
storm, catastrophic. See flood (or storm), catastrophic.
storm drain. See drain, storm.
storm duration. The period or length of storm.
stormflow. See runoff, direct (36).
storm hydrograph. See hydrograph, storm.
storm seepage. That part of precipitation that infiltrates the surface soil and moves toward the
streams as ephemeral, shallow, perched groundwater above the main groundwater level. Storm
seepage is usually part of the direct runoff (36).
storm sewer. See sewer, storm.
storm surge. Oceanic tidelike phenomenon resulting from wind and barometric pressure changes.
stormwater. See runoff, urban (1).
stratification. The act or process of becoming layered.
stream. A general term for a body of flowing water. In hydrology the term is generally applied to the
water flowing in a natural channel as distinct from a canal. More generally as in the term “stream
gaging,” it is applied to the water flowing in any channel, natural or artificial (36).
To flow in or as if in a stream. Depending on local or regional usage, a lesser form of a river. A body
of water that may range in size from a large river to a small rill flowing in a channel. See rill. By
extension, the term is sometimes applied to a natural channel or drainage course formed by flowing
water whether it is occupied by water or not. A body of flowing water, whether in an open or closed
conduit, a jet of water as from a nozzle; the term is incorrectly used to designate the conduit in which
the stream flows.
Streams in natural channels may be classified as follows (47) [these and other stream types are
defined in more detail on subsequent pages]:
Relation of a stream (or river) to time:
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Perennial. One that flows continuously, or
Ephemeral. One that flows only in direct response to precipitation and whose channel is at all
times above the water table;
Relation of a stream (or river) to space:
Continuous. One that does not have interruptions in space, or
Interrupted. One that contains alternating reaches, that are either perennial, intermittent, or
ephemeral; and
Relation of a stream (or river) to groundwater:
Gaining. A reach that receives water from the zone of saturation,
Losing. A reach that contributes water to the zone of saturation,
Insulated. A reach that neither contributes water to the zone of saturation nor receives water from
it; it is separated from the zones of saturation by an impermeable bed, or
Perched. A reach that is either a losing or insulated and is separated from the underlying
groundwater by a zone of aeration.
See channel and river. Compare with canal and ditch.
stream, anabranched. A stream or river whose flow is divided at normal and lower stages by large
islands or, more rarely, by large bars; the width of individual islands or bars is greater than
approximately three times the average bankfull water surface width, and the channels are more widely
and distinctly separated than those of a braided stream or river. See anabranch and braid. Compare
with stream, braided; and stream, anastomosing.
stream, anastomosing. Successive division and rejoining (of riverflow) with accompanying islands
is the important characteristic denoted by the synonymous terms, braided or anastomosing stream
(41). Not a common term. A braided stream is composed of anabranches. See anabranch and braid.
Compare with stream, braided; and stream, anabranched.
stream bank. See bank.
stream bank erosion. Removal of soil particles or a mass of particles from a bank surface due
primarily to water action. Other factors such as weathering, ice and debris abrasion, chemical
reactions, and land use changes may also directly or indirectly lead to stream bank erosion.
Compare with mass wasting and sloughing.
stream bank failure. See mass wasting and sloughing.
stream bank protection. Any technique used to prevent erosion or failure of a channel bank. See
and compare with armor, apron, blanket, channel lining, mattress, and revetment.
streambed. See bed.
Glossary
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stream, braided. A stream whose surface is divided at normal stage by small middle bars or small
islands. The individual width of bars and islands is less than three times the water width. A braided
stream has the aspect of a single large channel within which are subordinate channels. The visual
signature of an unstable channel. Braided streams have multiple subordinate channels that are within
the main stream channel. Anabranched streams have more than one channel. See anabranch and
braid. Compare with stream, anabranched; and stream, anastomosing.
stream, continuous. A stream that does not have interruptions in space (36).
stream contraction. A narrowing of a natural channels waterway. Usually in reference to a drainage
facility installed in the roadway embankment.
stream constriction. See stream contraction.
stream, effluent. See stream, gaining.
stream, entrenched. Stream or river cut into bedrock or consolidated deposits.
stream, ephemeral. A stream [or river] that flows only in direct response to precipitation and whose
channel is at all times above the water table (36). A stream or reach of a stream that does not flow
continuously for most of the year. A stream or reach of a stream that does not flow for parts of the
year. As used here, the term includes intermittent streams or rivers whose flow is less than perennial
but more then ephemeral.
stream, flashy. Stream characterized by a rapidly rising and falling stage, as indicated by a sharply
peaked hydrograph. Most flashy streams are ephemeral but some are perennial.
streamflow. The discharge that occurs in a natural channel. Although the term discharge can be
applied to the flow of a canal, the word streamflow uniquely describes the discharge in a surface
stream course. The term “streamflow” is more general than runoff, as streamflow may be applied to
discharge whether or not it is affected by diversion or regulation (36) (50). Compare with discharge
and runoff.
stream flow depletion. The amount of water that flows into a valley or onto a particular land area,
minus the water that flows out the valley or off from the particular land area (8).
streamflow record. A tabulation of the flow of a stream. Streamflow records are published annually
by the United States Geological Survey in their Water-Supply Papers. Such things as daily, monthly,
annual, and instantaneous extremes of discharge are shown therein, along with information about the
stream gage.
stream gage. See gage, stream; and stream-gaging station.
stream gaging. The process and art of measuring the depths, areas, velocities, and rates of flow in
natural or artificial channels (17).
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stream-gaging station. A gaging station where a record of discharge of a stream [or river] is
obtained. Generally, this term is used only for those gaging stations where a continuous record of
discharge is obtained [“General Introduction and Hydrologic Definitions” (36)].
stream, gaining. A stream or reach of a stream that receives water from the zone of saturation (36). A
stream or reach of a stream whose flow is being increased by inflow of groundwater (68). Also
known as an effluent stream (20).
stream, graded. See regime of a stream.
stream, incised. A stream or river that flows in an incised channel with high banks; say, banks that
stand above the 50-year to 100-year water surface stage are arbitrarily regarded as high. Sometimes
termed an entrenched stream or river. See incised channel and incised reach.
stream, influent. See stream, losing.
streaming flow. See flow, subcritical.
stream, insulated. A stream or reach of a stream that neither contributes water to the zone of
saturation nor receives water from it. It is separated from the zones of saturation by an impermeable
bed (36).
stream, intermittent. A stream that flows only at certain times of the year when it receives water
from springs or from some surface source such as melting snow in mountainous areas (36).
stream, interrupted. A stream that contains alternating reaches, that are either perennial,
intermittent, or ephemeral (36).
streamline flow. See flow, laminar.
stream, losing. A stream or reach of a stream that contributes water to the zone of saturation (36). A
stream or reach of a stream in which water flows from the channel bed into the ground (68). A stream
or reach of a stream that is losing water by seepage into the ground; also known as an influent stream.
stream, meandering. See meandering channel.
stream order. A method of numbering [ordering] streams as part of a drainage basin network. The
smallest, unbranched, mapped tributary is called first order, the stream receiving the tributary is called
second order and so on. It is usually necessary to specify the scale of the map used. A first-order
stream on a 1:62,500 map may be a third-order stream on a 1:12,000 map (40).
Tributaries that have no branches are designated as of the first order, streams that receive only
first-order tributaries are of the second order, larger branches that receive only first-order and
second-order tributaries are designated third order and so on. The main stream being always of the
highest order (25).
stream, perched. A perched stream is either a losing stream or an insulated stream that is separated
from the underlying groundwater by a zone of aeration [“General Introduction and Hydrologic
Definitions” (36)].
Glossary
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stream, perennial. A stream or reach of a stream that flows continuously for all or most of the year.
stream, poised. A term used by river engineers as applying to a stream that over a period of time is
neither degrading nor aggrading its channel. A stream nearly in equilibrium as to sediment transport
and supply. See regime theory; regime channel; channel, stable; and regime of a stream.
stream power. An expression used in predicting bed forms and hence bed load transport in alluvial
channels—a parameter comprised of the mean velocity, the specific weight of the water-sediment
mixture, the normal depth of flow, and the channel slope. A parameter that reflects the ability of a
stream to distort its bed to produce such things as bed forms, scour, or deposition.
stream reach. A length of stream channel selected for use in hydraulic or other computations (67).
stream regime. See regime of a stream.
stream response. Changes in the dynamic equilibrium of a stream by any one or combination of
various causes.
stream, seasonal. See stream, intermittent.
stream, stable. See channel, stable.
street. A general term denoting a public way for purposes of vehicular travel, usually including curb
and gutters, to include the entire area within the right-of-way. Compare with road.
structural measures. Methods of reducing damage from floods include [such things as] dams and
reservoirs, levees, dikes, floodwalls, diversion channels, bridge modifications, channel alterations,
pumping stations, and land treatment. [Also] for flood prevention work, any form of earthwork (dam,
ditch, levee, etc.) or installation of concrete, masonry, metal, or other material (drop spillway, jetties,
riprap, etc.); or installation for forest fire protection (firetowers, roads, firebreaks); or, in some cases,
a special planting for nonfarm purposes (stabilization of critical sediment-producing area, etc.) (32).
Structural measures reduce the frequency of damaging overflows. Different types of structural flood
damage reduction measures have different primary and secondary impacts on flooding. Plan
formulation and impact assessment should take into account all impacts and residual flooding from all
sources. (The dominant flooding may be from a different source without and with project conditions.)
In project planning, both the primary beneficial effects and the secondary effects of the alternatives
must be borne in mind and appropriately accommodated. Compare with nonstructural measures.
structural methods. See structural measures.
structures. Such things as bridges, culverts, catch basins, drop inlets, retaining walls, cribbing, access
holes, endwalls, buildings, storm drains, service pipes under drains, foundation drains, and other
appurtenant features.
student’s t distribution . See probability distribution.
sub-base. The layer or layers of specified or selected material of designed thickness placed on a
subgrade to support a base course. Compare with sub-bed material and subgrade.
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sub-bed material. Material underlying that portion of the channel bed that is subject to the direct
action of the flow. Compare with subgrade and sub-base.
subcritical flow. See flow, subcritical.
subgrade. The top surface of a roadbed upon which the pavement structure and shoulders are
constructed. Compare with sub-bed material and sub-base.
sublimate. In the hydrology sense, the loss of moisture from snow before it can infiltrate due to
evaporation and wind.
submeander. Small meander contained within the low-flow banks of a main channel, associated with
relatively low discharges and rills.
submerged inlet. Inlets of culvert like structures having a headwater greater than approximately
1.2 D, where D is the culvert rise. See rise.
submerged orifice. An orifice that in use is drowned by having the tailwater higher than all parts of
the opening. See orifice.
submerged outlet. Submerged outlets are those culvert-like outlets having a tailwater elevation
greater than the soffit of the culvert.
submerged weir. See weir, submerged.
submergence. The ratio of the tailwater elevation to the headwater elevation, when both are higher
than the crest, the overflow crest of the structure being the datum of reference. The distances
upstream or downstream from the crest at which headwater and tailwater elevations are measured are
important, but have not been standardized. In culvert terminology, the condition where tailwater or
headwater elevation are greater than elevation of the conduit top (soffit).
subsection. Part of a cross section; for example, the left floodplain section of a total cross section.
Commonly defined as those contiguous portions of the total cross section having the same
conveyance characteristics.
subsoil. The material lying below the surface soil, generally devoid of humus or organic matter.
See soil.
substitute wetlands. See wetlands, substitute.
substrate. A surface (such as a bed or bank) within surface waters, or beds, banks, or shores that are
periodically covered by surface waters on which a plant or animal may live, grow, or be attached.
Compare with bed.
substructure. All of the structure below the bearings of simple and continuous spans, skewbacks of
arches and tops of footings or rigid frames, together with the backwalls, wingwalls, and wing
protection railings.
Glossary
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subsurface float. A submerged body that is attached by a line to and the movement of which is
indicated by a surface float; used for the purpose of observing velocities or the direction of flow.
subsurface runoff. See runoff, subsurface.
subsurface water. See water, subsurface.
subwatershed. A watershed that is part of a larger watershed [or drainage area]. It is worked on
[analyzed] separately when necessary to improve computational accuracy for results on a whole
watershed basis or to get results for that [sub]area only (67).
sump inlet. See inlet, sump.
sump, wet well. See wet well sump.
superelevation. The increase in water surface elevation at the outside of open-channel bendways.
May also be a transverse tilting of the channel bed (in lined channels with predominately supercritical
flow) or the increase in the elevation at the outside edge of a road or traveled way located in a
horizontal curve.
superflood. Any flood or tidal flow with a flow rate greater than that of the 100-year flood. Flood
used to evaluate the effects of a rare flow event; a flow exceeding the 100-year flood. It is
recommended that the superflood be on the order of the 500-year event or a flood 1.7 times the
magnitude of the 100-year flood if the magnitude of the 500-year flood is not known or predictable
with available hydrology methods. Compare with flood (or storm), review; flood (or storm),
catastrophic; flood, five-hundred-year; flood, one-thousand-year; flood, probable maximum; and
design discharge.
superstructure. The portion of a structure above the substructure. See substructure.
supplemental irrigation. See irrigation, supplemental.
supplemental sources. See irrigation, supplemental sources.
surface area (lake). See lake surface area.
surface curve. See water surface profile.
surface float. A float on a water surface used to indicate velocity or direction of flow.
surface runoff. See runoff, surface.
surface slope. See slope, surface.
surface storage. Natural or constructed roughness of a land surface, which stores some or all of the
surface runoff of a storm. [Such things as] natural depressions, contour furrows, and terraces are
usually considered as producing surface storage, but stock ponds, reservoirs, stream channel storage,
etc., are generally excluded (67).
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Stormwater that is contained in surface depressions or basins. Compare with storage, basin.
surface water course, natural waterway. See channel, natural.
surface water enhancement. Improving existing or new surface water functions and values with
practicable measures.
surface water function. See functions of surface waters.
surface water mitigation. The on-site and/or off-site construction of new surface waters,
enhancement of existing surface waters, acquisition and enhancement of existing surface waters, or
combinations thereof.
surface water quality. The findings from an evaluation of the importance and degree of excellence
of surface water functions, values, and features.
surface waters. Water on the surface of the earth (67). Any stream, river, lake, pond, or reservoir.
Some include wetlands in surface waters.
For regulatory purposes, navigable waters of the United States as currently defined by the USACE.
See navigable waters.
A more legally based description might be, depending on the context, water appearing on the land
surface in a diffused state for a considerable time, with no permanent source of supply or regular
course; as distinguished from water appearing in watercourses, lakes, or ponds. Sometimes
considered as overland flow or surface flow. See flow, overland. Compare with streamflow,
discharge, and runoff.
surface waters, jurisdictional. See navigable waters.
surface waters, rare. Surface waters or wetlands having features, functions, values, or quality that
are uncommon, unique, or seldom occur in the ecoregion. Compare with surface waters, sensitive.
surface waters, sensitive. Those surface waters or wetlands that, by their nature and setting, are
inherently important, unique, or rare due to such things as their environment, public use, and flood
control function. Waters that, without mitigation measures, would be threatened by a highway action.
Compare with surface waters, rare.
surface waters threat. The likelihood that surface waters or wetlands, or a portion thereof, will
be destroyed, degraded, or otherwise adversely impacted, directly or indirectly, through a
highway action.
surface waters, unique. See surface waters, rare.
surface water value. The various essential and nonessential aesthetics, products and services of
sometimes definable value that surface waters provide to society, including such things as fish and
wildlife habitat, water supply, improvement of water quality, flood control, bank erosion and
shoreline protection, outdoor recreation opportunities, education and research, and beauty.
Glossary
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The value, economic or environmental, of a surface water function. Compare with functions of
surface waters.
surficial bed material. The part (30 mm to 60 mm (1.2 in. to 2.4 in.) [in dimension]) of the bed
material that is sampled using U.S. Series Bed Material Samplers (50).
surf zone. See breakers.
suspended. Used in tables of chemical analyses, this term refers to the amount (concentration) of
undissolved material in a water-sediment mixture. It is associated with the material retained on a
0.45-micrometer filter (50).
suspended sediment discharge. See sediment discharge, suspended.
suspended sediment load. See sediment load, suspended.
swale. A wide, shallow ditch usually grassed or paved and without well-defined bed and banks. A
slight depression in the ground surface where water collects and that may be transported as a stream.
Often vegetated and shaped so as not to provide a visual signature of a bank or shore.
synthetic filter. Fabric of synthetic material that serves the same purpose as a granular filter blanket.
synthetic hydrograph. See hydrograph, synthetic.
synthetic mattress, matting, or tubing. A grout, or sand-filled, manufactured, semiflexible casing
placed on a channel bank to prevent erosion. See blanket and mattress. Compare with apron.
synthetic series. A storm or flood series obtained by taking selected values from a frequency line
based on historical data according to the National Engineering Council.
t distribution. See probability distribution.
tailwater. Tailwater (TW) is the depth of flow in the channel directly downstream of a drainage
facility. Often calculated for the discharge flowing in the natural stream without the highway effect
(but may include other local effects from development), unless there is a significant amount of
temporary storage that will be (or is) caused by the highway facility; in which case, a flood routing
analysis may be required. The tailwater is usually used in such things as culvert and storm drain
design and is the depth measured from the downstream flow line of the culvert or storm drain to the
water surface. May also be the depth of flow in a channel directly downstream of a drainage facility
as influenced by the backwater curve from an existing downstream drainage facility. With such things
as releases from a dam, the water just downstream from a structure.
talus. Rock debris collecting at the base of a cliff.
talus slope. Slope (talus slope) formed by an accumulation of rock debris at the base of a cliff.
tank. An artificial reservoir for stock water; local usage generally in the Southwest United
States (36).
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tapered entrance. See inlet, tapered.
temporary snowline. See snowline, temporary.
terrace. A berm or discontinuous segments of a berm, in a valley at some height above the
floodplain, representing a former abandoned floodplain of the stream (36).
test of significance. A [statistical] test made to determine the probability that a result is accidental or
that a result differs from another result. For all the many types of tests there are standard formulas and
tables. In making a test it is necessary to choose a “level of significance,” the choice being arbitrary
but generally not less than the low level of 10 percent nor more than the high level of one
percent. (33).
tetrahedron. Component of river training works made of six steel or concrete struts fabricated in the
shape of a pyramid.
tetrapod. Bank protection component of precast concrete consisting of four legs joined at a central
joint, with each leg making an angle of 109.5 degrees with the other three.
Texas crossing. A low-class road crossing of a channel designed to pass low, frequently occurring
flows through a relatively small culvert type opening with large, more rare floods being conveyed
over the road with little or no road damage; such damage often being precluded by: (1) an erosion
protection blanket on the downstream fillslope of the road (keyed into the floodplain and channel
bottom); and (2) the blanket being monolithic with the shoulder.
thalweg. The line or path (such as a rill) connecting the lowest flow points along the bed of a
channel. The line does not include local depressions. The path very low flows would follow in
proceeding down a stream, river, swale, or channel. The line extending along a channel profile that
follows the lowest elevation of the bed.
thalweg, wandering. A thalweg whose position in the channel shifts during floods and typically
serves as an inset channel (or rill) that transmits all or most of the channel flow at normal or
lower stages.
thermal stratification. Vertical temperature stratification [in a lake or pond] that shows the
following: (1) the upper layer of the lake, known as the epilimnion, in which the water temperature is
virtually uniform; (2) a stratum next below, known as the thermocline, in which there is a marked
drop in temperature per unit of depth; and 3. the lowermost region or stratum, known as the
hypolimnion, in which the temperature from its upper limit to the bottom is nearly uniform (71).
thermocline. A central layer of rapid temperature transition that is located between the epilimnion
and the hypolimnion of a lake or pond. See thermal stratification.
threat, surface waters. See surface waters threat.
threshold value. The value beyond which a significant, adverse effect or impact would probably
occur; generally a term applicable to the quality of surface waters.
Glossary
G-137
throughfall. In a vegetated area, the precipitation that falls directly to the ground or the rainwater or
snowmelt that drops from twigs or leaves (23). See interception. Compare with stemflow.
throughflow. The lateral movement of water in an unsaturated zone during and immediately after a
precipitation event. The water from throughflow seeps out at the base of slopes and then flows across
the ground surface as return flow, ultimately reaching a stream or lake (20).
tidal amplitude. Generally, half of the tidal range.
tidal cycle. One complete rise and fall of the tide.
tidal inlet. A body of water with an opening to the sea, but otherwise enclosed.
tidal passage. A tidal channel connecting with the sea at both ends.
tidal period. Duration of one complete tidal cycle.
tidal prism. Volume of water contained in a tidal inlet or estuary, between low and high tide levels.
tidal range. Vertical difference between specified low and high tide levels.
tides, astronomical. Variations in sea level due to the motion of heavenly bodies.
tieback. Structure connected to such things as a retard, or revetment paralleling a bank, or a blanket
located on a bank. Tiebacks extend into or are otherwise tied to the bank to prevent flanking by
streamflow.
timber or brush mattress. Such things as a revetment, blanket, or armor made of such things as
brush, poles, logs, or lumber interwoven or otherwise lashed together. The completed mattress is then
placed on the bank of a stream or river and weighted with ballast. See blanket and mattress.
time of concentration. The time [Tc] required for water to flow from the farthest point on the
watershed to the gaging station (53). The time that it takes for water to flow from the most distant part
of the drainage basin to the measuring point (20).
The time (Tc) it takes water from the most distant point (hydraulically) to reach a watershed outlet. Tc
varies, but [is] often used as a constant (67).
The estimated time required for runoff to flow from the most remote section of the drainage area to
the point at which the discharge is to be determined. Stated another way, the time it takes water from
the most distant point (hydraulically) to reach a watershed outlet.
Not synonymous with travel time. Compare with travel time.
tipping bucket. See gage, tipping bucket.
toe. That portion of a stream cross section where the lower bank terminates and the channel bottom or
the opposite lower bank begins.
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Highway Drainage Guidelines
toe dike. See dike, toe.
topsoil. Surface soil that is suitable for the germination of seeds and the support of vegetative growth.
torrential flow. See flow, supercritical.
tort. A private or civil wrong committed upon the person or property independent of contract. The
elements of every tort action are: (1) existence of legal duty from defendant to plaintiff; (2) breach of
duty; and (3) damage as proximate result.
tort action. See tort.
tortuous channel. See channel, tortuous.
tortuous flow. See flow, turbulent.
total energy. See energy.
total fish length. The greatest possible length of a fish between the mouth with mouth closed and
compressed (i.e., squeezed together) and the tail fin to give the maximum overall measurement. See
fish; and fish, design.
total head. See head, total.
total head line. See energy grade line.
totally bounded distribution. See probability distribution.
total sediment discharge. See sediment discharge, total.
total storage. See storage, total.
tractive force. The drag on a stream bank caused by passing water that tends to pull soil particles
along with the streamflow. The force or drag developed at the channel bed by flowing water. For
uniform flow, this force is equal to a component of the gravity force acting in a direction parallel to
the channel bed on a unit wetted area. Usually expressed in units of stress, i.e., force per unit area.
The force per unit area on a stationary boundary exerted by a fluid flowing past that boundary.
Compare with critical shear stress.
training dike. See dike, training.
tranquil flow. See flow, subcritical.
transformation. The change of numerical values of data to make later computations easier, to
linearize a plot or to normalize a skewed distribution by making it more nearly a normal distribution.
The most common transformation are those changing ordinary numerical values into their logarithms,
square roots, or cube roots; many others are possible (33).
Glossary
G-139
transition. A short conduit and/or channel uniting two other conduits and/or channels having
different hydraulic elements; a conversion; a variable conduit or channel section connecting one
uniform conduit or channel to another of different cross section form.
transmission loss. A reduction in volume of flow in a stream, canal, [channel] or other waterway, due
to infiltration or seepage into the channel bed and banks. Evaporation is also a transmission loss, but
it is ordinarily neglected under the assumption that it is small (67).
transpiration. The quantity of water absorbed and transpired and used directly in the building of
plant tissue in a specified time. It does not include soil evaporation (8).
The process by which water vapor escapes from the living plant, principally the leaves and enters the
atmosphere. The process by which plants give off water vapor through their leaves (20).
trapezoidal weir. See weir, trapezoidal.
trash rack. A device used to capture debris, either floating, suspended, or rolling and saltating along
the bed, before it enters a drainage facility. A grid or screen across a stream or entrance to a drainage
facility designed to catch debris.
travel time. The average time for water to flow through a reach or other stream or valley length that
is less than the total [stream or valley] length. A travel time is part of a Tc [Time of Concentration] but
never the whole Tc (67).
The average time for water to flow through a reach or other stream or valley length.
Not synonymous with time of concentration. Compare with time of concentration.
traveled way. That portion of the roadway for the movement of vehicles, exclusive of shoulders and
auxiliary lanes (such as turning lanes and parking lanes).
trench-filled revetment. Stone, concrete, or masonry material placed in a trench dug behind and
parallel to an eroding stream bank. When the erosive action of the stream reaches the trench, the
material placed in the trench armors the bank and thus retards further erosion. Compare with windrow
revetment and revetment.
trend. A statistical term referring to the direction or rate of increase or decrease in magnitude of the
individual members of a time series of data when random fluctuations of individual members are
disregarded (36). Compare with probability distribution and cycle.
triangular weir. See weir, triangular.
tributaries. Lesser branches of the watershed stream system. Compare with main stem.
trough. The long, narrow depression between waves.
tsunami. A gravity wave caused by an underwater seismic disturbance (such as sudden faulting,
landsliding, or volcanic activity). Long period, ocean wave resulting from earthquake or other seismic
disturbance. Waves created by earthquakes or other tectonic disturbance on the ocean bottom.
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Highway Drainage Guidelines
Tukey distribution. See probability distribution.
turbidity. Muddy water, having sediment or foreign particles stirred up or suspended. Measured
by the Jackson Turbidity Unit (JTU) or Nephelometric Turbidity Unit (NTU). NTU is current
(1992) practice.
turbulence. Motion of fluids in which local velocities and pressures fluctuate irregularly in a random
manner as opposed to laminar flow where all particles of the fluid move in distinct and separate
streamlines. A state of flow wherein the water is agitated by cross-currents and eddies; opposed to a
condition of flow that is quiet or quiescent.
turbulent flow. See flow, turbulent; and turbulence.
turbulent velocity. That velocity above which, in a particular conduit, turbulent flow will always
exist and below which the flow may be either turbulent or laminar, depending on circumstances.
turnover. See lake turnover.
TW. Acronym for tailwater (depth). [Also an acronym for] actual warning time that is something less
than potential warning time (TWP) (32).
two-dimensional water surface profile. An estimated water surface profile that recognizes flow only
in the upstream-downstream and transverse direction; vertical velocity vector components are
ignored. Compare with one-dimensional water surface profile.
TWP. [Acronym for] The maximum time available for warning local communities prior to impending
flooding (32).
uncontrolled crossing. A bridge crossing that imposes no constraints on the natural width of the
stream or on its ability to shift its channel.
uncontrolled spillway. See spillway, uncontrolled.
underflow. The downstream flow of water through the permeable deposits that underlie a stream and
that are more or less limited by rocks of low permeability. See discharge.
underground watercourse. See watercourse, underground.
undeveloped floodplain. See floodplain, undeveloped.
ungaged stream sites. Locations at which no systematic records are available regarding actual stream
flows or water quality information.
uniform channel. See channel, uniform.
uniform flow. See flow, uniform.
unique surface waters. See surface waters, rare.
Glossary
G-141
unit discharge. See discharge, unit.
unitgraph. See hydrograph, unit.
unit hydrograph. See hydrograph, unit.
unit shear force. See tractive force.
unsaturated zone. See zone, unsaturated.
unsteady flow. See flow, unsteady.
uplift. The upward water-pressure force on the base of a structure.
upper bank. See bank, upper.
urban. As used in this [Glossary] this term includes suburban areas (1).
urban runoff. See runoff, urban.
usable storage. See storage, usable.
USACE. Acronym for U.S. Army Corp of Engineers.
USFS. Acronym for U.S. Forest Service.
USGS. [Acronym for] United States Department of the Interior, Geological Survey (67).
USWB. [Acronym for] United States Department of Commerce, Weather Bureau. [Weather Bureau
changed to National Oceanic and Atmospheric Administration, National Weather Service (see
NWS) (67)].
vadose zone. See zone, unsaturated (68).
value of surface waters. See surface water value.
variable. A quantity susceptible of fluctuating in value or magnitude under different conditions
according to the Reader’s Digest Great Encyclopedic Dictionary. Given an equation or relation
involving two or more variables (X, Y, Z . . . n), the variable(s) that are assigned values
{y = f(x, z . . . n)—in this case x, z . . . n} are called independent variables and the variable that takes
on values (in this case y) is called the dependent variable. Compare with parameter.
variable, dependent, or independent. See variable.
variance. A [statistical] measure of the amount of spread or dispersion of a set of values around their
mean, obtained by calculation of the mean value of the squares of the deviations from the mean and
hence equal to the square of the standard deviation (33).
varied flow. See flow, unsteady.
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Highway Drainage Guidelines
vegetation. Woody or nonwoody plants. Such vegetation is commonly used to stabilize a channel
bank, floodplains, and other ground areas exposed during highway construction to retard erosion.
velocity. The rate of motion of a stream or river or of the objects or particles transported therein,
usually expressed in distance per time. Rate of travel; distance per unit of time.
velocity, average. Velocity at a given cross section determined by dividing the total discharge at that
point by the total cross section area.
velocity, cross section average. See velocity, average.
velocity, fall. The velocity of a particle falling alone in quiescent, distilled water of infinite extent.
velocity head. See head, velocity.
velocity head coefficient. (α) A correction factor, ґα (alpha) applied to the velocity head for the mean
velocity, to correct for non-uniformity of velocity in a cross section. The factor is 1.0 where velocities
are identical across a section and greater than 1.0 where velocities vary across a section. Regular
channels have coefficients as low as 1.10 whereas overflooded river valleys may have coefficients as
high as 2 (13). Compare with momentum coefficient, β (beta).
velocity, local average. Local discharge intensity divided by depth of flow.
velocity, mean. See velocity, average.
velocity of approach. The mean velocity in the conduit or channel immediately upstream from a
weir, dam, Venturi throat orifice, or other structure; the mean velocity in the approach section.
velocity, permissible. The highest velocity at which water may be carried safely in a canal or other
conduit without channel bed scour or bank erosion.
velocity-weighted sediment concentration. The dry weight of sediment discharged through a cross
section during a unit of time.
vena contracta. The most contracted section area of a stream, jet, or nappe beyond the plane of the
constriction through which it issues.
Venturi flume. See flume, Venturi.
verification. The process of testing a model to an observed set of data using the model parameters
derived during calibration. Model calibration and verification must be performed on separate sets
of data (1).
vertical abutment. An abutment, usually with wingwalls, that has no fill slope on its channel side.
See abutment. Compare with spillthrough abutment.
vertical-velocity curve. A graph of the relation between depth and velocity along a vertical line in a
stream, as determined by a set of observations or estimated by computations.
Glossary
G-143
vice. “Instead of,” or “in lieu of.”
viscous flow. See flow, laminar.
vistas. The passage framing the approach to such things as a scene, scenic landscape, or surface
waterscape.
V-notch weir. See weir, triangular.
wandering channel. See channel, wandering.
wandering thalweg. See thalweg, wandering.
wash. Term of local geographic usage. See channel.
wash load. That part of the total sediment discharge that is composed of particle sizes finer than those
found in appreciable quantities in the bed material. Large quantities of fine materials that could be
carried easily by streamflow. That part of the total sediment load that is composed of particle sizes
finer than those represented in the bed. Normally the fine-sediment load is finer than 0.062 mm
(0.0024 in.) for a sand bed channel. Silts, clays, and sand could be considered as wash load in coarse
gravel and cobble bed channels. Wash load sediments commonly originate on uplands and are carried
into a channel by overland flows. Compare with bed load; sediment load, suspended; sediment
discharge, total; and sediment discharge, suspended.
water balance. See hydrologic budget.
water budget. An evaluation of all the sources of supply and the corresponding discharges with
respect to an aquifer or a drainage basin (20). See hydrologic budget.
water content. The amount of water lost from the soil after drying it to constant weight at 105°C
(221°F), expressed either as the weight of water per unit weight of dry soil or as the volume of water
per unit bulk volume of soil (68).
The ratio of the volume of soil moisture to the total volume of the soil. This is the volumetric water
content; also called volume wetness (20).
water content of snow. See water equivalent of snow.
watercourse. A stream, river, or channel in which a flow of water occurs, either continuously or
intermittently, with some degree of regularity. See channel, river, and stream.
watercourse, natural. See channel, natural.
watercourse, underground. A geological formation that contains water flowing in a known and
defined channel. A water right for water in underground watercourses are in most States similar to the
water right for water in natural surface watercourses.
water crop. See runoff, annual.
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Highway Drainage Guidelines
water cushion. A pool of water maintained to take the impact of water overflowing a dam, chute,
drop, or other spillway structure.
Water Drainage Rights. The right that a land owner has, under the law, to dispose of excess or
surplus water that accumulates upon his land, over the lands of others. Such rights are of several
classes—see Common Enemy Doctrine or Rule; Civil Law Doctrine or Rule; Natural Drainage
Doctrine or Rule; and Reasonable Use Doctrine or Rule.
Not to be confused with water right. Compare with water right and flood right.
water equivalent of snow. Amount of water that would be obtained if the snow should be completely
melted. Water content may be merely the amount of liquid water in the snow at the time of
observation (36).
The depth of water obtained by melting a given thickness of snow (20). The depth of water, in
millimeters [inches], that results from melting a given depth of snow (67).
water law. See doctrine, rule, Riparian Doctrine or Rule, water right, flood right, common law, civil
law, Common Enemy Doctrine or Rule, Civil Law Doctrine or Rule, Natural Drainage Doctrine or
Rule, Reasonable Use Doctrine or Rule, rule of law, and Water Drainage Rights.
water level. A water surface; also, its elevation above any datum; gage height; stage.
water-level (stage) recorder. A device for producing a graphic record of the rise and fall of a water
surface with respect to time. Compare with gage, crest-stage.
water loss. The difference between the average precipitation over a drainage basin and the water
yield from the basin for a given period. The basic concept is that water loss is equal to
evapotranspiration, that is water that returns to the atmosphere and thus is no longer available for use.
However, the term is also applied to differences between measured inflow and outflow even where
part of the difference may be seepage (36).
Variable meaning, depending on personal interest of water users. Farmers and ranchers usually think
of flood runoff as a water loss; many river engineers think of infiltration as a water loss (36).
Compare with hydrology, loss.
water requirement. The quantity of water, regardless of its source, required by a crop in a given
period of time, for its normal growth under field conditions. It includes surface evaporation and other
economically unavoidable wastes (8).
water, return. See flow, return.
water right. An adjudication of waters (usually by a State agency) to a specified user for a beneficial
purpose. See beneficial use. Compare with flood right.
watershed. See drainage area.
Glossary
G-145
watershed measures. Any vegetative or structural means (including earthwork) of directly
improving or conserving the soil and water resources of a watershed. See land treatment measure and
structural measures (67).
Waters of the United States. See navigable waters.
water, subsurface. All water that occurs below the land surface (68).
Water Supply Paper (WSP). An annual publication of the USGS, in which streamflow for the water
year is given for all gaged streams in a geographical region of the United States.
water surface profile. A graph of water levels plotted against stream distance at a particular time or
for a particular condition, such as for a flood peak or for a low-flow period. Two other definitions are
appropriate: (1) the longitudinal profile assumed by the surface of a stream of water flowing in an
open conduit (the surface curve of a stream of water is the curve of equilibrium of all forces acting on
the flowing water); (2) the hydraulic grade line. See hydraulic grade line. Water-surface curves or
profiles are generally catalogued into twelve classifications, three of which are designated strictly as
backwater curves. The classifications are accounted for by the different bottom slopes and relative
values of normal and critical depth. The curves are classified by the nomenclature; M1, M2, and
M3 for mild slope (backwater curves); C1 and C3 for critical slope; H2 and H3 for horizontal
(zero slope); S1, S2, and S3 for steep slope; and A2 and A3 for adverse slope. Compare with
backwater curve.
water table. The upper surface of a zone of saturation. No water table exists where that surface is
formed by an impermeable body (36) (46).
The upper surface of a zone of saturation except where that surface is formed by a confining unit. The
upper surface of the zone of saturation on which the water pressure in the porous medium equals
atmospheric pressure. Means that surface in a groundwater body at which the water pressure is
atmospheric. Upper surface of a zone of saturation, where the body of groundwater is not confined by
an overlying impermeable zone.
The surface in an unconfined aquifer or confining bed at which the pore water pressure is
atmospheric. It can be measured by installing shallow wells extending a few meters [feet] into the
zone of saturation and then measuring the water level in those wells (20).
The upper surface of groundwater (67).
The upper surface of a zone of saturation in soil or in permeable strata or beds. The upper surface of
the zone of saturation, except where that surface is formed by an impermeable body. Compare with
groundwater, perched.
waterway. Any stream, river, lake, pond, or ocean that can be traversed for purposes of commerce or
recreation. May also refer to a channel. See channel and watercourse.
waterway opening. See waterway opening area.
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Highway Drainage Guidelines
waterway opening area. Area of bridge opening at (below) a specified stage, measured normal to
principal direction of flow.
waterway opening depth. Depth corresponding to a waterway opening area. Requires clarification in
some cases as to whether this is the average depth, maximum depth, minimum depth, normal depth,
or some other measure of depth.
waterway opening width. Width corresponding to a waterway opening area. Requires clarification
in some cases as to whether this is a top width, bottom width, average width, or some other measure
of width.
water year. In the Federal agency reports dealing with surface-water supply, the 12-month period,
October 1 through September 30. The water year is designated by the calendar year in which it ends
and that includes 9 of the 12 months. Thus, the year ended September 30, 1959, is called the “1959
water year” (36).
The year taken as beginning October 1. Often used for a convenience in streamflow work, because in
many areas streamflow is at its lowest at that time. Used by the U.S. Geological Survey in their WSP
National Engineering Handbook (67).
water yield. See runoff, annual.
wave attack. Impact of waves on a channel bank or shore.
wave celerity. See celerity, wave.
wave frequency. Wave frequency, n, is the number of waves passing a point in the liquid per unit
time (14).
wave period. Time period between arrivals of successive wave crests at a point.
wave run-up. Height to which water rises above still-water level when waves meet such things as a
beach, wall, embankment, or causeway. Compare with wave set-up and wind set-up.
wave set-up. The creation of waves from wind. Compare with wind set-up, wind set-down, and
wave run-up.
wave, shallow water. Water of such a depth that waves are noticeably affected by bottom conditions;
customarily, water shallower than half the wavelength.
wave, significant. A statistical finding for denoting surface water waves with the average height and
period of the one-third highest wave of a given wave group.
weephole. An opening left in such things as an impermeable wall, revetment, apron, lining, or
foundation, to relieve the neutral stress or pore water pressure and permit drainage.
Weibull distribution. See probability distribution.
Glossary
G-147
weighted mean. [Statistical] value [or statistic] obtained by multiplying each of a series of values by
its assigned weight and dividing the sum of those products by the sum of the weights (33).
weir. A dam across a channel for diverting flows or for measuring the flow.
weir, broad-crested. An overflow structure on which the nappe is supported for an appreciable
length; a weir with a significant dimension in the direction of the stream. Highways generally
function as broad-crested weirs when overtopped by floodwaters.
weir, Cipolletti. A contracted measuring weir, in which each side of the notch has a slope of one
horizontal to four vertical, to compensate for end contractions; named after Caesar Cipolletti, an
Italian engineer.
weir flow. Free surface flow over a control surface that has a defined discharge versus depth
relationship.
weir, measuring. A device for measuring the flow of water. It generally consists of rectangular,
trapezoidal, triangular, or other shaped notch in a thin plate in a vertical plane through which the
water flows. The weir head is an index of the rate of flow. Unless a suppressed weir is specified
the term may be taken to mean a contracted weir. See weir, Cipolletti; weir, rectangular; and
weir, triangular.
weir, rectangular. A contracted measuring weir with a rectangular notch.
weir, sharp-crested. A contracted measuring weir with its crest at the upstream edge or corner of a
relatively thin plate, generally of metal.
weir, submerged. A weir that in use has the tailwater level equal to, or higher than the weir crest. See
weir, measuring.
weir, trapezoidal. A contracted measuring weir with a trapezoidal notch. See weir, Cipolletti.
weir, triangular. A contracted measuring weir notch with sides that form an angle with its apex
downward; the crest is the apex of the angle; a V-notch weir.
wells. Shallow to deep vertical excavations, generally with perforated or slotted pipe backfilled with
selected aggregate. The bottom of the excavation terminates in pervious strata below the water table.
wetland acquisition. As used in the National Wetlands Priority Conservation Plan, any purchase of
complete or partial interest in a wetland site obtained with total or partial government funding.
wetland bank. That document maintained to reflect wetland credit that may be available for use in
the mitigation of a wetland impacted by a future highway action.
wetland banking. Although not required at the time of a highway action, the process of creating,
restoring, or enhancing a wetland in conjunction with the highway action to compensate for
unavoidable and at the time unknown wetland impacts from some future highway action and then
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obtaining approval from the cognizant resource and regulatory agencies for this work as a wetland
credit to be entered into the wetland bank.
wetland credit. The credited value in the wetland bank for substitute wetlands in a particular
geographic area and biologic region as approved by the cognizant resource and regulatory agencies.
wetland loss index. Measure of loss of a wetland type within an ecoregion expressed by the equation:
(Y
− X ) x 100
N
(Unit
( Net
x
Loss x 100 )
(Y
National Loss ))
− X ) x 100
Y
x
(Unit
Loss x 100 )
1954 Unit Base
where:
Y
X
Y–X
N
Unit
Base
=
=
=
=
=
=
1954 Unit Base km2 (mi2) per wetland type and unit area;
1974 Remaining km2 (mi2) per wetland type and unit area;
Unit Loss (e.g., 1954–74 State loss per wetland type);
1954–74 Net National Loss per wetland type;
Area of comparison (e.g., ecoregion, State); and
km2 (mi2) of wetlands in 1954 for the unit.
wetland losses, historic. The losses of wetlands from a particular site or loss of a specific type
of wetlands within a region from the time of European settlement of the United States through
the present.
wetlands. Those lands having wetland hydrology, hydric soils, and hydrophyte-type vegetation as
delineated by current editions of the Federal Manual for Identifying and Delineating Jurisdictional
Wetlands. These include wetlands subject to Federal law regardless of whether they involve Federal,
State, or private lands.
More generally, an area that is inundated or saturated by surface waters or groundwater at a frequency
and duration sufficient to support and, under normal circumstances, does support a prevalence of
vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps,
marshes, bogs, and similar areas. Mudflats, sand flats, rocky shores, gravel beaches, and sand bars,
although they often do not support vegetation, can also be considered wetlands. Wetlands typically
have hydric soils, phreatic vegetation, and wetland hydrology.
Some regulatory agencies may prefer land that has a predominance of hydric soils that is inundated or
saturated by surface or groundwater at a frequency and duration sufficient to support “and that under
normal circumstances does support” a prevalence of hydrophytic vegetation typically adapted for life
in saturated soil conditions.
wetlands, jurisdictional. See navigable waters.
Glossary
G-149
wetlands, replacement. Replacement wetlands provided through mitigation methods to replace
wetlands to be destroyed or adversely impacted on a specific highway project. Not synonymous with
wetlands, substitute. Compare with wetlands, substitute.
wetlands, substitute. Wetlands that were created or acquired and developed (enhanced) in perpetuity
through application of wetland mitigation methods on previously constructed highway projects. Not
synonymous with wetlands, replacement. Compare with wetlands, replacement.
wetted perimeter. The boundary over which water flows in a channel, stream, river, swale, or
drainage facility such as a culvert or storm drain. The boundary is taken normal to the flow direction
of the discharge in question. The length of the wetted contact between a stream of water and its
containing conduit, measured along a plane at right angles to the flow in question; that part of the
periphery of the cross section area of a stream in contact with its container. See hydraulic radius.
wet well sump. The feature in a pump station in which runoff waters are temporarily stored.
width constriction. A constriction whereby the flow is contracted in the horizontal plane only; no
bottom contraction; a bridge with approach embankments across a floodplain is an example of a
width constriction.
wildlife. Living things that are neither human nor domesticated; mammals, birds, and fishes [that may
be] hunted by man according to the Webster’s New Collegiate Dictionary. Compare with livestock
and aquatic life.
windrow revetment. A row of stone (called a windrow) placed on top of the bank landward of an
eroding stream bank. As bank erosion and mass wasting continues the windrow is eventually
undercut, launching the stone downslope, thus armoring the bank face. Compare with trench-filled
revetment and revetment.
wind set-down. Lowering of the level of surface waters due to wind action. Corresponding fall in
level at the windward side of a surface water body due to wind stresses on the surface. Compare with
wind set-up.
wind set-up. Raising of the level of surface waters due to wind action. Rise in level at the leeward
side of a surface water body due to wind stresses on the surface. Compare with wind set-down, wave
run-up, and wave set-up.
wire-enclosed riprap. See riprap, wire-enclosed.
wire mesh. Wire woven to form a mesh, the openings of which are of a suitable size, shape and
durability to serve as an enclosure for such things as rock, rubble, or broken concrete used as a
mattress, gabion, or wire enclosed riprap. Also used as fencing on riparian spur dikes and retards.
withdrawal use of water. The water removed from the ground or diverted from a stream or lake
for use (44).
WSP. Acronym for Water Supply Paper. See Water Supply Paper or water surface profile.
G-150
Highway Drainage Guidelines
X-percent chance flood. A flood magnitude that has X chances in 100 of being exceeded in any
one-year period. The occurrence of floods is assumed to be random in time; no schedule or regularity
of occurrence is implied. The exceedance of a one percent chance flood is no guarantee, therefore,
that a similar size flood will not occur next week or next year. Over time periods longer than one
year, the risks of experiencing large floods increase in a nonadditive fashion. For example, the risk of
exceeding a one percent chance flood one or more times during a three-year period is 25 percent and
during a 70-year period is 50 percent (30). See probability distribution. Compare with flood,
one-hundred year; flood, one-thousand year; and flood-frequency.
year. See climatic year and water year (36).
zone, coastal. See coastal zone.
zone, flood. See flood zone.
zone, limnetic. See limnetic zone.
zone, littoral. See littoral zone.
zone of aeration. The zone above the water table. Water in the zone of aeration does not flow into a
well (36).
zone of breakup. See breakers.
zone of saturation. The zone in which the functional permeable rocks are saturated with water under
hydrostatic pressure (46). Water in the zone of saturation will flow into a well and is called
groundwater (36). The Groundwater Subcommittee offers three definitions: (1) those parts of the
earth’s crust in which all voids are filled with water under pressure greater than atmospheric; (2) that
part of the earth’s crust beneath the regional water table in which all voids, large and small, are filled
with water under pressure greater than atmospheric; (3) that part of the earth’s crust beneath the
regional water table in which all voids, large and small, are ideally filled with water under pressure
greater than atmospheric.
The zone in which the voids in the rock or soil are filled with water at a pressure greater than
atmospheric. The water table is the top of the saturated zone in an unconfined aquifer (20).
zone, profundal. See profundal zone.
zone, recovery. See recovery zone.
zone, unsaturated. The Groundwater Subcommittee offers five definitions: (1) the zone between the
land surface and the water table; (2) the zone between the land surface and the deepest water table
that includes the capillary fringe (generally water in this zone is under less than atmospheric pressure
and some of the voids may contain air or other gases at atmospheric pressure; also, beneath flooded
areas or in perched water bodies, the water pressure locally may be greater than atmospheric); (3) the
zone between the land surface and the regional water table (generally, water in this zone is under less
than atmospheric pressure and some of the voids may contain air or other gases at atmospheric
pressure; also, beneath flooded areas or in perched water bodies, the water pressure locally may be
Glossary
G-151
greater than atmospheric); (4) the zone between the land surface and the regional water table
(generally, fluid pressure in this zone is less than atmospheric pressure and some of the voids may
contain air or other gases at atmospheric pressure; also, beneath flooded areas or in perched water
bodies the fluid pressure locally may be greater than atmospheric); (5) the zone between the land
surface and the water table (generally, water in this zone is under less than atmospheric pressure and
some of the voids may contain air and other gases at atmospheric pressure; also, beneath flooded
areas or in perched water bodies, the water pressure locally may be greater than atmospheric).
The zone between the land surface and the water table. It includes the root zone, intermediate zone,
and capillary fringe. The pore spaces contain water at less than atmospheric pressure, and air and
other gases. Saturated bodies, such as perched groundwater, may exist in the unsaturated zone (20).
G.3 REFERENCES
References include two types:
cited, and
uncited.
G.3.1 Cited References
(1)
Alley, W. M., ed. Guide for Collection, Analysis and Use of Urban Stormwater Data.
Conference Report presented November 28–December 3, 1976 at Easton, Maryland.
Cosponsored by the Urban Water Resources Research Council, American Society of Civil
Engineers, 115 p., 1976.
(2)
ASCE. Hydrology Handbook. Manuals Eng. Practice, No. 28, American Society of Civil
Engineers, 184 p., 1949.
(3)
ASCE. Report of the Subcommittee on the Joint Division Committee on Floods, Vol. 118.
American Society of Civil Engineers, p. 1220–1230, 1953.
(4)
ASCE. Nomenclature for Hydraulics. Manuals and Reports on Engineering Practice, No. 43,
American Society of Civil Engineers, 1962.
(5)
Barnes, H. T. Ice Engineering. Renouf Publishing Co., Montreal, Canada, 364 p., 1928.
(6)
Barrows, H. K. Floods, Their Hydrology and Control. McGraw-Hill Book Co., New York,
791 p., 1943.
(7)
Bergsten, Folke. The Seiches of Lake Vetter, Centraltryckeriet. Stockholm, Sweden,
72 p., 1926.
(8)
Blaney, H. F. Consumptive Use of Water. Proceedings, Vol. 77, American Society of Civil
Engineers, 19 p., 1951.
G-152
Highway Drainage Guidelines
(9)
Blaney, H. F. Use of Water by Irrigated Crops in California. American Water Works Journal,
Vol. 43, No. 3, p. 189–200, 1951.
(10)
Blench, Thomas. Regime of Behavior of Canals and Rivers, London, England and Toronto,
Canada. Butterworths Sci. Pub., 138 p., 1957.
(11)
Bryan, Kirk. Erosion and Sedimentation in the Papago Country, Arizona, with a Sketch of the
Geology. U.S. Geol. Survey Bulletin 730-B, p. 19–90, 1922.
(12)
Carter, R. W., and Godfrey, R. G. Storage and Flood Routing. U.S. Geol. Survey
Water-Supply Paper 1543-B, p. 81–104.
(13)
Chow, V. T. Open Channel Hydraulics. McGraw-Hill Book Company, New York, 1959.
(14)
Chow, V. T. Handbook of Applied Hydrology. McGraw-Hill Book Company, New York, 1964.
(15)
Colby, B. R., Hembree, C. H., and Jochens, E. R. Chemical Quality of Water and
Sedimentation in the Moreau River Drainage Basin, South Dakota. U.S. Geol. Survey Circ.
270, 53 p., 1953.
(16)
Colson, B. E. Personnel Communication to the Advisory Committee on Water Data—For
Discussion Only. 1989.
(17)
Corbett, D. M., et al. Stream-Gaging Procedure, a Manual Describing Methods and Practices
of the Geological Survey. U.S. Geol. Survey Water-Supply Paper 888, 243 p., 1943.
(18)
Dalrymple, Tate. Flood-Frequency Analysis. U.S. Geol. Survey Water-Supply Paper 1543-A,
p. 1–79, 1960.
(19)
Erbe, N. A., and Flores, D. T. Iowa Drainage Laws (annot.). Iowa Highway Research Board,
Bull. 6, 870 p., 1957.
(20)
Fetter, C. W., Jr. Applied Hydrology. Charles E. Merrill Publishing Co., Columbus, Ohio,
488 p., 1980.
(21)
FHWA. Drainage of Highway Pavements. Hydraulic Engineering Circular No. 12, TS-84-202,
Federal Highway Administration, U.S. Department of Transportation, Washington, DC, 1984.
(22)
Harrold, L. L. and F. R. Dreibelbis. Agricultural Hydrology as Evaluated by Monolith
Lysimeters. USDA Tech. Bull. 1050. U.S. Department of Agriculture, Washington, DC,
149 p., 1951.
(23)
Hoover, M. D. Interception of Rainfall in a Young Loblolly Pine Plantation. U.S. Forest
Service, Southeastern Forest Expt. Sta. Paper 21, 13 p., 1953.
(24)
Horton, R. E. Surface Runoff Phenomena, Part 1, Analysis of the Hydrograph. Vorheesville,
N.Y., Horton Hydrol. Lab. Pub. 101, 73 p., 1935.
Glossary
G-153
(25)
Horton, R. E. Remarks on Hydrologic Terminology. Am. Geophys. Union Trans., Vol. 23,
Part 1, p. 479–482, 1942.
(26)
Houk, Ivan. Irrigation Engineering. John Wiley & Sons, New York, 545 p., 1951.
(27)
Hoyt, W. G., and Langbein, W. B. Floods. Princeton Univ. Press, Princeton, N.J., 469 p., 1955.
(28)
Hoyt, W. G., et al. Studies of Relations of Rainfall and Runoff in the United States. U.S. Geol.
Survey Water-Supply Paper 772, 30 p., 1936.
(29)
Huffman, R. E. Irrigation Development and Public Water Policy. Ronald Press Co., New
York, 336 p., 1953.
(30)
Huffman, R. G. Personnel Communication to the Hydrology Subcommittee of the Federal
Interagency Advisory Committee on Water Data. 1990.
(31)
Hutchinson, G. Evelyn. A Treatise on Limnology, Volume 1, Geography, Physics and
Chemistry. John Wiley & Sons, New York, 1016 p., 1957.
(32)
Hydrology Subcommittee of the Federal Interagency Advisory Committee on Water Data.
Guidelines on Community Local Flood Warning and Response Systems. U.S. Geological
Survey, Office of Water Data Coordination, 104 p., 1985.
(33)
Interagency Advisory Committee on Water Data. Guidelines for Determining Flood Flow
Frequency. Bulletin 17B of the Hydrology Subcommittee, U.S. Geology Survey, Office of
Water Data Coordination, 183 p., 1982.
(34)
Jarvis, C. S., et al. Floods in the United States, Magnitude and Frequency. U.S. Geol. Survey
Water-Supply Paper 771, 497 p., 1936.
(35)
Langbein, W. B., and Hardison, C. H. Extending Streamflow Data. Proceedings, Vol. 81, No.
826, American Society of Civil Engineers, p. 826-1 to 826-13., 1955.
(36)
Langbein, W. B., and Iseri, K. T. General Introduction and Hydrologic Definitions. Manual of
Hydrology: Part 1. General Surface-Water Techniques. U.S. Geological Survey Water Supply
Paper 1541-A, 29 p., 1960.
(37)
Langbein, W. B., et al. Major Winter and Nonwinter Floods in Selected Basins in New York
and Pennsylvania. U.S. Geol. Survey Water-Supply Paper 915, 139 p., 1947.
(38)
Lee, C. H. Transpiration and Total Evaporation in Hydrology. In Part 9 of Physics of the Earth.
Meinzer, O. E., ed., Dover Pubs., New York, p. 259–330, 1949.
(39)
Leopold, L. B., and Maddock, Thomas, Jr. The Flood Control Controversy. Ronald Press Co.,
New York, 278 p., 1954.
(40)
Leopold, L. B., and Miller, J. P. Ephemeral Streams—Hydraulic Factors and Their Relation to
the Drainage Net. U.S. Geol. Survey Prof. Paper 282-A p. 1–37, 1956.
G-154
Highway Drainage Guidelines
(41)
Leopold, L. B., and Wolman, M. G. River Channel Patterns—Braided, Meandering and
Straight. U.S. Geol. Survey Prof. Paper 282-B, p. 39–85, 1957.
(42)
Linsley, R. K., Jr. River Forecasting Methods. U.S. Weather Bur., Washington, 100 p., 1942.
(43)
Linsley, R. K., Jr., Kohler, M. A., and Paulhus, J. L. H. Applied Hydrology. McGraw-Hill
Book Co., New York, 689 p., 1949.
(44)
MacKichan, K. A. Estimated Use of Water in the United States, 1955. U.S. Geol. Survey
Circ. 398, 18 p., 1957.
(45)
Matthes, F. E. Glaciers. In Hydrology, Part 9 of Physics of the Earth. Meinzer, O. E., ed.,
Dover Pub., New York, p. 149–219, 1949.
(46)
Meinzer, O. E. Outline of Ground-Water Hydrology, with Definitions. U.S. Geol. Survey
Water-Supply Paper 494, 70 p., 1923.
(47)
Meinzer, O. E. Hydrology. Part 9 of Physics of the Earth. Dover Pub., New York, 712 p.,
1949.
(48)
Meyer, A. F. Elements of Hydrology, 2d ed. revised. John Wiley & Sons, New York,
522 p., 1928.
(49)
Miller, I., and Freund, J. E. Probability and Statistics for Engineers. Prentice-Hall, Inc., 1965.
(50)
Novak, C. E. WRD Data Reports Preparation Guide, 1985 Edition. U.S. Geological Survey,
321 p., 1985.
(51)
Paulsen, C. G. Overall Trends. American Water Works Association Journal, Vol. 42, No. 9,
p. 800–804, 1950.
(52)
Pillsbury, A. F., Compton, O. C., and Picker, W. E. Irrigation-Water Requirements of Citrus in
the South Coastal Basin of California. Univ. of Calif., Berkely, Agr. Expt. Sta. Bull. 686,
19 p., 1944.
(53)
Ramser, C. E. Runoff from Small Agricultural Areas. Agriculture Research Journal, Vol. 34,
No. 9, p. 797–823, 1927.
(54)
Rouse, Hunter. Engineering Hydraulics. John Wiley and Sons, New York, London, Third
Printing, 1961.
(55)
Schaefer, V. J. The Formation of Frazil and Anchor Ice in Cold Water. Am. Geophys. Union
Trans., Vol.31, No. 6, p. 885–893, 1950.
(56)
Searcy, J. K. Flow-Duration Curves. U.S. Geol. Survey Water-Supply Paper 1542-A,
p. 1–33, 1959.
(57)
Searcy, J. K. Graphical Correlation of Gaging-Station Records. U.S. Geol. Survey
Water-Supply Paper 1541-C, p. 67–100, 1960.
Glossary
G-155
(58)
Searcy, J. K., and Hardison, C. H. Double-Mass Curves. U.S. Geol. Survey Water-Supply
Paper 1541-B, p. 31–66, 1960.
(59)
Sherman, L. K. The Unit Hydrograph Method. In Hydrology, Part 9 of Physics of the Earth.
Meinzer, O.E., ed., Dover Pub., New York, p. 514–525, 1949.
(60)
Simons, W. D. Irrigation and Streamflow Depletion in Columbia River Basin Above the
Dalles, Oreg. U.S. Geol. Survey Water-Supply Paper 1220, 126 p., 1953.
(61)
Thomas, H. E. The Conservation of Groundwater. McGraw-Hill Book Co., New York,
327 p., 1951.
(62)
Thomas, N. O., and Harbeck, G. E. Reservoirs in the United States. U.S. Geol. Survey
Water-Supply Paper 1360-A, p. 1–99, 1956.
(63)
Thornthwaite, C. W. Report of Committee on Transpiration and Evaporation. In Am. Geophys.
Union Trans., Vol. 25, Part 5, p. 683–693, 1944.
(64)
Troxell, H. E., et al. Hydrology of the San Bernardino and Eastern San Gabriel Mountains,
Calif. U.S. Geol. Survey. Hydrol. Inv. Atlas HA-1, 1954.
(65)
U.S. Bureau of Reclamation. Use of Water on Federal Irrigation Projects. U.S. Bureau of
Reclamation, Denver, Colo., 79 p., August 1952.
(66)
U.S. Department of Agriculture Soil Conserv. Service and Nevada State Engineer.
Federal-State Cooperative Snow Surveys—Summary for Nevada, 1910–1948. U.S. Dept.
Agriculture, Soil Conserv. Service and Nevada State Engineer, Reno, Nev., 88 p., 1948
(67)
USGS. National Engineering Handbook, Section 4 Hydrology, Chapter 22, Glossary. U.S.
Geological Survey, Office of Water Data Coordination, 1956, Reprinted with minor
revisions, 1971.
(68)
USGS. Groundwater Subcommittee of the Federal Interagency Advisory Committee on Water
Data, Federal Glossary of Selected Terms: Subsurface-Water Flow and Solute Transport. U.S.
Geological Survey, Office of Water Data Coordination, 38 p., 1989.
(69)
Viessman, Jr., W., G. L. Lewis, and J. W. Knapp. Introduction to Hydrology. Harper and Row,
New York, 1989.
(70)
Waugh, A. E. Elements of Statistical Method, 3d ed. McGraw-Hill Book Company, Inc., New
York, 1952.
(71)
Welch, P. S. Limnology, 2d ed. McGraw-Hill Book Co., New York, 537 p., 1952.
(72)
White, G. F. Human Adjustments to Floods. Univ. Chicago Press, Chicago, 225 p., 1945.
(73)
Willett, H. C. Patterns of World Weather Changes. Am. Geophys. Union Trans., Vol. 29,
No. 6, pp. 803–809, 1948.
Highway Drainage Guidelines
G-156
(74)
Woolley, R. R. Cloudburst Floods in Utah, 1850–1938. U.S. Geol. Survey Water-Supply
Paper 994, 128 p., 1946.
(75)
Yevjevich, V. Probability and Statistics in Hydrology. Water Resource Publications, Ft.
Collins, Colo., 1972.
G.3.2 Uncited
AASHTO. Model Drainage Manual. American Association of State Highway and Transportation
Officials, Washington, DC, 2005.
FHWA. Hydraulics of Bridge Waterways. Hydraulic Design Series 1, FHWA-EPD-86-101,
Federal Highway Administration, U.S. Department of Transportation, Washington, DC, 1978.
FHWA. 23 CFR 650, Subpart A, “Location and Hydraulic Design of Encroachments on
Floodplains,” 44 FR 67580. Federal Highway Administration, U.S. Department of
Transportation, Washington, DC, November 26, 1979.
FHWA. Hydraulic Design of Energy Dissipators for Culverts and Channels. Hydraulic
Engineering Circular No. 14, FHWA-EPD-86-110, Federal Highway Administration, U.S.
Department of Transportation, Washington, DC, 1983.
FHWA. Design of Roadside Channels with Flexible Linings. Hydraulic Engineering Circular
No. 15, FHWA-IP-87-7, Federal Highway Administration, U.S. Department of Transportation,
Washington, DC, 1988.
FHWA. Design of Riprap Revetment. Hydraulic Engineering Circular No. 11, FHWA-IP-89-016,
Federal Highway Administration, U.S. Department of Transportation, Washington, DC, 1989.
FHWA. Highways in the River Environment—Participant Notebook. FHWA-HI-90-016. Federal
Highway Administration, U.S. Department of Transportation, Washington, DC, February 1990.
FHWA. Design of Urban Highway Drainage. Implementation Report, FHWA-NHI-01-021,
Federal Highway Administration, U.S. Department of Transportation, Washington, DC, 2001.
FHWA. Evaluating Scour at Bridges. Hydraulic Engineering Circular No. 18,
FHWA-NHI-01-001, Federal Highway Administration, U.S. Department of Transportation,
Washington, DC, 2001.
FHWA. River Engineering for Highway Encroachments. Hydraulic Design Series 6, FHWANHI-01-004, Federal Highway Administration, U.S. Department of Transportation, Washington,
DC, 2001.
FHWA. Hydraulic Design of Highway Culverts. Hydraulic Design Series 5, FHWA-NHI-01-020,
Federal Highway Administration, U.S. Department of Transportation, Washington, DC, 2001.
FHWA. Stream Stability at Highway Structures. Hydraulic Engineering Circular No. 20, FHWANHI-01-002, Federal Highway Administration, U.S. Department of Transportation, Washington,
DC, 2001.
FHWA. Highway Hydrology. Hydraulic Design Series 2, FHWA-NHI-02-001, Federal Highway
Administration, U.S. Department of Transportation, Washington, DC, 2002.
Richardson, E. V., S. Karaki, K. Mahmood, D. B. Simons, and M. A. Stevens. Highways in the
River Environment—Hydraulic and Environmental Design Considerations. Colorado State
University for FHWA, Fort Collins, CO, 1975.
AASHTO Highway Drainage Guidelines
Table of Contents
Chapter
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Appendix
Title
Hydraulic Considerations in Highway Planning and Location
Hydrology
Erosion and Sediment Control in Highway Construction
Hydraulic Design of Culverts
The Legal Aspects of Highway Drainage
Hydraulic Analysis and Design of Open Channels
Hydraulic Analysis for the Location and Design of Bridges
Hydraulic Aspects in Restoration and Upgrading of Highways
Storm Drain Systems
Evaluating Highway Effects on Surface Water Environments
Highways along Coastal Zones and Lakeshores
Stormwater Management
Training and Career Development of Hydraulics Engineers
Culvert Inspection, Material Selection, and Rehabilitation
Guidelines for Selecting and Utilizing Hydraulics Engineering Consultants
Glossary of Highway-Related Drainage Terms
xi
CHAPTER 1
TABLE OF CONTENTS
1.1
INTRODUCTION......................................................................................................... 1-1
1.2
GENERAL CONSIDERATIONS ............................................................................... 1-1
1.2.1
1.2.2
1.2.3
1.2.4
1.2.5
1.3
1.3.1
Definitions ................................................................................................................... 1-2
1.2.1.1 Planning ......................................................................................................... 1-2
1.2.1.2 Location ......................................................................................................... 1-2
Coordination ................................................................................................................ 1-3
1.2.2.1 Coordination within the Transportation Agency............................................ 1-4
1.2.2.2 Coordination with Other Agencies................................................................. 1-4
1.2.2.3 Public Involvement ........................................................................................ 1-5
Legal Considerations ................................................................................................... 1-5
1.2.3.1 Permits ........................................................................................................... 1-6
1.2.3.2 Regulations..................................................................................................... 1-6
1.2.3.3 Laws ............................................................................................................... 1-7
1.2.3.4 Federal Emergency Management Agency ..................................................... 1-7
Related Considerations ................................................................................................ 1-8
1.2.4.1 Design-Related Considerations...................................................................... 1-9
1.2.4.2 Construction-Related Considerations............................................................. 1-9
1.2.4.3 Maintenance-Related Considerations........................................................... 1-10
Environmental Considerations................................................................................... 1-10
1.2.5.1 Water Quality............................................................................................... 1-10
1.2.5.2 Fish and Wildlife.......................................................................................... 1-11
1.2.5.3 Other Environmental Considerations........................................................... 1-12
SPECIFIC CONSIDERATIONS............................................................................... 1-12
Stream Geomorphology ............................................................................................. 1-13
1.3.1.1 Types of Streams.......................................................................................... 1-13
1.3.1.1.1 Braided Streams ........................................................................... 1-14
1.3.1.1.2 Straight Streams ........................................................................... 1-14
1.3.1.1.3 Meandering Streams .................................................................... 1-14
1.3.1.2 Islands .......................................................................................................... 1-15
1.3.1.3 Delta Formations and Alluvial Fans ............................................................ 1-16
1.3.1.4 Aggradation and Degradation ...................................................................... 1-16
1.3.2 Highway Alignment................................................................................................... 1-17
1.3.2.1 Horizontal Alignment .................................................................................. 1-17
1.3.2.1.1 Existing Alignment ...................................................................... 1-17
1.3.2.1.2 New Location or Relocation ........................................................ 1-18
1.3.2.2 Vertical Alignment....................................................................................... 1-18
1.3.3 Location of Stream Crossings .................................................................................... 1-19
1.3.3.1 Physical Considerations ............................................................................... 1-19
1.3.3.1.1 Confluences.................................................................................. 1-19
1.3.3.1.2 Tidal Areas................................................................................... 1-20
1-iv
1.3.4
1.3.5
1.3.6
1.3.7
1.3.8
1.4
Highway Drainage Guidelines
1.3.3.2 Land Use Considerations.............................................................................. 1-20
1.3.3.3 Type of Structure .......................................................................................... 1-21
Encroachments ...........................................................................................................1-21
1.3.4.1 Longitudinal Encroachments ........................................................................ 1-21
1.3.4.2 Transverse Encroachments ........................................................................... 1-23
Ice and Debris.............................................................................................................1-24
Location of Storm Drainage Facilities........................................................................1-24
Location of Utilities....................................................................................................1-25
Floodplain Development and Use ..............................................................................1-26
PRELIMINARY SURVEYS ......................................................................................1-27
1.4.1
1.4.2
1.4.3
Topographic Data .......................................................................................................1-27
Channel Characteristics ..............................................................................................1-28
Hydrologic Data .........................................................................................................1-28
1.4.3.1 Basin Characteristics .................................................................................... 1-28
1.4.3.2 Precipitation.................................................................................................. 1-29
1.4.3.3 Flood Data .................................................................................................... 1-29
1.4.3.4 Highwater Information ................................................................................. 1-29
1.4.3.5 Existing Structures........................................................................................ 1-30
1.4.4 Environmental Data....................................................................................................1-30
1.4.4.1 Fish and Wildlife .......................................................................................... 1-30
1.4.4.2 Vegetation..................................................................................................... 1-30
1.4.4.3 Water Quality ............................................................................................... 1-31
1.4.5 Field Review...............................................................................................................1-31
1.5
PRELIMINARY HYDRAULIC REPORTS.............................................................1-32
1.6
REFERENCES ............................................................................................................1-32
APPENDIX 1A ....................................................................................................................1-34
CHAPTER 2
TABLE OF CONTENTS
2.1 INTRODUCTION........................................................................................................... 2-1
2.2 FACTORS AFFECTING FLOOD RUNOFF .............................................................. 2-2
2.2.1 Physiographic Characteristics ........................................................................................ 2-2
2.2.1.1 Drainage Area .................................................................................................. 2-3
2.2.1.2 Shape Factor..................................................................................................... 2-4
2.2.1.3 Slope ................................................................................................................ 2-4
2.2.1.4 Land Use .......................................................................................................... 2-4
2.2.1.5 Soil and Geology.............................................................................................. 2-5
2.2.1.6 Storage Area - Volume .................................................................................... 2-5
2.2.1.7 Elevation .......................................................................................................... 2-6
2.2.1.8 Orientation of the Basin ................................................................................... 2-7
2.2.1.9 Configuration of Channel and Floodplain Geometry....................................... 2-7
2.2.1.10 Stream and Drainage Densities ....................................................................... 2-7
2.2.2 Site-Specific Characteristics .......................................................................................... 2-8
2.2.2.1 Aggradation and Degradation .......................................................................... 2-8
2.2.2.2 Ice and Debris .................................................................................................. 2-8
2.2.2.3 Seasonal and Progressive Changes in Vegetation............................................ 2-9
2.2.2.4 Channel Modifications..................................................................................... 2-9
2.2.3 Meteorological Characteristics ...................................................................................... 2-9
2.2.3.1 Rainfall........................................................................................................... 2-10
2.2.3.2 Snow .............................................................................................................. 2-11
2.2.3.3 Temperature, Wind, Evaporation and Transpiration...................................... 2-11
2.2.3.4 Mixed Population Floods ............................................................................... 2-12
2.3 DATA SOURCES ......................................................................................................... 2-12
2.3.1 Categories of Hydrologic Data .................................................................................... 2-13
2.3.2 Sources of Hydrologic Data......................................................................................... 2-13
2.3.2.1 Runoff Data.................................................................................................... 2-13
2.3.2.2 Rainfall Data .................................................................................................. 2-15
2.3.2.3 Flood History and Historical Floods .............................................................. 2-16
2.3.2.4 Flood History of Existing Structures.............................................................. 2-16
2.3.2.5 Paleoflood Data.............................................................................................. 2-17
2.4 ELEMENTS OF RUNOFF PROCESS....................................................................... 2-17
2.4.1
2.4.2
2.4.3
2.4.4
Infiltration .................................................................................................................... 2-18
Detention and Depression Storage............................................................................... 2-20
Stream Flow and Flood Hydrograph............................................................................ 2-20
Hydrograph Parameters ............................................................................................... 2-22
2.4.4.1 Time of Concentration ................................................................................... 2-22
2.4.4.1.1 Overland Flow................................................................................ 2-22
2.4.4.1.2 Swale, Ditch or Stream Channel Flow........................................... 2-23
2-iv
Highway Drainage Guidelines
2.4.4.1.3 Storm Drain or Culvert Flow..........................................................2-24
2.4.4.2 Lag Time, Rise Time and Time to Peak .........................................................2-24
2.4.5 Unit Hydrographs .........................................................................................................2-25
2.5 MEASUREMENTS OF FLOOD MAGNITUDES.....................................................2-25
2.5.1 Direct Measurements of Flood Magnitudes .................................................................2-25
2.5.2 Indirect Measurements of Flood Magnitudes ...............................................................2-26
2.5.3 Ordinary Highwater and Mean Annual Flood ..............................................................2-26
2.6 FLOOD PROBABILITY AND FREQUENCY AS APPLIED TO
HIGHWAY HYDROLOGY.........................................................................................2-27
2.6.1 Concepts of Probability and Frequency Analysis.........................................................2-27
2.6.2 Floods Considered in Hydrologic and Hydraulic Analysis ..........................................2-28
2.6.2.1 Base Flood and Super Flood...........................................................................2-28
2.6.2.2 Overtopping Flood..........................................................................................2-28
2.6.2.3 Design Flood...................................................................................................2-28
2.6.2.4 Maximum Historical Flood.............................................................................2-29
2.6.2.5 Probable Maximum Flood ..............................................................................2-29
2.6.3 Design Flood Frequency...............................................................................................2-29
2.6.3.1 Policy Alternative ...........................................................................................2-30
2.6.3.2 Economic Assessment Alternative .................................................................2-30
2.6.3.3 Highway Classification...................................................................................2-31
2.6.3.4 Flood Hazard Criteria .....................................................................................2-31
2.6.3.4.1 Sensitivity to Increased Flood Magnitude ......................................2-32
2.6.3.4.2 Loss of Life.....................................................................................2-32
2.6.3.4.3 Property Damages...........................................................................2-32
2.6.3.4.4 Traffic Interruption .........................................................................2-33
2.6.3.4.5 Economics and Budgetary Constraints...........................................2-33
2.7 METHODS FOR ESTIMATING FLOOD PEAKS, DURATIONS
AND VOLUMES ............................................................................................................2-33
2.7.1 Individual Station Flood Frequency Analysis ..............................................................2-34
2.7.1.1 Development of Flood-Frequency Curve .......................................................2-34
2.7.1.1.1 Graphical Method ...........................................................................2-34
2.7.1.1.2 Mathematical Method.....................................................................2-35
2.7.1.2 Extrapolating Flood-Frequency Curves..........................................................2-35
2.7.1.3 Transfer of Data..............................................................................................2-36
2.7.2 Regional Flood-Frequency Analysis ............................................................................2-36
2.7.2.1 Index-Flood Method .......................................................................................2-36
2.7.2.2 Multiple Regression Analysis—Watershed Characteristics ...........................2-36
2.7.2.2.1 USGS-FHWA Urban Method ........................................................2-37
2.7.2.2.2 USGS Regional or Local Rural Methods .......................................2-38
2.7.2.3 Multiple Regression Analysis—Channel/Characteristics Method .................2-38
2.7.3 Empirical Hydrologic Methods ....................................................................................2-39
2.7.3.1 Rational Method .............................................................................................2-39
2.7.3.2 British Method................................................................................................2-40
Hydrology
2.7.4
2.7.5
2.7.6
2.7.7
2.7.3.3 NRCS T.R. 55 Method................................................................................... 2-40
Unit Hydrograph Methods ........................................................................................... 2-40
2.7.4.1 Finite Time Unit Hydrograph......................................................................... 2-41
2.7.4.2 Synthetic Unit Hydrograph ............................................................................ 2-41
2.7.4.2.1 Ten-Minute Unit Hydrographs....................................................... 2-42
2.7.4.2.2 Dimensionless Hydrograph............................................................ 2-42
Regional Hydrographs ................................................................................................. 2-42
Mathematical Models................................................................................................... 2-43
2.7.6.1 HYDRAIN Computer System........................................................................ 2-44
2.7.6.2 HEC-1/HEC-HMS Models ............................................................................ 2-44
2.7.6.3 NRCS TR-20 Method .................................................................................... 2-45
2.7.6.4 The Stormwater Management Model (SWMM)............................................ 2-45
2.7.6.5 The Stanford Watershed or Hydrocomp (HSP) Model.................................. 2-45
2.7.6.6 Penn State Urban Runoff Model .................................................................... 2-46
2.7.6.7 The Massachusetts Institute of Technology Catchment
(The MITCAT) Model .................................................................................... 2-46
2.7.6.8 USACE STORM Model ................................................................................ 2-46
2.7.6.9 ILLUDAS Model ........................................................................................... 2-46
2.7.6.10 USGS “Dawdy” Model ................................................................................. 2-47
Accuracy of Methods for Estimating Peak Discharges................................................ 2-47
2.8 CHARACTERISTICS AND ANALYSIS OF LOW FLOWS .................................. 2-48
2.9 STORAGE AND FLOOD ROUTING FOR STORMWATER MANAGEMENT . 2-49
2.9.1 Storage Characteristics................................................................................................. 2-49
2.9.2 Storage Size and Location ........................................................................................... 2-50
2.9.3 Determination of Storage Volume and Flood Routing Procedures ............................. 2-51
2.10 DOCUMENTATION................................................................................................... 2-51
2.11 REFERENCES............................................................................................................. 2-52
2-v
CHAPTER 3
TABLE OF CONTENTS
3.1 INTRODUCTION........................................................................................................... 3-1
3.2 PURPOSE AND OBJECTIVES .................................................................................... 3-2
3.3 EROSION AND SEDIMENT-RELATED PLANNING AND LOCATION
CONSIDERATIONS ...................................................................................................... 3-4
3.3.1 Identification of Erosion Sensitive Areas ...................................................................... 3-5
3.3.2 Identification of Sediment Sensitive Areas.................................................................... 3-5
3.3.3 Coordination .................................................................................................................. 3-6
3.3.3.1 Coordination Within the Transportation Agency.............................................. 3-6
3.3.3.2 Coordination with Other Agencies.................................................................... 3-6
3.4 EROSION AND SEDIMENT-RELATED GEOMETRIC CONSIDERATIONS .... 3-6
3.4.1 Alignment and Grade..................................................................................................... 3-7
3.4.2 Cross Section ................................................................................................................. 3-7
3.5 PLAN DEVELOPMENT ............................................................................................... 3-8
3.5.1 Temporary Erosion and Sediment Control Measures .................................................... 3-9
3.5.1.1 Ground Cover.................................................................................................... 3-9
3.5.1.2 Channel Liners ................................................................................................ 3-10
3.5.1.3 Diversion Dikes and Ditches........................................................................... 3-13
3.5.1.4 Filter Berms..................................................................................................... 3-15
3.5.1.5 Temporary Slope Drains ................................................................................. 3-16
3.5.1.6 Brush Barriers ................................................................................................. 3-18
3.5.1.7 Silt Fences ....................................................................................................... 3-19
3.5.1.8 Check Dams .................................................................................................... 3-21
3.5.1.9 Straw Bales ..................................................................................................... 3-23
3.5.1.10 Riprap............................................................................................................. 3-23
3.5.1.11 Sediment Basins ............................................................................................. 3-26
3.5.1.11.1 Planning and Location .................................................................. 3-26
3.5.1.11.2 Design ........................................................................................... 3-29
3.5.1.12 Phased Erosion and Sediment Control Plans ................................................. 3-30
3.5.2 Permanent Erosion and Sediment Control Measures................................................... 3-30
3.5.2.1 Vegetation ....................................................................................................... 3-30
3.5.2.2 Slopes .............................................................................................................. 3-31
3.5.2.3 Channels.......................................................................................................... 3-32
3.5.2.3.1 Sizing and Shape............................................................................. 3-33
3.5.2.3.2 Alignment and Grade ...................................................................... 3-33
3.5.2.3.3 Linings ............................................................................................ 3-34
3.5.2.3.4 Grade Control Structures ................................................................ 3-36
3.5.2.4 Shoulder Drains............................................................................................... 3-38
3-iv
Highway Drainage Guidelines
3.5.2.5 Culverts............................................................................................................ 3-38
3.5.2.6 Underdrains ..................................................................................................... 3-40
3.6 CONSTRUCTION ........................................................................................................3-41
3.6.1
3.6.2
3.6.3
3.6.4
3.6.5
3.6.6
3.6.7
3.6.8
3.6.9
Scheduling Operation ...................................................................................................3-41
Clearing and Grubbing .................................................................................................3-42
Construction Operations in Rivers, Streams, and Impoundments ................................3-42
Excavation and Embankment Construction .................................................................3-44
Bridge Construction .....................................................................................................3-45
Culvert Construction ....................................................................................................3-46
Borrow Pits, Waste Areas, and Haul Roads .................................................................3-48
Maintenance of Control Features .................................................................................3-48
Enforcement .................................................................................................................3-52
3.7 REFINEMENT OF METHODS ..................................................................................3-53
3.7.1 Research and Development ..........................................................................................3-53
3.7.2 Feedback.......................................................................................................................3-53
3.8 REFERENCES ..............................................................................................................3-54
CHAPTER 4
TABLE OF CONTENTS
4.1
INTRODUCTION......................................................................................................... 4-1
4.2
DATA COLLECTION ................................................................................................. 4-2
4.2.1
4.2.2
4.2.3
4.2.4
4.2.5
4.2.6
4.2.7
4.3
4.3.1
4.3.2
4.4
4.4.1
4.4.2
4.4.3
4.5
4.5.1
4.5.2
4.5.3
4.5.4
4.5.5
Topographic Features................................................................................................... 4-2
Drainage Area .............................................................................................................. 4-2
Channel Characteristics ............................................................................................... 4-2
Fish Life....................................................................................................................... 4-3
Highwater Information................................................................................................. 4-3
Existing Structures ....................................................................................................... 4-3
Field Review ................................................................................................................ 4-4
CULVERT LOCATION .............................................................................................. 4-4
Plan .............................................................................................................................. 4-5
Profile........................................................................................................................... 4-6
CULVERT TYPE ......................................................................................................... 4-8
Shape and Cross Section.............................................................................................. 4-8
4.4.1.1 Circular......................................................................................................... 4-8
4.4.1.2 Pipe Arch and Elliptical ............................................................................... 4-8
4.4.1.3 Box or Rectangular....................................................................................... 4-8
4.4.1.4 Arches........................................................................................................... 4-9
4.4.1.5 Multiple Barrels............................................................................................ 4-9
Materials ...................................................................................................................... 4-9
End Treatments .......................................................................................................... 4-10
4.4.3.1 Projecting ................................................................................................... 4-10
4.4.3.2 Mitered ....................................................................................................... 4-10
4.4.3.3 Pipe End Sections....................................................................................... 4-10
4.4.3.4 Headwalls and Wingwalls .......................................................................... 4-11
HYDRAULIC DESIGN.............................................................................................. 4-12
Design Flood Discharge............................................................................................. 4-12
Headwater Elevation.................................................................................................. 4-12
Tailwater .................................................................................................................... 4-14
Outlet Velocity........................................................................................................... 4-14
Culvert Hydraulics ..................................................................................................... 4-14
4.5.5.1 Conditions of Flow..................................................................................... 4-15
4.5.5.1.1 Inlet Control............................................................................. 4-15
4.5.5.1.2 Outlet Control .......................................................................... 4-16
4.5.5.2 Performance Curves ................................................................................... 4-16
4-iv
Highway Drainage Guidelines
4.5.6
Entrance Configurations.............................................................................................4-18
4.5.6.1 Conventional............................................................................................... 4-18
4.5.6.2 Beveled ....................................................................................................... 4-21
4.5.6.3 Side-Tapered Inlets ..................................................................................... 4-22
4.5.6.4 Slope-Tapered Inlets ................................................................................... 4-24
Barrel Characteristics .................................................................................................4-24
Outlet Design..............................................................................................................4-25
4.5.7
4.5.8
4.6
SPECIAL HYDRAULIC CONSIDERATIONS .......................................................4-26
4.6.1
4.6.2
Anchorage ..................................................................................................................4-26
Piping..........................................................................................................................4-28
4.6.2.1 Joints ........................................................................................................... 4-29
4.6.2.2 Anti-Seep Collars........................................................................................ 4-29
4.6.2.3 Weep Holes................................................................................................. 4-29
Junctions and Bifurcations .........................................................................................4-30
Training Walls ............................................................................................................4-30
Sag Culverts ...............................................................................................................4-31
Irregular Alignment ....................................................................................................4-31
Cavitation ...................................................................................................................4-31
Tidal Effects and Flood Protection.............................................................................4-31
4.6.3
4.6.4
4.6.5
4.6.6
4.6.7
4.6.8
4.7
MULTIPLE-USE CULVERTS ..................................................................................4-32
4.7.1
4.7.2
4.7.3
4.7.4
Utilities .......................................................................................................................4-32
Stock and Wildlife Passage ........................................................................................4-32
Land Access................................................................................................................4-32
Fish Passage ...............................................................................................................4-32
4.8
IRRIGATION ..............................................................................................................4-35
4.9
DEBRIS CONTROL ...................................................................................................4-35
4.9.1
4.9.2
Debris Control Structure Design ................................................................................4-36
Maintenance ...............................................................................................................4-36
4.10
SERVICE LIFE .........................................................................................................4-36
4.10.1
4.10.2
Abrasion ...................................................................................................................4-37
Corrosion ..................................................................................................................4-38
4.11
SAFETY .....................................................................................................................4-39
4.12
DESIGN DOCUMENTATION ................................................................................4-39
4.12.1
4.12.2
Compilation of Data .................................................................................................4-39
Retention of Records ................................................................................................4-40
Hydraulic Design of Culverts
4.13
4.13.1
4.13.2
4.13.3
4.14
4.14.1
4.14.2
4.14.3
4.15
HYDRAULIC-RELATED CONSTRUCTION CONSIDERATIONS................. 4-40
Verification of Plans ................................................................................................ 4-40
Temporary Erosion Control ..................................................................................... 4-40
Construction and Documentation ............................................................................ 4-41
HYDRAULIC-RELATED MAINTENANCE CONSIDERATIONS................... 4-41
Maintenance Inspections.......................................................................................... 4-41
Flood Records .......................................................................................................... 4-41
Reconstruction and Repair....................................................................................... 4-41
REFERENCES.......................................................................................................... 4-42
4-v
CHAPTER 5
TABLE OF CONTENTS
5.1
INTRODUCTION......................................................................................................... 5-1
5.2
LAWS IN GENERAL................................................................................................... 5-2
5.3
FEDERAL LAWS......................................................................................................... 5-3
5.3.1
5.3.2
5.3.3
5.3.4
5.3.5
5.3.6
5.3.7
5.4
5.4.1
5.4.2
National Environmental Policy Act (NEPA) ............................................................... 5-4
Flood Insurance............................................................................................................ 5-5
Navigable Waters......................................................................................................... 5-6
Fish and Wildlife ......................................................................................................... 5-8
Tennessee Valley Authority (TVA)............................................................................. 5-9
Coastal Zone Management .......................................................................................... 5-9
Executive Orders........................................................................................................ 5-10
STATE LAWS............................................................................................................. 5-11
Common Law............................................................................................................. 5-11
5.4.1.1 Classification of Waters ............................................................................. 5-11
5.4.1.1.1 Surface Waters......................................................................... 5-12
5.4.1.1.2 Stream Waters.......................................................................... 5-12
5.4.1.1.3 Floodwaters ............................................................................. 5-12
5.4.1.1.4 Groundwaters........................................................................... 5-13
5.4.1.2 Surface Water Rules and Applications....................................................... 5-13
5.4.1.2.1 Civil Law Rule (Natural Drainage Rule)................................. 5-13
5.4.1.2.2 Application of the Civil Law Rule........................................... 5-14
5.4.1.2.3 Common Enemy Doctrine ....................................................... 5-15
5.4.1.2.4 Application of the Common Enemy Doctrine ......................... 5-15
5.4.1.2.5 Reasonable Use Rule ............................................................... 5-15
5.4.1.2.6 Application of the Reasonable Use Rule ................................. 5-16
5.4.1.3 Stream Water Rules.................................................................................... 5-16
5.4.1.4 Floodwater Rule ......................................................................................... 5-17
5.4.1.5 Groundwater Rules..................................................................................... 5-17
Statutory Law............................................................................................................. 5-18
5.4.2.1 Eminent Domain ........................................................................................ 5-18
5.4.2.2 Water Rights............................................................................................... 5-18
5.4.2.2.1 Riparian Doctrine .................................................................... 5-19
5.4.2.2.2 The Doctrine of Prior Appropriation ....................................... 5-19
5.4.2.3 Flood Control, Drainage and Irrigation Districts ....................................... 5-20
5.4.2.4 Agricultural Drainage Law......................................................................... 5-20
5.4.2.5 Environmental Laws .................................................................................. 5-20
5.4.2.6 Highway Agency Rules.............................................................................. 5-21
5-iv
5.5
5.5.1
5.5.2
5.6
5.6.1
5.6.2
5.6.3
5.6.4
5.6.5
5.6.6
5.7
5.7.1
5.7.2
5.7.3
5.7.4
5.7.5
5.8
5.8.1
5.8.2
5.8.3
5.8.4
5.8.5
5.8.6
5.8.7
5.9
Highway Drainage Guidelines
LOCAL LAWS ............................................................................................................5-21
Local Ordinances........................................................................................................5-21
Flood Disaster Protection Act of 1973 .......................................................................5-22
COMMON DRAINAGE COMPLAINTS .................................................................5-22
Diversion ....................................................................................................................5-23
Collection and Concentration .....................................................................................5-23
Augmentation .............................................................................................................5-23
Obstruction .................................................................................................................5-24
Erosion and Sedimentation.........................................................................................5-24
Groundwater Interference...........................................................................................5-24
LEGAL REMEDY ......................................................................................................5-25
Inverse Condemnation................................................................................................5-25
Injunction....................................................................................................................5-25
Legislative Claims ......................................................................................................5-26
Tort Claims.................................................................................................................5-26
Tort Liability of State Highway Agencies..................................................................5-27
INVOLVEMENT OF THE HYDRAULICS ENGINEER.......................................5-28
Planning and Location Considerations.......................................................................5-29
Design Considerations................................................................................................5-29
5.8.2.1 Documentation............................................................................................5-29
5.8.2.2 Engineer Liability .......................................................................................5-30
Liaison with Legal Staff .............................................................................................5-30
Engineering Evidence.................................................................................................5-31
Negotiation .................................................................................................................5-31
The Engineer as a Witness .........................................................................................5-31
5.8.6.1 Engineering Testimony...............................................................................5-31
5.8.6.2 Conduct When a Witness............................................................................5-32
Engineer’s Conduct Toward the Opposing Party .......................................................5-33
REFERENCES ............................................................................................................5-33
CHAPTER 6
TABLE OF CONTENTS
6.1
INTRODUCTION......................................................................................................... 6-1
6.2
CONSIDERATIONS FOR ESTABLISHING CRITERIA....................................... 6-2
6.3
PLANNING AND LOCATION ................................................................................... 6-2
6.3.1
6.3.2
6.4
6.4.1
6.4.2
6.4.3
6.4.4
6.4.5
Planning ....................................................................................................................... 6-3
6.3.1.1 Coordination with Other Agencies............................................................... 6-3
6.3.1.1.1 Local Drainage Systems ............................................................ 6-5
6.3.1.1.2 Flood Control............................................................................. 6-5
6.3.1.1.3 Floodplain Management ............................................................ 6-5
6.3.1.1.4 Conservation.............................................................................. 6-6
6.3.1.1.5 Fish and Wildlife ....................................................................... 6-6
6.3.1.1.6 Irrigation .................................................................................... 6-7
6.3.1.1.7 Permits ....................................................................................... 6-7
6.3.1.2 Cooperative Projects .................................................................................... 6-7
Location ....................................................................................................................... 6-8
6.3.2.1 Longitudinal Encroachments........................................................................ 6-8
6.3.2.2 Transverse Encroachments......................................................................... 6-10
SURVEYS.................................................................................................................... 6-11
Topographic Features................................................................................................. 6-20
Channel Characteristics ............................................................................................. 6-20
Fish and Wildlife ....................................................................................................... 6-21
Highwater Information............................................................................................... 6-22
Hydrologic Data......................................................................................................... 6-23
6.5
HYDROLOGY............................................................................................................ 6-23
6.6
HYDRAULICS OF OPEN CHANNELS .................................................................. 6-23
6.6.1
6.6.2
6.6.3
Types of Flow ............................................................................................................ 6-24
Open Channel Equations............................................................................................ 6-25
Analysis of Open-Channel Flow................................................................................ 6-27
6.6.3.1 Factors Affecting Open-Channel Flow ...................................................... 6-27
6.6.3.2 Stable Stage-Discharge Relationships........................................................ 6-28
6.6.3.2.1 Single-Section Analysis........................................................... 6-29
6.6.3.2.2 Water Surface Profiles............................................................. 6-29
6.6.3.2.3 Control Sections ...................................................................... 6-30
6.6.3.3 Unstable Stage-Discharge Relationships.................................................... 6-30
6.6.3.4 Flow and Velocity Distribution.................................................................. 6-35
Highway Drainage Guidelines
6-iv
6.6.4
6.7
Special Analysis Techniques......................................................................................6-36
6.6.4.1 Two-Dimensional Analysis ........................................................................ 6-36
6.6.4.2 Water and Sediment Routing ...................................................................... 6-36
6.6.4.3 Unsteady Flow Analysis ............................................................................. 6-38
FLUVIAL GEOMORPHOLOGY .............................................................................6-38
6.7.1
Alluvial Streams .........................................................................................................6-39
6.7.1.1 Stream Types .............................................................................................. 6-39
6.7.1.1.1 Straight Streams ....................................................................... 6-41
6.7.1.1.2 Braided Streams ....................................................................... 6-41
6.7.1.1.3 Meandering Streams................................................................. 6-42
6.7.1.2 Graded or Poised Streams........................................................................... 6-44
6.7.1.3 Stream System Response ............................................................................ 6-44
Nonalluvial Channels .................................................................................................6-46
Stream Classification Methods...................................................................................6-46
6.7.2
6.7.3
6.8
THE EFFECTS OF CHANNEL ALTERATIONS ..................................................6-51
6.8.1
Channel Realignment .................................................................................................6-52
6.8.1.1 Slope Modification ..................................................................................... 6-52
6.8.1.2 Section Modification................................................................................... 6-53
Conveyance Modification ..........................................................................................6-54
6.8.2
6.9
CHANNEL STABILIZATION AND BANK PROTECTION.................................6-56
6.9.1
6.9.2
6.9.3
6.9.4
Stabilization Considerations.......................................................................................6-56
Selection of Protective Measures ...............................................................................6-57
Revetments .................................................................................................................6-57
Using Vegetation for Stream Bank Stabilization .......................................................6-59
6.10
ROADSIDE DRAINAGE CHANNELS ..................................................................6-59
6.10.1
6.10.2
6.10.3
6.10.4
6.10.5
6.10.6
6.10.7
6.10.8
6.10.9
Safety and Aesthetics ...............................................................................................6-60
Shape ........................................................................................................................6-60
Lining .......................................................................................................................6-60
Superelevation ..........................................................................................................6-63
6.10.4.1 Supercritical Flow.................................................................................... 6-64
6.10.4.2 Subcritical Flow....................................................................................... 6-64
Chutes and Flumes ...................................................................................................6-64
Grade Control Structures..........................................................................................6-65
Transitions ................................................................................................................6-65
6.10.7.1 Supercritical Flow Transitions................................................................. 6-66
6.10.7.2 Subcritical Flow Transitions .................................................................... 6-66
Confluences ..............................................................................................................6-66
Bends and Curves .....................................................................................................6-67
Hydraulic Analysis and Design of Open Channels
6.11
6.11.1
6.11.2
6.11.3
6.11.4
STRUCTURAL CONSIDERATIONS.................................................................... 6-68
Subsurface Investigations ........................................................................................ 6-69
Reinforcement for Rigid Linings ............................................................................. 6-69
Buoyancy and Heave ............................................................................................... 6-70
Seepage Control Filter Blankets .............................................................................. 6-70
6.12
CONSTRUCTION-RELATED HYDRAULIC CONSIDERATIONS................. 6-71
6.13
MAINTENANCE-RELATED HYDRAULIC CONSIDERATIONS................... 6-71
6.13.1
6.13.2
6.14
Maintenance during Contract Period ....................................................................... 6-72
Hydraulic-Related Maintenance Considerations ..................................................... 6-72
REFERENCES.......................................................................................................... 6-72
6-v
CHAPTER 7
TABLE OF CONTENTS
7.1
INTRODUCTION......................................................................................................... 7-1
7.2
PLANNING AND LOCATION................................................................................... 7-1
7.2.1
7.2.2
7.2.3
7.2.4
7.2.5
7.2.6
7.2.7
7.2.8
7.2.9
7.3
7.3.1
7.3.2
7.3.3
7.3.4
7.3.5
7.3.6
7.3.7
7.3.8
Location of Stream Crossing ....................................................................................... 7-2
Structure Type.............................................................................................................. 7-3
Environmental Considerations..................................................................................... 7-3
Coordination with Other Agencies .............................................................................. 7-4
7.2.4.1 Water Resources Agencies........................................................................... 7-5
7.2.4.2 Permits and Approvals ................................................................................. 7-5
Stream Morphology ..................................................................................................... 7-6
7.2.5.1 Types of Streams .......................................................................................... 7-7
7.2.5.1.1 Braided Streams......................................................................... 7-7
7.2.5.1.2 Straight Streams......................................................................... 7-8
7.2.5.1.3 Meandering Streams ................................................................ 7-10
7.2.5.2 Islands, Delta Formations, and Alluvial Fans ............................................ 7-12
7.2.5.3 Aggradation and Degradation .................................................................... 7-14
Confluences ............................................................................................................... 7-15
Tidal Areas................................................................................................................. 7-17
Floodplain Levees and Encroachments...................................................................... 7-17
Replacement, Repair, and Rehabilitation................................................................... 7-18
DATA COLLECTION ............................................................................................... 7-19
Topographic Features................................................................................................. 7-19
Land Use and Culture ................................................................................................ 7-19
Hydrologic Data......................................................................................................... 7-20
7.3.3.1 Flood Data .................................................................................................. 7-20
7.3.3.2 Basin Characteristics .................................................................................. 7-20
7.3.3.3 Precipitation ............................................................................................... 7-22
7.3.3.4 Highwater Information ............................................................................... 7-22
Existing Structures ..................................................................................................... 7-23
Channel Characteristics ............................................................................................. 7-24
Environmental Data ................................................................................................... 7-24
Site Plan ..................................................................................................................... 7-25
Field Reviews............................................................................................................. 7-26
7.4
HYDROLOGIC ANALYSIS ..................................................................................... 7-26
7.5
HYDRAULIC ANALYSIS OF THE STREAM....................................................... 7-28
7.5.1
7.5.2
Stage-Discharge Relationships .................................................................................. 7-28
Types of Stage-Discharge Relationships ................................................................... 7-30
7.5.2.1 Stable.......................................................................................................... 7-30
7.5.2.2 Unstable...................................................................................................... 7-30
7-iv
7.6
7.6.1
7.6.2
7.6.3
7.6.4
7.6.5
7.7
7.7.1
7.7.2
7.8
7.8.1
Highway Drainage Guidelines
STREAM-CROSSING DESIGN ............................................................................... 7-31
Criteria ....................................................................................................................... 7-31
Highway-Stream Crossing Systems ........................................................................... 7-33
7.6.2.1 Highway Profile and Alignment................................................................. 7-34
7.6.2.2 Waterway Openings ................................................................................... 7-39
7.6.2.2.1 Location ................................................................................... 7-39
7.6.2.2.2 Size .......................................................................................... 7-43
7.6.2.2.3 Auxiliary Openings.................................................................. 7-44
7.6.2.2.4 Replacement Bridges ............................................................... 7-45
7.6.2.3 Structural Alternatives................................................................................ 7-45
7.6.2.3.1 Bridge or Culvert ..................................................................... 7-45
7.6.2.3.2 Piers ......................................................................................... 7-47
7.6.2.3.3 Abutments................................................................................ 7-48
7.6.2.3.4 Foundations.............................................................................. 7-49
7.6.2.3.5 Superstructures......................................................................... 7-50
7.6.2.4 Channel Modification ................................................................................. 7-54
Analysis of the Stream Crossing System ................................................................... 7-56
7.6.3.1 Hydraulic Performance of the Crossing System......................................... 7-57
7.6.3.2 Backwater ................................................................................................... 7-58
7.6.3.3 Flow Distribution........................................................................................ 7-59
7.6.3.4 Velocity ...................................................................................................... 7-60
7.6.3.5 Scour........................................................................................................... 7-61
7.6.3.5.1 General Scour .......................................................................... 7-63
7.6.3.5.2 Local Scour .............................................................................. 7-65
7.6.3.5.3 Natural Scour ........................................................................... 7-65
7.6.3.5.4 In-Stream Borrow Areas, Commercial Mining, and
Dredging for Navigation and Flood Control............................ 7-66
7.6.3.5.5 Combined Effects of General, Local, and Natural Scour......... 7-68
7.6.3.6 Stream Environment................................................................................... 7-68
7.6.3.7 Economic Analysis ..................................................................................... 7-69
Protective and Preventive Measures .......................................................................... 7-71
7.6.4.1 Pier Foundations......................................................................................... 7-71
7.6.4.1.1 Preventive Measures at Piers ................................................... 7-72
7.6.4.1.2 Protective Measures at Piers .................................................... 7-75
7.6.4.2 Abutments and Approach Fills ................................................................... 7-76
7.6.4.2.1 Protective and Preventive Measures along Embankments ...... 7-77
7.6.4.2.2 Protective and Preventive Measures at Abutments.................. 7-78
7.6.4.3 Bank Stabilization and River Training ....................................................... 7-81
7.6.4.4 Buoyant, Drag, Debris, and Ice Forces on Bridge Superstructures ............ 7-82
Dolphins and Fender Systems .................................................................................... 7-83
DECK DRAINAGE .................................................................................................... 7-85
Deck Inlets ................................................................................................................. 7-85
Bridge End Drains...................................................................................................... 7-86
DESIGN DOCUMENTATION.................................................................................. 7-86
Design Data for Permanent Records .......................................................................... 7-87
Hydraulic Analysis for the Location and Design of Bridges
7.8.2
7.9
7-v
Post-Construction Data .............................................................................................. 7-87
CONSTRUCTION-RELATED HYDRAULIC CONSIDERATIONS................... 7-88
7.9.1
7.9.2
7.9.3
7.9.4
Hydrologic Information.............................................................................................. 7-88
Cofferdams, Caissons, and Falsework ....................................................................... 7-88
Contractor Crossings and Work Areas....................................................................... 7-88
Borrow Areas ............................................................................................................. 7-89
7.10
HYDRAULIC-RELATED CONSTRUCTION CONSIDERATIONS................. 7-90
7.10.1
7.10.2
7.10.3
7.10.4
7.10.5
7.10.6
Verification of Plans................................................................................................. 7-90
Plan Changes ............................................................................................................ 7-90
Borrow Areas ........................................................................................................... 7-90
Detours ..................................................................................................................... 7-91
Environmental and Ecological Aspects.................................................................... 7-91
Feedback................................................................................................................... 7-92
7.11
MAINTENANCE-RELATED HYDRAULIC CONSIDERATIONS................... 7-92
7.12
HYDRAULIC-RELATED MAINTENANCE CONSIDERATIONS................... 7-93
7.12.1
7.12.2
7.13
Maintenance Inspections .......................................................................................... 7-93
Flood Damages......................................................................................................... 7-93
7.12.2.1 Remedial Construction and Repair .......................................................... 7-94
7.12.2.2 Recurring Damage ................................................................................... 7-94
REFERENCES .......................................................................................................... 7-95
CHAPTER 8
TABLE OF CONTENTS
8.1
INTRODUCTION......................................................................................................... 8-1
8.2
PLANNING CONSIDERATIONS.............................................................................. 8-1
8.2.1
8.2.2
8.2.3
8.2.4
8.3
8.3.1
8.3.2
8.3.3
8.3.4
8.3.5
8.4
8.4.1
8.4.2
8.4.3
8.5
8.5.1
8.5.2
8.5.3
8.5.4
8.6
8.6.1
8.6.2
Coordination and Cooperation ..................................................................................... 8-2
8.2.1.1 Other Agencies ............................................................................................. 8-2
8.2.1.2 Utilities ......................................................................................................... 8-3
8.2.1.3 Property Owners........................................................................................... 8-3
Permits and Approvals................................................................................................. 8-3
8.2.2.1 National Flood Insurance Program............................................................... 8-4
8.2.2.2 Federal Water Pollution Control Act (FWPCA)—Section 404 ................... 8-5
8.2.2.3 National Pollutant Discharge Elimination System (NPDES)....................... 8-5
8.2.2.4 Navigation .................................................................................................... 8-6
8.2.2.5 Coastal Zone Management Act .................................................................... 8-7
8.2.2.6 State Permits................................................................................................. 8-7
Replace or Rehabilitate Structures............................................................................... 8-7
Environmental Considerations..................................................................................... 8-8
TYPES OF RESTORATION AND UPGRADING ................................................... 8-9
Horizontal or Vertical Alignment Adjustments ........................................................... 8-9
Cross Section Changes................................................................................................. 8-9
8.3.2.1 Widening Roadway .................................................................................... 8-10
8.3.2.2 Flattening Foreslopes ................................................................................. 8-10
8.3.2.3 Adding Traffic Lanes ................................................................................. 8-10
Urban Reconstruction ................................................................................................ 8-11
Safety Projects ........................................................................................................... 8-11
Rehabilitation and Replacement of Bridges and Culverts ......................................... 8-12
DATA COLLECTION ............................................................................................... 8-12
Condition Survey of Existing Structures ................................................................... 8-12
Past Hydraulic Performance....................................................................................... 8-13
Identification of Other Problems with Existing Structures ........................................ 8-14
HYDROLOGIC CONSIDERATIONS ..................................................................... 8-15
Evaluation of Observed Flood Discharge .................................................................. 8-15
Changes in Hydrologic Methods................................................................................ 8-16
Land Use Changes ..................................................................................................... 8-17
Hydrologic and Hydraulic Investigations .................................................................. 8-19
HYDRAULIC CONSIDERATIONS......................................................................... 8-19
Economic Analysis of Alternatives............................................................................ 8-20
Consequences of Change in Hydraulic Performance of Existing Structures ............. 8-21
8-iv
Highway Drainage Guidelines
8.6.2.1
8.6.3
8.6.4
8.6.5
8.6.6
Adding Traffic Lanes..................................................................................8-21
8.6.2.1.1 With Median.............................................................................8-21
8.6.2.1.2 Without Median........................................................................8-22
8.6.2.2 Safety Projects ............................................................................................8-24
8.6.2.3 Culvert Replacement and Rehabilitation ....................................................8-25
8.6.2.3.1 Culvert Replacement ................................................................8-25
8.6.2.3.2 Culvert Rehabilitation ..............................................................8-25
8.6.2.4 Bridge Replacements ..................................................................................8-26
8.6.2.5 Changes in Type of Structure .....................................................................8-27
8.6.2.6 Culvert Extensions ......................................................................................8-28
8.6.2.7 Structures in Urban Areas ...........................................................................8-29
Influence of Adjacent Structures ................................................................................8-30
8.6.3.1 Upstream Structures....................................................................................8-30
8.6.3.2 Downstream Structures...............................................................................8-31
Improvements in Hydraulic Performance of Existing Structures...............................8-31
8.6.4.1 Improved Inlets ...........................................................................................8-32
8.6.4.2 Adding Guide Banks...................................................................................8-32
8.6.4.3 Channel Modifications................................................................................8-33
8.6.4.4 Bridge Modification....................................................................................8-35
8.6.4.5 Debris Control.............................................................................................8-35
Obsolete Structures.....................................................................................................8-35
Added Structures ........................................................................................................8-36
8.7 CONSTRUCTION-RELATED CONSIDERATIONS DURING
HYDRAULIC DESIGN .............................................................................................8-37
8.7.1
8.7.2
8.7.3
Providing for Drainage during Construction ..............................................................8-38
8.7.1.1 Contractor Crossings...................................................................................8-38
8.7.1.2 Detours........................................................................................................8-39
Stage Construction......................................................................................................8-40
Roadway Realignment................................................................................................8-40
8.8
EROSION AND SEDIMENTATION CONTROL...................................................8-41
8.9
REFERENCES ............................................................................................................8-41
CHAPTER 9
TABLE OF CONTENTS
9.1
INTRODUCTION......................................................................................................... 9-1
9.2
PLANNING AND COORDINATION ........................................................................ 9-2
9.2.1
9.2.2
9.3
9.3.1
9.3.2
9.4
9.4.1
9.5
9.5.1
9.5.2
Drainage Involvement by Others ................................................................................. 9-2
Cooperative Projects .................................................................................................... 9-3
DRAINAGE DATA ...................................................................................................... 9-3
Sources of Drainage Data ............................................................................................ 9-3
Types of Drainage Data ............................................................................................... 9-4
HYDROLOGY.............................................................................................................. 9-5
Calibration of Computations........................................................................................ 9-5
9.4.1.1 Hydrologic Calibration................................................................................. 9-5
9.4.1.2 Hydraulic Calibration................................................................................... 9-5
ELEMENTS OF SYSTEM DESIGN .......................................................................... 9-6
Pavement Drainage ...................................................................................................... 9-6
9.5.1.1 Curb and Gutter Flow................................................................................... 9-6
9.5.1.2 Surface Drainage of Pavements ................................................................... 9-8
9.5.1.2.1 Pavement Texture ...................................................................... 9-8
9.5.1.2.2 Mechanics of Flow on Pavements ............................................. 9-9
9.5.1.2.3 Ruts and Puddles ..................................................................... 9-12
9.5.1.3 Inlets ........................................................................................................... 9-12
9.5.1.3.1 Inlet Location........................................................................... 9-12
9.5.1.3.2 Inlet Size .................................................................................. 9-14
9.5.1.3.3 Grate Inlets .............................................................................. 9-14
9.5.1.3.4 Curb-Opening Inlets ................................................................ 9-17
9.5.1.3.5 Slotted Drain Inlets.................................................................. 9-18
9.5.1.3.6 Combination Inlets .................................................................. 9-19
9.5.1.3.7 Bridge Deck Inlets ................................................................... 9-20
9.5.1.4 Design Criteria ........................................................................................... 9-21
9.5.1.4.1 Flood Frequency ...................................................................... 9-22
9.5.1.4.2 Allowable Water Spread.......................................................... 9-23
9.5.1.4.3 Surface Water Depth................................................................ 9-24
Storm Drains .............................................................................................................. 9-24
9.5.2.1 Hydraulics of Storm Drains........................................................................ 9-25
9.5.2.1.1 Open-Channel vs. Pressure Flow............................................. 9-25
9.5.2.1.2 Storm Drain Outfalls................................................................ 9-27
9.5.2.1.3 Head Losses in Storm Drains .................................................. 9-28
9.5.2.1.4 Hydraulic Gradeline ................................................................ 9-29
Highway Drainage Guidelines
9-iv
9.5.2.2
9.5.3
9.5.4
9.5.5
Storm Drain Design Criteria ....................................................................... 9-30
9.5.2.2.1 Flood Frequency....................................................................... 9-31
9.5.2.2.2 Maximum Highwater ............................................................... 9-32
9.5.2.2.3 Minimum Velocities................................................................. 9-32
9.5.2.3 Design Process and System Planning ......................................................... 9-32
9.5.2.4 Appurtenant Structures ............................................................................... 9-34
9.5.2.4.1 Access Holes ............................................................................ 9-34
9.5.2.4.2 Junction Chambers ................................................................... 9-36
Roadside Channels .....................................................................................................9-37
Storage Facilities ........................................................................................................9-37
9.5.4.1 Types of Facilities....................................................................................... 9-38
9.5.4.1.1 Detention Facilities .................................................................. 9-39
9.5.4.1.2 Retention Facilities................................................................... 9-41
9.5.4.1.3 Recharge Facilities ................................................................... 9-42
9.5.4.2 Inflow and Outflow Structures.................................................................... 9-42
9.5.4.3 Safety .......................................................................................................... 9-43
Stormwater Pumping Stations ....................................................................................9-44
9.5.5.1 General Considerations............................................................................... 9-44
9.5.5.2 Location ...................................................................................................... 9-44
9.5.5.3 Hydrology ................................................................................................... 9-45
9.5.5.4 Storm Drain Collection Systems................................................................. 9-46
9.5.5.5 Types of Stations ........................................................................................ 9-46
9.5.5.6 Station Design............................................................................................. 9-47
9.5.5.6.1 Number and Capacity of Pumps............................................... 9-47
9.5.5.6.2 Sump Pump .............................................................................. 9-48
9.5.5.6.3 Types of Pumps........................................................................ 9-48
9.5.5.6.4 Types of Power ........................................................................ 9-48
9.5.5.6.5 Sump Design ............................................................................ 9-50
9.5.5.6.6 Pump On-Off Settings.............................................................. 9-50
9.5.5.6.7 Pump Controls.......................................................................... 9-51
9.5.5.6.8 Discharge Piping ...................................................................... 9-51
9.5.5.6.9 Flap Gates and Check Valves................................................... 9-51
9.5.5.6.10 Roof Hatches and Monorails.................................................. 9-51
9.5.5.6.11 Trash Racks and Grit Chambers............................................. 9-52
9.5.5.6.12 Ventilation.............................................................................. 9-52
9.5.5.6.13 Telemetering .......................................................................... 9-52
9.5.5.7 Specifications.............................................................................................. 9-52
9.5.5.8 Equipment Certification and Testing .......................................................... 9-52
9.5.5.9 Construction................................................................................................ 9-53
9.5.5.10 Maintenance.............................................................................................. 9-53
9.5.5.11 Retrofitting Existing Stations.................................................................... 9-53
9.5.5.12 Architectural Considerations .................................................................... 9-53
9.5.5.13 Total Annual Cost..................................................................................... 9-54
9.5.5.14 Safety ........................................................................................................ 9-54
9.5.5.15 Reliability ................................................................................................. 9-54
Storm Drain Systems
9.5.6
Outfalls....................................................................................................................... 9-54
9.5.6.1 General Outfall Considerations .................................................................. 9-55
9.5.6.2 Design and Analysis of Outfalls................................................................. 9-57
9.5.6.2.1 Hydrology................................................................................ 9-57
9.5.6.2.2 Hydraulics................................................................................ 9-58
Siphons....................................................................................................................... 9-59
9.5.7.1 Systems for Draining Subsurface Water .................................................... 9-60
9.5.7.2 Horizontal Drains ....................................................................................... 9-62
9.5.7.3 Pipe Underdrains ........................................................................................ 9-63
9.5.7.4 Vertical Wells............................................................................................. 9-65
9.5.7.5 Subgrade Drainage Systems....................................................................... 9-66
9.5.7.6 Edge Drain Collector Systems ................................................................... 9-67
9.5.7
9.6
FACTORS AFFECTING DESIGN........................................................................... 9-68
9.6.1
9.6.2
9.6.3
9.6.4
9.6.5
9.6.6
9.7
Utilities....................................................................................................................... 9-68
Easements .................................................................................................................. 9-70
Service Life of Drainage Pipe.................................................................................... 9-71
Erosion, Sedimentation, and Water Quality............................................................... 9-74
Attractive Nuisance Mitigations ................................................................................ 9-74
Legal Factors.............................................................................................................. 9-75
COMPUTER MODELS ............................................................................................. 9-76
9.7.1
9.7.2
General Features ........................................................................................................ 9-76
Hydrology .................................................................................................................. 9-77
9.7.2.1 Peak Discharge Determination ................................................................... 9-77
9.7.2.2 Surface Hydrologic Simulation .................................................................. 9-78
Storm Drain Inlets...................................................................................................... 9-78
Underground Appurtenances ..................................................................................... 9-79
9.7.4.1 Conduit Systems......................................................................................... 9-79
9.7.4.1.1 Hydraulic Gradeline ................................................................ 9-79
9.7.4.1.2 Supercritical Flow.................................................................... 9-80
9.7.4.2 Other Losses............................................................................................... 9-80
Water Quality............................................................................................................. 9-80
Cost Estimating.......................................................................................................... 9-80
9.7.3
9.7.4
9.7.5
9.7.6
9.8
DOCUMENTATION.................................................................................................. 9-81
9.9
CONSTRUCTION ...................................................................................................... 9-82
9.9.1
9.9.2
Construction-Related Hydraulic Considerations ....................................................... 9-82
Hydraulic-Related Construction Considerations ....................................................... 9-85
9.10
MAINTENANCE...................................................................................................... 9-86
9.10.1
9.10.2
Maintenance-Related Hydraulic Considerations ..................................................... 9-86
Hydraulic-Related Maintenance Considerations ..................................................... 9-87
9.10.2.1 Maintenance Inspections .......................................................................... 9-87
9-v
Highway Drainage Guidelines
9-vi
9.10.2.2
9.11
Performance Evaluation............................................................................ 9-87
REFERENCES ..........................................................................................................9-87
CHAPTER 10
TABLE OF CONTENTS
10.1
INTRODUCTION..................................................................................................... 10-1
10.1.1 Purpose of the Chapter.............................................................................................. 10-2
10.1.2 Role of the Hydraulics Engineer............................................................................... 10-2
10.1.3 Environmentally Sensitive Surface Water Environments......................................... 10-4
10.1.4 Surface Water Effect and Significance ..................................................................... 10-4
10.1.5 Threshold Value........................................................................................................ 10-5
10.1.6 Reversible and Irreversible Effects ........................................................................... 10-5
10.1.7 Legal Constraints ...................................................................................................... 10-5
10.1.8 Organization of the Chapter...................................................................................... 10-6
10.1.9 Survey and Data Collection ...................................................................................... 10-7
10.1.10 Implementing Environmental Plans........................................................................ 10-7
10.2
ESTIMATING SURFACE WATER EFFECTS.................................................... 10-7
10.2.1 Quantity Effects ........................................................................................................ 10-7
10.2.1.1 Surface Water Inventory .......................................................................... 10-8
10.2.1.2 Floodplain Inventory.............................................................................. 10-10
10.2.1.3 Runoff Peaks and Volumes.................................................................... 10-10
10.2.2 Quality Effects ........................................................................................................ 10-16
10.2.2.1 Sediment ................................................................................................. 10-16
10.2.2.1.1 Sediment Transport.............................................................. 10-16
10.2.2.1.2 Sediment Data...................................................................... 10-17
10.2.2.1.3 Estimating Existing Sediment Regimes............................... 10-18
10.2.2.1.4 Construction Erosion ........................................................... 10-19
10.2.2.1.5 Sediment Effects.................................................................. 10-21
10.2.2.1.6 Significance of Sediment Effects......................................... 10-26
10.2.2.2 Mineral and Chemical............................................................................. 10-26
10.2.2.3 Character and Aesthetics......................................................................... 10-35
10.2.2.3.1 Character.............................................................................. 10-35
10.2.2.3.2 Aesthetics............................................................................. 10-35
10.2.3 Climatological Effects ............................................................................................ 10-36
10.2.3.1 Data......................................................................................................... 10-36
10.2.3.2 Temperature ............................................................................................ 10-37
10.2.3.3 Season Delimitation ................................................................................ 10-38
10.2.3.4 Precipitation ............................................................................................ 10-38
10.2.4 Biological Effects.................................................................................................... 10-40
10.2.5 Socioeconomic Effects............................................................................................ 10-41
10.3
MITIGATING SURFACE WATER EFFECTS .................................................. 10-41
10.3.1 Quantity Effects ...................................................................................................... 10-42
10.3.1.1 Surface Water Inventory ......................................................................... 10-43
10.3.1.2 Floodplains.............................................................................................. 10-43
10.3.1.3 Runoff Peaks and Volumes..................................................................... 10-44
10-iv
Highway Drainage Guidelines
10.3.2 Quality Effects.........................................................................................................10-47
10.3.2.1 Erosion and Sediment.............................................................................10-47
10.3.2.2 Mineral and Chemical ............................................................................10-47
10.3.2.3 Character and Aesthetics ........................................................................10-51
10.3.3 Climatological Effects .............................................................................................10-51
10.4
STREAM MODIFICATION AND MITIGATIVE PRACTICES......................10-51
10.4.1 Need for Mitigative Practices ..................................................................................10-51
10.4.2 Design Discharge for Fish Migration ......................................................................10-52
10.4.3 Fish Passage through Highway Structures ..............................................................10-54
10.4.3.1 Fish Migrations........................................................................................10-55
10.4.3.2 Swimming Capability of Fish..................................................................10-56
10.4.3.3 Types of Stream Fisheries .......................................................................10-58
10.4.3.3.1 Tributary Streams to Large Lakes and Rivers......................10-58
10.4.3.3.2 Steep Mountain Streams.......................................................10-58
10.4.3.3.3 Intermittent Streams .............................................................10-58
10.4.3.3.4 Groundwater Streams...........................................................10-58
10.4.3.3.5 Estuarine Streams.................................................................10-59
10.4.3.4 Data Collection for Stream Modifications...............................................10-59
10.4.3.5 Fish Passage Design ................................................................................10-60
10.4.3.5.1 Flow Depths .........................................................................10-61
10.4.3.5.2 Structures with Natural Substrate Bottoms ..........................10-63
10.4.3.5.3 Baffled and Sill Structures ...................................................10-65
10.4.3.5.4 Slot Orifice Structures..........................................................10-67
10.4.4 Mitigating Channel Modification Effects................................................................10-68
10.4.4.1 Amount of Habitat...................................................................................10-69
10.4.4.2 Quality of Habitat ....................................................................................10-69
10.4.4.2.1 Geomorphology....................................................................10-70
10.4.4.2.2 Pool and Riffle Sequence .....................................................10-72
10.4.4.2.3 Substrate ...............................................................................10-74
10.4.4.2.4 Stream and Bank Cover........................................................10-75
10.4.4.2.5 Minimum Flow Requirements .............................................10-78
10.4.4.2.6 Riparian Vegetation..............................................................10-78
10.4.4.2.7 Water Quality .......................................................................10-79
10.4.4.2.8 Construction Coordination ...................................................10-80
10.5
10.5.1
10.5.2
10.5.3
10.5.4
LAKE AND POND MODIFICATIONS AND MITIGATING PRACTICES ...10-80
Features ...................................................................................................................10-80
Ecosystems ..............................................................................................................10-81
Water Characteristics...............................................................................................10-82
Classifications..........................................................................................................10-83
10.5.4.1 Source.....................................................................................................10-84
10.5.4.2 Physical ..................................................................................................10-84
10.5.4.3 Size .........................................................................................................10-85
10.5.4.4 Productivity ............................................................................................10-85
10.5.5 Reservoir-Related Problems ....................................................................................10-86
Evaluating Highway Effects on Surface Water Environments
10.5.6 Sedimentation ......................................................................................................... 10-87
10.5.7 Nutrient Enrichment................................................................................................ 10-88
10.5.8 Other Pollutants ...................................................................................................... 10-89
10.5.9 Circulation............................................................................................................... 10-90
10.5.10 Sensitivity ............................................................................................................. 10-90
10.5.11 Evaluating Effects................................................................................................. 10-91
10.5.12 Data Collection for Lakes and Ponds.................................................................... 10-93
10.5.13 Mitigating Measures ............................................................................................. 10-94
10.5.13.1 Migration Routes .................................................................................. 10-94
10.5.13.2 Lake Size............................................................................................... 10-95
10.5.13.3 Littoral Zone Substrate ......................................................................... 10-95
10.5.13.4 Diversion............................................................................................... 10-96
10.5.14 Ponds..................................................................................................................... 10-96
10.6
WETLANDS MODIFICATIONS AND MITIGATING PRACTICES............. 10-96
10.6.1 Introduction............................................................................................................. 10-96
10.6.1.1 Function of Wetlands.............................................................................. 10-98
10.6.1.2 Regulatory Requirements for Wetlands Considerations ......................... 10-98
10.6.2 Classification of Wetlands .................................................................................... 10-100
10.6.3 Wetlands Hydrology ............................................................................................. 10-103
10.6.4 Potential Highway Effects to Wetlands ................................................................ 10-103
10.6.4.1 Water Depth .......................................................................................... 10-103
10.6.4.2 Seasonal Patterns .................................................................................. 10-104
10.6.4.3 Erosion and Sedimentation ................................................................... 10-105
10.6.4.4 Suspended Solid Concentration ............................................................ 10-105
10.6.4.5 Chemical Regime.................................................................................. 10-106
10.6.4.6 Dissolved Oxygen (DO) Regime .......................................................... 10-107
10.6.4.7 Nutrient Loading................................................................................... 10-107
10.6.4.8 Pollutants .............................................................................................. 10-108
10.6.5 Estimating Highway Effects on Wetlands ............................................................ 10-109
10.6.5.1 Baseline Data ........................................................................................ 10-109
10.6.5.2 Highway Location................................................................................. 10-110
10.6.5.3 Water Quality........................................................................................ 10-110
10.6.5.4 Wetlands Hydrology ............................................................................. 10-111
10.6.5.5 Evaluation of Mitigation Alternatives................................................... 10-111
10.6.6 Mitigation Measures ............................................................................................. 10-112
10.6.6.1 Location ................................................................................................ 10-112
10.6.6.2 Tidal Wetlands Crossings ..................................................................... 10-113
10.6.6.3 Inland Wetlands Crossing Hydraulics................................................... 10-115
10.6.6.4 Erosion and Sedimentation ................................................................... 10-117
10.6.6.5 Migration of Aquatic Biota................................................................... 10-117
10.6.6.6 Constructing New Habitat..................................................................... 10-117
10.6.6.7 Compensation ....................................................................................... 10-119
10.6.7 Wetlands Banking................................................................................................. 10-119
10.7
SURVEYS AND DATA COLLECTION ............................................................ 10-120
10-v
10-vi
Highway Drainage Guidelines
10.7.1 Determining the Need............................................................................................10-121
10.7.1.1 Project Nature........................................................................................10-121
10.7.1.2 Public Input ...........................................................................................10-121
10.7.1.3 Development by Others.........................................................................10-122
10.7.2 Level of Effort .......................................................................................................10-122
10.7.3 Office Survey and Data Collection........................................................................10-122
10.7.3.1 Data Availability ...................................................................................10-122
10.7.3.2 Preliminary Environmental Estimates ...................................................10-124
10.7.4 Limited Field Studies ............................................................................................10-124
10.7.5 Complete Field Studies..........................................................................................10-125
10.7.5.1 Determining General Objectives ...........................................................10-125
10.7.5.2 Selecting Study Parameters ...................................................................10-126
10.7.5.3 Selecting Sampling Sites .......................................................................10-126
10.7.5.4 Temporal Sample Distribution ..............................................................10-127
10.7.5.5 Sampling Frequency..............................................................................10-127
10.7.6 Study Level............................................................................................................10-127
10.7.6.1 Planning Level.......................................................................................10-127
10.7.6.2 Project Level..........................................................................................10-128
10.7.6.3 Construction Level ................................................................................10-129
10.8
REFERENCES ......................................................................................................10-129
CHAPTER 11
TABLE OF CONTENTS
11.1
INTRODUCTION..................................................................................................... 11-1
11.1.1 Purpose of this Chapter ............................................................................................. 11-1
11.1.2 Areas Covered by this Chapter ................................................................................. 11-1
11.1.2.1 Coastal Zone ............................................................................................ 11-1
11.1.2.2 Lakeshore................................................................................................. 11-1
11.1.3 Role of the Hydraulics Engineer............................................................................... 11-1
11.1.3.1 Highway Planning Considerations........................................................... 11-2
11.1.3.2 Highway Location Considerations........................................................... 11-2
11.1.3.3 Legal Considerations ............................................................................... 11-2
11.1.3.4 Construction Considerations.................................................................... 11-2
11.1.4 References................................................................................................................. 11-2
11.2
DATA COLLECTION ............................................................................................. 11-3
11.2.1 Hydrologic and Hydraulic Data ................................................................................ 11-3
11.2.1.1 Meteorological Data................................................................................. 11-3
11.2.1.2 Hydraulic Data ......................................................................................... 11-3
11.2.2 Engineering Data ...................................................................................................... 11-4
11.2.3 Environmental Data .................................................................................................. 11-4
11.2.4 Regulatory Data ........................................................................................................ 11-4
11.3
SHORELINE TOPOGRAPHY ............................................................................... 11-4
11.3.1 Beaches ..................................................................................................................... 11-5
11.3.1.1 Sand Beaches ........................................................................................... 11-6
11.3.1.2 Sand Dunes .............................................................................................. 11-6
11.3.1.3 Non-Sand Beaches ................................................................................... 11-7
11.3.1.4 Beach Sediment Variability ..................................................................... 11-7
11.3.2 Bluffs and Headlands................................................................................................ 11-7
11.3.3 Mud Flats .................................................................................................................. 11-8
11.3.4 Wetlands ................................................................................................................... 11-8
11.3.5 Estuaries.................................................................................................................... 11-9
11.3.6 Inlets.......................................................................................................................... 11-9
11.3.7 Lake Shoreline ........................................................................................................ 11-10
11.4
CHARACTERISTICS OF WAVES AND CURRENTS ..................................... 11-11
11.4.1 Overview................................................................................................................. 11-11
11.4.2 Gravity Waves ........................................................................................................ 11-11
11.4.2.1 Wave Characteristics ............................................................................. 11-11
11.4.2.2 Relative Depth ....................................................................................... 11-12
11.4.2.3 Wave Generation ................................................................................... 11-12
11.4.3 Wave Transformations............................................................................................ 11-13
11.4.3.1 Shoaling ................................................................................................. 11-13
11-iv
11.4.4
11.4.5
11.4.6
11.4.7
11.4.8
11.4.9
11.5
Highway Drainage Guidelines
11.4.3.2 Breaking ..................................................................................................11-13
11.4.3.3 Refraction ................................................................................................11-13
11.4.3.4 Diffraction ...............................................................................................11-14
11.4.3.5 Decay.......................................................................................................11-14
Tsunamis..................................................................................................................11-14
Tides ........................................................................................................................11-14
Currents ...................................................................................................................11-15
Storm Surges ...........................................................................................................11-15
Longshore Current...................................................................................................11-16
Longshore Transport ...............................................................................................11-16
DESIGN CONSIDERATIONS ..............................................................................11-16
11.5.1 Introduction .............................................................................................................11-16
11.5.2 Design Waves..........................................................................................................11-17
11.5.2.1 Wave Distribution ...................................................................................11-17
11.5.2.2 Source of Wave Observation Data ..........................................................11-17
11.5.2.3 Wind and Wave Setup .............................................................................11-17
11.5.2.4 Waves over Flooded Areas......................................................................11-18
11.5.3 Wave Action on Lakes and Tidal Basins.................................................................11-18
11.5.4 Tidal Flow through Structures.................................................................................11-18
11.5.5 Scour........................................................................................................................11-19
11.5.6 Environmental Concerns .........................................................................................11-20
11.6
SHORE PROTECTION DEVICES.......................................................................11-20
11.6.1 Seawalls...................................................................................................................11-21
11.6.2 Revetments ..............................................................................................................11-22
11.6.2.1 Rigid Revetment......................................................................................11-22
11.6.2.2 Flexible Revetment..................................................................................11-23
11.6.3 Retards.....................................................................................................................11-24
11.6.4 Jetties .......................................................................................................................11-24
11.6.4.1 Stone........................................................................................................11-25
11.6.4.2 Precast Concrete Blocks ..........................................................................11-25
11.6.5 Groins ......................................................................................................................11-26
11.6.5.1 Materials ..................................................................................................11-26
11.6.5.1.1 Stone....................................................................................11-27
11.6.5.1.2 Concrete ..............................................................................11-27
11.6.5.1.3 Steel.....................................................................................11-27
11.6.5.1.4 Timber .................................................................................11-27
11.6.5.2 Alignment ................................................................................................11-28
11.6.5.3 Permeability.............................................................................................11-28
11.6.6 Breakwater Systems ................................................................................................11-28
11.6.7 Bulkheads ................................................................................................................11-30
11.6.8 Beach Nourishment .................................................................................................11-31
11.7
PLANNING FOR SHORELINE CHANGES .......................................................11-31
11.7.1 Shoreline Changes ...................................................................................................11-31
Highways along Coastal Zones and Lakeshores
11.7.2 Rate of Shoreline Change ....................................................................................... 11-32
11.7.3 Sea Level Rise......................................................................................................... 11-32
11.7.4 Considerations in Planning for the Future .............................................................. 11-33
11.7.4.1 Condition Assessment............................................................................ 11-33
11.7.4.2 Objectives of a Vulnerability Study....................................................... 11-33
11.7.4.3 Coastal Highway Vulnerability Model .................................................. 11-34
11.7.4.4 Emergency Evacuation Plan .................................................................. 11-35
11.8
CONSTRUCTION AND MAINTENANCE CONSIDERATIONS ................... 11-35
11.8.1 Construction-Related Considerations ..................................................................... 11-35
11.8.2 Maintenance-Related Considerations ..................................................................... 11-35
11.8.2.1 Maintenance during Construction.......................................................... 11-36
11.8.2.2 Perpetual Maintenance........................................................................... 11-36
11.9
REFERENCES........................................................................................................ 11-36
11.9.1 Cited References ..................................................................................................... 11-36
11.9.2 Additional Publications........................................................................................... 11-37
11.9.3 Web-Related References......................................................................................... 11-39
11-v
CHAPTER 12
TABLE OF CONTENTS
12.1
INTRODUCTION..................................................................................................... 12-1
12.1.1 History ...................................................................................................................... 12-1
12.1.2 Potential Impacts....................................................................................................... 12-2
12.2
REGULATIONS ....................................................................................................... 12-4
12.2.1 Federal Regulations .................................................................................................. 12-4
12.2.2 State and Local Regulations...................................................................................... 12-6
12.3
IMPLEMENTATION OF STORMWATER MANAGEMENT DESIGN.......... 12-7
12.3.1 Best Management Practices ...................................................................................... 12-9
12.3.2 Facility Types.......................................................................................................... 12-10
12.3.2.1 Enhanced Ponds: Wet Detention/Extended Detention ............................ 12-10
12.3.2.1.1 Wet Detention Ponds ........................................................... 12-10
12.3.2.1.2 Extended Detention Ponds................................................... 12-11
12.3.2.2 Stormwater Wetland Basins .................................................................... 12-11
12.3.2.3 Infiltration Facilities ................................................................................ 12-12
12.3.2.4 Filtering Devices ..................................................................................... 12-13
12.3.2.5 Water Quality Inlets ................................................................................ 12-13
12.3.2.5.1 Vegetative Systems.............................................................. 12-14
12.3.3 Design Aspects........................................................................................................ 12-15
12.3.3.1 Ponds ....................................................................................................... 12-15
12.3.3.2 Infiltration Facilities ................................................................................ 12-17
12.3.3.3 Filtering Devices ..................................................................................... 12-18
12.3.4 Documentation........................................................................................................ 12-19
12.4
CONSTRUCTION.................................................................................................. 12-20
12.4.1 Pre-Bid/Preconstruction Conference....................................................................... 12-21
12.4.2 Construction of Best Management Practices .......................................................... 12-21
12.4.2.1 Ponds/Basins ........................................................................................... 12-22
12.4.2.2 Infiltration Facilities/Filtering Devices ................................................... 12-23
12.4.2.3 Vegetative Systems ................................................................................. 12-23
12.4.3 Grading ................................................................................................................... 12-24
12.4.4 Inspection/Quality Control...................................................................................... 12-25
12.4.5 Certification and Documentation ............................................................................ 12-25
12.4.6 Training and Communication ................................................................................. 12-26
12.5
12.5.1
12.5.2
12.5.3
12.5.4
MAINTENANCE.................................................................................................... 12-26
Maintenance/Inspection Programs.......................................................................... 12-27
Documentation........................................................................................................ 12-28
Maintainability........................................................................................................ 12-29
Budgetary Requirements......................................................................................... 12-30
12-iv
Highway Drainage Guidelines
12.5.5 Training ...................................................................................................................12-30
12.6 FUTURE ISSUES.....................................................................................................12-31
12.7 CITED REFERENCES............................................................................................12-32
12.8 BIBLIOGRAPHY.....................................................................................................12-32
APPENDIX 12A COMMON HIGHWAY RUNOFF CONSTITUENTS AND
THEIR PRIMARY SOURCES ........................................................12-35
APPENDIX 12B DAM INSPECTION CHECKLIST...................................................12-36
APPENDIX 12C A COMPARATIVE ASSESSMENT OF THE
EFFECTIVENESS OF CURRENT URBAN BEST
MANAGEMENT PRACTICES .........................................................12-41
CHAPTER 13
TABLE OF CONTENTS
13.1
INTRODUCTION................................................................................................... 13-1
13.2
RECOMMENDED KNOWLEDGE OF RELATED DISCIPLINES ................ 13-1
13.2.1
13.2.2
13.2.3
13.2.4
13.2.5
13.2.6
13.2.7
13.2.8
13.2.9
13.2.10
13.2.11
13.2.12
13.2.13
13.2.14
13.3
13.3.1
13.3.2
13.3.3
13.3.4
13.3.5
13.3.6
13.3.7
13.4
13.4.1
13.4.2
13.4.3
13.4.4
13.5
Planning and Environmental Assessment .............................................................. 13-2
Right-of-Way ......................................................................................................... 13-2
Surveying ............................................................................................................... 13-2
Agency Policy and Procedures .............................................................................. 13-3
Roadway Design .................................................................................................... 13-3
Pavement Design ................................................................................................... 13-3
Structural Design ................................................................................................... 13-4
Environmental Design ........................................................................................... 13-4
Geotechnical Design .............................................................................................. 13-4
Material Properties................................................................................................ 13-5
Quantity and Cost Estimation ............................................................................... 13-5
Construction.......................................................................................................... 13-5
Maintenance.......................................................................................................... 13-5
Legal Issues........................................................................................................... 13-6
CAREER DEVELOPMENT ................................................................................. 13-6
Mastery of Basic Drainage Design Technology .................................................... 13-6
Experience in Basic Drainage Design.................................................................... 13-7
Professional Engineer Status.................................................................................. 13-7
Training and Education in Advanced Hydraulics Engineering.............................. 13-7
Instructing and Public Speaking ............................................................................ 13-8
Experience in Complex Hydraulic Design............................................................. 13-9
Management Training and Experience .................................................................. 13-9
OTHER ISSUES ................................................................................................... 13-10
Research and Development.................................................................................. 13-10
Continuing Education Credit ............................................................................... 13-10
Record Keeping ................................................................................................... 13-11
Engineer Versus Technician ................................................................................ 13-12
CONCLUSION ..................................................................................................... 13-12
APPENDIX 13A BASIC HIGHWAY DRAINAGE DESIGN INTRODUCTION .. 13-14
APPENDIX 13B GENERAL HIGHWAY DRAINAGE DESIGN ............................ 13-17
APPENDIX 13C URBAN STORM DRAIN DESIGN................................................ 13-31
APPENDIX 13D BRIDGE WATERWAYS ANALYSIS ........................................... 13-36
13-iv
Highway Drainage Guidelines
APPENDIX 13E RIVER MECHANICS ......................................................................13-39
CHAPTER 14
TABLE OF CONTENTS
14.1
INTRODUCTION..................................................................................................... 14-1
14.2
INSPECTION............................................................................................................ 14-2
14.2.1 Safety ........................................................................................................................ 14-2
14.2.1.1 OSHA Regulations .................................................................................... 14-2
14.2.1.2 Special Considerations .............................................................................. 14-2
14.2.2 Frequency of Inspection............................................................................................ 14-3
14.2.3 Inspection Protocol ................................................................................................... 14-4
14.2.3.1 Preliminary Information ............................................................................ 14-4
14.2.3.2 Equipment Needed .................................................................................... 14-5
14.2.3.3 Inspector Qualifications ............................................................................ 14-5
14.2.4 Site Visit ................................................................................................................... 14-6
14.2.4.1 Roadway Surface and Side Slopes ............................................................ 14-6
14.2.4.2 Waterway .................................................................................................. 14-7
14.2.4.2.1 Culvert Inlet ........................................................................... 14-7
14.2.4.2.2 Culvert Outlet ........................................................................ 14-7
14.2.4.2.3 Inside of Culvert .................................................................... 14-8
14.2.5 Special Investigation Procedures ............................................................................ 14-10
14.2.5.1 Failure of a Culvert during Construction................................................. 14-10
14.2.5.2 Failure of an Existing Culvert ................................................................. 14-12
14.2.5.3 Special Techniques.................................................................................. 14-13
14.2.6 Inspection Report Form and Documentation .......................................................... 14-13
14.2.7 Distribution of Information..................................................................................... 14-13
14.3
FACTORS INFLUENCING SERVICE LIFE ..................................................... 14-14
14.3.1 Corrosion ................................................................................................................ 14-15
14.3.1.1 Hydrogen Ion Concentration (pH)........................................................... 14-16
14.3.1.2 Soil Resistivity ........................................................................................ 14-16
14.3.1.3 Chlorides ................................................................................................. 14-17
14.3.1.4 Sulfates .................................................................................................... 14-17
14.3.1.5 Industrial Effluents .................................................................................. 14-17
14.3.1.6 Stray Electrical Current ........................................................................... 14-17
14.3.2 Abrasion.................................................................................................................. 14-18
14.3.2.1 Debris ...................................................................................................... 14-18
14.3.2.2 Bedload.................................................................................................... 14-19
14.3.3 Loss of Structural Integrity ..................................................................................... 14-20
14.3.3.1 Joint Separation ....................................................................................... 14-20
14.3.3.2 Misalignment........................................................................................... 14-21
14.3.3.3 Deflection ................................................................................................ 14-21
14.3.3.4 Seam Defects........................................................................................... 14-22
14.3.3.5 Stream/Roadway Embankment Erosion.................................................. 14-22
14-iv
14.4
Highway Drainage Guidelines
MATERIAL SELECTION AND ESTIMATING SERVICE LIFE ...................14-22
14.4.1 Culvert Materials .....................................................................................................14-23
14.4.1.1 Concrete Culverts..................................................................................... 14-23
14.4.1.2 Corrugated Metal Pipe ............................................................................. 14-24
14.4.1.2.1 Corrugated Steel Pipe........................................................... 14-24
14.4.1.2.2 Corrugated Aluminum Pipe ................................................. 14-25
14.4.1.3 Plastic Pipe............................................................................................... 14-25
14.4.1.3.1 High-Density Polyethylene .................................................. 14-26
14.4.1.3.2 Polyvinyl Chloride ............................................................... 14-26
14.4.1.4 Other ........................................................................................................ 14-27
14.4.2 Protective Coatings..................................................................................................14-27
14.4.2.1 Zinc Galvanizing...................................................................................... 14-27
14.4.2.2 Aluminizing ............................................................................................. 14-27
14.4.2.3 Asphaltic Coatings ................................................................................... 14-28
14.4.2.3.1 Bituminous ........................................................................... 14-28
14.4.2.3.2 Fiber-Bonded Bituminous .................................................... 14-29
14.4.2.3.3 Asphalt Mastic...................................................................... 14-29
14.4.2.3.4 Polymerized Asphalt ............................................................ 14-29
14.4.2.4 Polymeric Sheet Coating.......................................................................... 14-29
14.4.2.5 Concrete Coatings.................................................................................... 14-29
14.4.3 Alternative Materials ...............................................................................................14-30
14.5
CULVERT REPAIR ...............................................................................................14-30
14.5.1 Rehabilitation Methods ...........................................................................................14-31
14.5.1.1 Grouting ................................................................................................... 14-31
14.5.1.2 Invert Paving and Plating......................................................................... 14-31
14.5.1.3 Sliplining.................................................................................................. 14-32
14.5.1.4 Trenchless Relining ................................................................................. 14-34
14.5.1.4.1 Cured-in-Place Pipe Lining .................................................. 14-35
14.5.1.4.2 Fold and Form Lining .......................................................... 14-36
14.5.1.4.3 Polyvinyl Chloride (PVC) Pipe Lining System ................... 14-37
14.5.1.4.4 Pipe Bursting ........................................................................ 14-37
14.5.1.4.5 Joint Sealing ......................................................................... 14-38
14.5.1.5 Cement Mortar Lining ............................................................................. 14-38
14.5.1.6 Epoxy Lining ........................................................................................... 14-38
14.5.2 Replacement Methods .............................................................................................14-38
14.5.2.1 Open Cut (Trench) Method...................................................................... 14-39
14.5.2.2 Trenchless Excavation Construction (TEC) Methods ............................. 14-39
14.5.2.2.1 Pipe Jacking ......................................................................... 14-39
14.5.2.2.2 Horizontal Earth Boring (HEB) ........................................... 14-41
14.5.2.2.3 Utility Tunnels (UT)............................................................. 14-42
14.6
REFERENCES ........................................................................................................14-43
APPENDIX 14A .................................................................................................................14-45
14A.1 Culvert Inspection Rating Form (Adapted from Reference (12))............................14-45
Culvert Inspection, Material Selection, and Rehabilitation Guideline
14A.2 CalTrans Guide for the Protection of Reinforced and Unreinforced
Concrete against Acid and Sulfate Exposure Conditions ........................................ 14-46
14A.3 U.S. Bureau of Reclamation Criteria for Sulfate-Resisting Concrete Pipe............. 14-46
14A.4 CalTrans Service Life Chart.................................................................................... 14-47
14A.5 AISI Service Life Chart (22)................................................................................... 14-48
14-v
CHAPTER 15
TABLE OF CONTENTS
15.1
INTRODUCTION..................................................................................................... 15-1
15.1.1
15.2
TYPES OF CONSULTANT CONTRACTS........................................................... 15-2
15.2.1
15.2.2
15.2.3
15.2.4
15.2.5
15.2.6
15.2.7
15.3
15.3.4
15.3.5
15.3.6
Elements of RFP .......................................................................................... 15-7
Schedules ..................................................................................................... 15-7
Other Contract Requirements ...................................................................... 15-8
SCOPE OF SERVICES ............................................................................................ 15-8
15.5.1
15.5.2
15.5.3
15.5.4
15.5.5
15.5.6
15.5.7
15.5.8
15.5.9
15.6
Advertisement or Solicitation ...................................................................... 15-4
Expressions of Interest................................................................................. 15-5
Short Listing (or Qualification Screening) .................................................. 15-5
15.3.3.1 Criteria ...................................................................................... 15-5
Request for Proposals .................................................................................. 15-6
Review and Selection .................................................................................. 15-6
Debriefing .................................................................................................... 15-7
REQUEST FOR PROPOSALS (RFP) .................................................................... 15-7
15.4.1
15.4.2
15.4.3
15.5
Project Site-Specific Contracts .................................................................... 15-2
On-Call Contracts ........................................................................................ 15-2
Combination Contracts ................................................................................ 15-3
Support Services .......................................................................................... 15-3
General Engineering Consultants ................................................................ 15-3
Design-Build Contracts ............................................................................... 15-3
Inspection and Remedial Maintenance ........................................................ 15-4
CONSULTANT SELECTION PROCESS ............................................................. 15-4
15.3.1
15.3.2
15.3.3
15.4
In-house vs. Consultant ............................................................................... 15-2
Purpose ........................................................................................................ 15-8
Duration ....................................................................................................... 15-8
Elements of Work ........................................................................................ 15-8
Staffing Required......................................................................................... 15-9
Labor and Cost............................................................................................. 15-9
Support Services .......................................................................................... 15-9
Consultant Responsibilities ......................................................................... 15-9
Certification ............................................................................................... 15-10
Items Provided by the Agency................................................................... 15-10
QUALIFICATIONS OF CONSULTANT HYDRAULICS ENGINEERS ........ 15-10
15.6.1
15.6.2
15.6.3
15.6.4
Firm qualifications..................................................................................... 15-10
Hydraulics Engineering Group .................................................................. 15-10
Survey Support .......................................................................................... 15-12
CADD Support .......................................................................................... 15-12
15-iv
Highway Drainage Guidelines
15.6.5
15.6.6
15.6.7
15.6.8
15.6.9
15.6.10
15.7
TECHNICAL PROPOSAL REVIEW .................................................................. 15-13
15.7.1
15.7.2
15.7.3
15.7.4
15.7.5
15.7.6
15.7.7
15.8
Performance Specification......................................................................... 15-15
Mile Stone Review .................................................................................... 15-16
Regulatory Permits .................................................................................... 15-16
INSPECTION AND REMEDIATION (IR) CONTRACTS.............................. 15-16
15.10.1
15.10.2
15.10.3
15.10.4
15.11
Hydraulics Engineer in a Lead Role .......................................................... 15-15
Hydraulics Engineer in a Supporting Role ................................................ 15-15
DESIGN-BUILD CONTRACTS............................................................................ 15-15
15.9.1
15.9.2
15.9.3
15.10
Review Criteria.......................................................................................... 15-13
Review Team ............................................................................................. 15-13
Scoring Scale ............................................................................................. 15-14
Review Notes............................................................................................. 15-14
Interviews or Oral Presentation ................................................................. 15-14
Scoring....................................................................................................... 15-14
Consultant Selection .................................................................................. 15-14
PROJECT SPECIFIC CONTRACTS................................................................... 15-15
15.8.1
15.8.2
15.9
Structural and Geotechnical Engineering Support..................................... 15-12
Highway Engineering Support .................................................................. 15-12
Regulatory Permitting................................................................................ 15-12
Wetlands .................................................................................................... 15-12
Other Support ............................................................................................ 15-13
Past Performance ..................................................................................... 15-13
Inspection................................................................................................. 15-16
Remediation Design................................................................................. 15-17
Permits ..................................................................................................... 15-17
Implementation ........................................................................................ 15-17
ASSIGNMENTS TO ON-CALL CONSULTANTS........................................... 15-17
15.11.1
15.11.2
15.11.3
15.11.4
15.11.5
15.11.6
15.11.7
Task Scope............................................................................................... 15-17
Independent Estimate............................................................................... 15-17
Consultant Task Proposal and Cost Estimate .......................................... 15-18
Negotiation .............................................................................................. 15-18
Notice to Proceed..................................................................................... 15-18
Clear Direction ........................................................................................ 15-18
Project Management ................................................................................ 15-18
15.12
EVALUATION OF CONSULTING ENGINEERS’ WORK............................ 15-18
15.13
SUB-CONSULTANTS.......................................................................................... 15-19
15.14
DEVELOPMENT OF CONSULTANT BASE ................................................... 15-19
15.14.1
15.14.2
Progressive Contracts and Assignments .................................................. 15-20
Multiple Smaller Contracts...................................................................... 15-20
Guidelines for Selecting and Utilizing Hydraulics Engineering Consultants
15.14.3
15.15
15-v
Training ................................................................................................... 15-20
REFERENCES ...................................................................................................... 15-21
Appendix 15A
STANDARD FORM 330 FOR DOCUMENTING FIRM
QUALIFICATIONS............................................................................... 15-23
Appendix 15B
SAMPLE CRITERIA FOR EVALUATING FIRM
QUALIFICATIONS............................................................................... 15-37
Appendix 15C
SAMPLE EVALUATION AND RATING CRITERIA FOR
CONSULTANT PROPOSALS ............................................................. 15-38
Appendix 15D
SAMPLE DESIGN AND REPORT STANDARDS............................ 15-41
Appendix 15E
SAMPLE SCOPE OF SERVICES ....................................................... 15-62
GLOSSARY OF HIGHWAY-RELATED DRAINAGE TERMS
TABLE OF CONTENTS
G.1
OVERVIEW.............................................................................................................. G-1
G.2
GLOSSARY .............................................................................................................. G-3
G.3
REFERENCES...................................................................................................... G-151
G.3.1 Cited References ................................................................................................... G-151
G.3.2 Uncited .................................................................................................................. G-156
CHAPTER 1
HYDRAULIC CONSIDERATIONS IN
HIGHWAY PLANNING AND LOCATION
Chapter 1
Hydraulic Considerations in Highway
Planning and Location
1.1 INTRODUCTION
The planning and locating of highway facilities are the first steps in a challenging process of
providing a safe and efficient transportation system. Hydrologic and hydraulic requirements are
among the facets that must be considered during the early phases of the design process.
Water and its related resources are important considerations in the planning and locating of highways
and their appurtenant facilities. Although historically only major drainage features (e.g., large rivers,
environmentally sensitive areas) have been considered during these early stages, the overall drainage
solution must be visualized and studied so that substantial design and construction changes are not
required later. The possible effects that highway construction may have on existing drainage patterns,
river characteristics, potential flood hazards, and the environment in general, and the effects the river
and other water features may have on the highway, should be considered at this time.
Hydrologic and hydraulic specialists must be actively involved during the initial project phases to
ensure that proper consideration is being given to drainage aspects. This involvement should include
participation during the highway location selection phase. Early input from these specialists will
result in a better design, both hydraulically and economically.
It must be emphasized that early studies are not comprehensive, detailed, technical designs. Rather,
most are cursory studies to consider obvious drainage-related problems that may be encountered or
created and what type of data needs to be collected for evaluation of possible impacts. The degree and
extent of preliminary hydraulic studies should be proportionate with the cost and scope of the project
and the perceived flood hazards that may be encountered. This chapter presents a comprehensive
overview of possible considerations in the planning and locating of a highway.
1.2 GENERAL CONSIDERATIONS
There are many tasks and requirements that must be considered during the early phases of project
development. Coordination between the various divisions of the transportation agency that may be
involved with the project must be established. Notification of proposed projects must be made to
other agencies and the public. The permits and regulations applicable to the project should be
identified as soon as possible. Often, project delays are due to the legal process. Problems that may
arise during design, construction, or maintenance should be considered. In addition, environmental
data needs should be determined.
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Highway Drainage Guidelines
These, and other considerations, cannot always be separate items that will be performed during the
planning phase or during the location phase. Sometimes, considerations will occur during planning,
while at other times those same considerations will occur during the locating of the highway. Often,
there will be overlap, and sometimes the separation between the phases will be so indistinct that it
will be difficult to determine in which phase the consideration should be addressed. Even so, it is
helpful to understand what items are usually considered during each phase, especially because
planning and location are such commonly used and misunderstood terms.
1.2.1 Definitions
Different transportation agencies have various definitions of planning and location. What one State or
agency feels is a planning function may be performed during the location phase in another. This
AASHTO guideline will use the definitions of planning and location as given in A Policy on
Geometric Design of Highways and Streets, 2004 (1).1
1.2.1.1 Planning
AASHTO, in A Policy on Geometric Design of Highways and Streets, 2004 (1) defines planning as
the process that “includes the conduct of inventories, the preparation of mathematical models, the
forecasting of economic and demographic growth, the development and evaluation of alternative
transportation systems, the advising of those who will implement the selected plan, and the
surveillance and reappraisal of the planning process as a continuing function.”
During the planning process, the hydraulics engineer will be principally involved in the conduct of
hydraulic inventories, such as:
river basins;
wetlands;
water supplies;
dams;
bridges;
past flood events; and
water, sewer, and storm drains.
Such inventories should facilitate better hydraulic studies during the design phase.
1.2.1.2 Location
The AASHTO definition of location activity details what specifically takes place during this phase:
Location activity generally takes place after transportation planning and prior to
highway design, but blends into both. The highway location should satisfy both the
broad goals of the transportation system and the local goals of the immediate environs.
The usual steps followed in accomplishing location studies are:
1
Numbers in parentheses refer to publications in “References” (Section 1.6).
Hydraulic Considerations in Highway Planning and Location
1-3
(1)
Determine broad route requirements; i.e., type of highway needed, control points.
(2)
Select corridors and identify all major alternatives.
(3)
Examine planning reports and conduct preliminary surveys to gather information on population
densities and trends, land use development, travel patterns and trends, and economic, social, and
environmental conditions that should be considered in selecting alternative highway locations.
(4)
Prepare preliminary plan and profile layouts for each alternative route so that cost estimates can
be made and construction feasibility can be tested.
(5)
Evaluate alternatives to see which are worthy of further study and development.
(6)
Proceed with more complete location studies on the acceptable alternatives.
(7)
Determine and evaluate the economic and environmental effects of each alternative.
(8)
Prepare the route location report as an aid to the decision maker.
(9)
Conduct a corridor public hearing. It may precede or follow the submission of the route location
report.
(10) Review by decision maker to determine which alternative route should be advanced to the
design stage.
The participation of the hydraulics engineer during the location phase should ensure the proper
consideration of the many items that affect or are affected by drainage. These specific items that may
need to be considered are covered in detail in the following sections of this chapter.
1.2.2 Coordination
There are two types of coordination during the preliminary phases of a highway project. One is to
obtain or provide information. The hydraulics engineer needs to know the general scope of the
highway project and possible plans of other agencies and developers regarding future projects in the
watershed through which the highway may pass. Information from the general public is useful, most
particularly in the area of historical flood data. Information should be shared with regulatory agencies
that issue permits or implement decisions that could affect the project. The hydraulics engineer should
also provide appropriate data to these same sources, which is used to support any important hydraulic
recommendations.
The second type of coordination can provide substantial economic savings. This is the coordination or
combination of a highway project with a non-highway project. A joint project, such as a stormwater
retention facility, can result in savings and other benefits for all parties involved, usually by
eliminating the duplication of certain functions or by the simultaneous construction of projects.
Facilities can sometimes be combined or integrated resulting in the need for less right-of-way.
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Highway Drainage Guidelines
1.2.2.1 Coordination within the Transportation Agency
Early coordination between the planning and location engineers and the hydraulics engineer may help
minimize potential problems. Planning and location engineers can be alerted to unstable reaches of
streams that may be avoided by slight changes in the alignment. Critical areas sensitive to flooding
should be identified. Estimated structure sizes and costs can be provided. Expensive bridges or
extensive encroachments on sensitive environmental areas may provide cause to modify the
alignment.
It is important, therefore, for the hydraulics engineer to become involved not only with the alignments
to be studied, but with the corridors as well. There may be sections to avoid within a watershed (e.g.,
wetlands, water supplies, sewage works, environmentally sensitive areas). There may also be times
when entire watersheds should be avoided.
1.2.2.2 Coordination with Other Agencies
The hydraulics engineer should be involved in the coordination process with other agencies that may
have water resource data. These Federal, State, and local agencies have a wealth of information useful
to anyone involved in hydraulics or hydrology.
This coordination is necessary to find out about plans for water-related projects within the project
area and to inform other agencies about the highway. Because these often are the agencies who will
issue the various permits for the project, concerns can be more easily worked out at this stage. Often,
minor changes can be agreed upon by both agencies without considerable paperwork and formal
meetings. It is important for the hydraulics engineer, therefore, to not only coordinate with these
agencies, but also to establish a good working relationship with them.
Some agencies that may be involved or have interest in a project include:
Federal
Army Corps of Engineers
Bureau of Reclamation
Bureau of Land Management
Bureau of Indian Affairs
Coast Guard
Environmental Protection Agency
Federal Emergency Management Agency
Fish and Wildlife Service
Forest Service
Geological Survey
International Boundary and Water Commission
National Marine Fisheries Service
National Oceanic and Atmospheric Administration
Natural Resources Conservation Service
Tennessee Valley Authority
Hydraulic Considerations in Highway Planning and Location
1-5
State
Environmental Protection Agencies
Coastal Zone Management Agencies
Planning Agencies
Fish and Game Agencies
Floodplain Management Agencies
Water Resource Agencies
Local
Drainage Districts
Flood Control Districts
Irrigation Districts
Municipal Governments
Indian Councils
Planning Districts
Regional Water Quality Control Boards
Watershed Districts
Other
Historical Commissions
Private Citizens
Private Industry
River Basin Compacts, Commissions, Committees, and Authorities
Governmental Societies
Academic Institutions
1.2.2.3 Public Involvement
Much drainage information can be obtained by contacting and coordinating with the general public.
Interviews with local residents concerning the knowledge of past hydrologic events can be helpful,
though the recollections of more than just one person should be obtained.
Although local ordinances generally do not have the force of law for State agencies, coordination
with the local community or jurisdiction is always desirable and recommended. Community offices
may have histories that could yield valuable information of past flooding events or other drainage or
water resource problems.
Public involvement is not only the gaining of information from others, but the sharing of it as well.
Information on the project should be presented during the early stages of development so that the
public will be knowledgeable of the agency’s plans and not rely on rumors that may originate from
other sources. Those providing information must be careful though to emphasize its preliminary
nature.
1.2.3 Legal Considerations
Among the many considerations to be made in selecting highway route locations are those regarding
the various legal requirements and implications of the construction. The hydraulics engineer must
have an understanding of those as they pertain to drainage and water law at the national, State, and
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Highway Drainage Guidelines
local level. This includes the permits required, regulations to be followed, and the laws concerning
the potential liabilities involved when the highway alters drainage patterns. The hydraulics engineer
must then make known these requirements to those who will actually perform the specific tasks
covered by them.
For more detail on the many legal considerations that are involved, see Chapter 5, “The Legal
Aspects of Highway Drainage,” of the Highway Drainage Guidelines (2).
1.2.3.1 Permits
The number and type of permits required for highway construction varies throughout the country.
These permits address such items as erosion control, water quality, environmental needs, flood
control, and size and type of structure. It is important during the planning and location phase to
identify where and what type of permits are needed that may require hydraulic information. It is
during the early phases of project development that contact shall be made with those agencies that
will be issuing the permits. This early contact may facilitate their review process by clarifying the
transportation agency’s plans. It will be easier to make changes prior to requesting a permit should it
be found that the preliminary design concepts do not meet permit requirements.
Federal permits the hydraulics engineer may be involved with include:
USACE—Section 404 of the Clean Water Act of 1977 and Section 10 of the Rivers and Harbors
Act of 1899;
USCG—Section 9 of the Rivers and Harbors Act of 1899; and
State environmental agency—Section 401 Certification of the Clean Water Act of 1977.
Permits specific to the individual State include:
construction permits for work in a stream or coastal area,
U.S. FWS permits or certification,
approval of erosion and sediment pollution control plans,
stormwater management requirements,
best management practices for treatment of highway runoff, and
NPDES.
Often, local entities have permit requirements too. These will usually be similar to State permits.
Although State agencies may not be obligated to obtain local permits, the requirements or practices
within those permits should be complied with wherever possible.
Permits are further discussed in Chapter 5, “The Legal Aspects of Highway Drainage,” of the
Highway Drainage Guidelines (2).
1.2.3.2 Regulations
As with permits, the various regulations affecting highway drainage facilities must be understood.
The drainage concepts of preliminary plans that may not comply with regulations must be recognized
and alternative designs suggested.
Hydraulic Considerations in Highway Planning and Location
1-7
Continuous coordination should be maintained with the legal staff of the transportation agency to
keep the hydraulics engineer abreast of significant regulation changes and to obtain legal
interpretations of these changes.
Federal regulations are published in the Federal Register. This is a publication available to every
transportation agency. Significant regulations pertinent to the hydraulics engineer include:
Executive Order 11988 on floodplains and implementing regulations,
Executive Order 11990 on wetlands,
FHWA regulations,
FEMA regulations (discussed in Section 2.3.4), and
Jurisdictional wetlands permitting administered by USACE.
1.2.3.3 Laws
Probably the most important legal consideration during the drainage design of a transportation facility
is that of water law and related potential liability. Although water law varies throughout the United
States, responsibility for additional flood damage is usually placed on the person or agency that
changes the natural flow characteristics of a watercourse. Some changes in these are unavoidable, but
the hydraulics engineer can often design facilities that minimize or eliminate any adverse effects of
such changes.
Possible risks the engineer should be aware of include:
additional backwater caused by constricted flows;
velocity changes that may cause erosion or deposition;
diminishing or increasing downstream flow rates that could affect existing water uses;
degradation of water quality by roadway runoff or by infiltration into groundwater;
alteration of shallow groundwater flow; and
limitation to fish migration by in-stream facilities, such as poorly sited culverts.
The hydraulics engineer must provide input so that it can be determined whether construction of the
highway may cause these potential risks or if these conditions exist prior to construction of the
highway. For this reason, it is important to document existing conditions by photographs and
descriptions of the area under study.
1.2.3.4 Federal Emergency Management Agency
The Federal Emergency Management Agency (FEMA) promulgates regulations under the National
Flood Insurance Program (NFIP) of which the highway hydraulics engineer must be knowledgeable.
These regulations and those in the FHWA Location and Hydraulic Design of Encroachments on
Flood Plains, 23 CFR 650, Subpart A (4) and subsequent design memorandums address those
procedures to follow when a highway facility is to be located in an identified flood prone area. The
hydraulics engineer must review the NFIP studies so that he can determine if the location of a
highway is infringing upon a designated floodplain area. When there is infringement, it is necessary
to determine and document the effect.
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Highway Drainage Guidelines
The hydraulics engineer should be familiar with the joint agreement “Procedures for Coordinating
Highway Encroachments on Floodplains with FEMA.” This agreement was developed by FHWA in
conjunction with FEMA. This paper establishes procedures to be followed when highway projects
encroach on floodplains and floodways. Four circumstances are discussed where coordination with
FEMA will be necessary when:
(1) A crossing encroaches on a regulatory floodway and will require an amendment to the floodplain
map.
(2) A crossing encroaches on a floodplain where a detailed study has been made, but no floodway
has been designated and the crossing would create an increase in the base flood elevation greater
than 0.3048 m (1 ft).
(3) The community will enter into the flood insurance program shortly and detailed floodplain
studies are in progress.
(4) The community is in the emergency program and the crossing will increase the base flood
elevation by more than 0.3048 m (1 ft) near insurable buildings.
The three types of NFIP maps are also discussed in the agreement. These include the Flood Hazard
Boundary Map (FHBM), which is based on approximate studies only; the Flood Boundary and
Floodway Map (FBFM), which is obtained from a detailed hydraulic study of water surface profiles;
and the Flood Insurance Rate Map (FIRM), which shows boundaries for the different insurance rate
schedules. Of these, the FBFM is the most valuable to the hydraulics engineer.
Procedures exist to change FEMA flood area designations when it is determined that they are
incorrect. These procedures, which are mentioned in both the FHWA–FEMA joint agreement and the
FHWA regulation, require a study using the same hydraulic model as was used in the original study.
Studies changing previous designations must contain the reasons why the FEMA criteria are
demonstrably inappropriate.
1.2.4 Related Considerations
During the planning and location phase of project development, the flow characteristics at highwaystream crossings should be considered, not only to determine the effects of the highway upon the
stream and its floodplain, but also the effects of the stream upon the highway. This includes the
existing conditions and those that will result from the proposed project. Such a determination can
assist in identifying those locations where difficult and costly construction or maintenance problems
could be encountered. Sometimes, a minor change in roadway location or structure alignment can
resolve these problems. If possible, several alternative solutions should be considered.
If it appears that solutions may require major changes, studies should be expanded and become more
detailed, even at these early stages. Only by enlarging the studies can the agency be assured that
practical alternatives are possible. If, on the other hand, the studies do not identify a practical
solution, documentation should be provided to the planning or location engineer supporting the
determination of that particular location as unacceptable.
Hydraulic Considerations in Highway Planning and Location
1-9
1.2.4.1 Design-Related Considerations
Problems may arise during design that were overlooked during the preliminary phases of planning
and location. Examples include:
lateral encroachments on a channel;
disruption of water supplies, irrigation facilities, or storm drainage systems;
encroachments into environmentally sensitive areas; and
failure to plan for right-of-way needs.
Often, the planner does not have field surveys available and what appears feasible in the field or from
large-scale maps cannot be done when more detailed information is available. Recommendations
made during the planning or location phase should not be accepted as final solutions, nor should any
binding commitments be made at this time. On the other hand, as noted in the previous section,
studies of sufficient detail should be made of problem areas to ensure that a reasonable design
solution is achievable. Careful attention to these areas during the preliminary phases should keep
problems during this phase to a minimum.
1.2.4.2 Construction-Related Considerations
Problems during construction will be minimized when important drainage or other water-related
factors are considered during the location and planning phases of the project. The occurrence of
erosion and sediment, and how to control it, must be considered, at least in broad terms, during the
early phases of location. The hydraulics engineer, along with other specialists, may be involved in the
identification of groundwater flows and potential unstable slopes because of underground water so
that proper measures can be taken to prevent problems before they occur.
The time of the year and the total construction time should be taken into consideration in considering
impacts. Certain elements, such as embankments along a stream, should be completed before the
anticipated flood season. In some sections of the country, work cannot be performed in some streams
during the spawning season of sensitive fish species. In other areas, the stream may also serve as an
irrigation supply requiring that flows not be interrupted and that pumping and distribution systems not
be contaminated with sediment.
The use of temporary structures must also be planned. Often, a temporary crossing can be smaller
than normal if it is only going to be utilized during the dry summer months. If it will be used for more
than one year, perhaps it needs to be sized for a flood of greater magnitude. This consideration may
change the concept of the project or at least the type of structure designed.
Many construction-related hydraulic problems are ones of scheduling. Although they will be studied
in more detail during the design phase, they should be initially considered, at least in a preliminary
manner, as early as possible. Commitments regarding water resource related items made in the
Environmental Impact Statement (EIS) must be made known to the personnel who will be involved in
the actual construction. Some commitments that “sound nice” may not be feasible to build. In other
cases, construction occurs so long after the EIS has been prepared that those commitments are
forgotten or not included in the plans or contract documents. A “commitment list” that follows the
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Highway Drainage Guidelines
project through the various stages of development should be prepared to ensure that these items are,
in fact, incorporated into the project.
1.2.4.3 Maintenance-Related Considerations
Planning and location studies should consider the effects the drainage will have on the completed
highway. Although problems such as erosion and sedimentation may be temporarily controlled during
the construction phase, these same problems must be minimized even after the project is opened to
traffic.
Any change to the natural contours or drainage system regardless of how minor, usually entail certain
maintenance responsibilities. These responsibilities can include many items from mowing grassed
banks to clearing the channel of debris or ice.
The identification of potential maintenance problems is most easily done by allowing maintenance
personnel the opportunity to review the preliminary plans and locations and asking for their advice
concerning potential problem areas. Reference to maintenance and flood reports, newspapers, and
interviews with local residents can also be helpful in identifying and evaluating potential maintenance
problems.
Once the possible problems are identified, the hydraulics engineer can suggest modifications to lessen
the effects or to avoid the problems completely, or at least he can emphasize the problems and their
anticipated impacts.
1.2.5 Environmental Considerations
Along with all the other considerations made during the planning and location phases, the effects of
the highway on the environment must be evaluated. The hydraulics engineer may assist in answering
questions about:
roadway runoff and its effect on the quality of the receiving water,
effect of construction of channel relocations and culverts on fish and other aquatic life, and
displacement of wetlands and any overall effect on water resources within the highway corridor.
The hydraulics engineer should also review any proposed hydraulic-related mitigative measures and
ensure that they are directly related to impacts caused by the highway and can be constructed in a
realistic and cost-effective manner.
For more detail on the many environmental considerations that must be made, see Chapter 10,
“Guidelines for Evaluating Highway Effects on Surface Water Environments,” of the Highway
Drainage Guidelines (2).
1.2.5.1 Water Quality
The conservation of water and the maintenance of its quality are of primary concern. Droughts in
some parts of the country and water pollution problems affecting entire river systems in other areas
have emphasized that water is not a limitless resource. Most research suggests that runoff from most
highways does not contain pollutants in sufficient quantities to cause adverse effects. However, there
Hydraulic Considerations in Highway Planning and Location
1-11
are areas within highway corridors that should be given special consideration because of the risk of
pollution. The identification and evaluation of these areas should be made during the preliminary
location stage. Then, steps can be taken to minimize or eliminate any harmful effects or to select
alternative routes that avoid the sensitive area.
Areas that should be considered include roadways adjacent to lakes or ponds, outlets of closed storm
drain systems, and areas where there are many springs or wells along the highway. Another area of
concern is potential high-accident locations, particularly on highways where many chemical or fuel
trucks might be traveling.
Some States and local communities may have requirements influencing how the highway agency may
discharge the roadway runoff into a stream. To comply with these requirements, the agency may have
to show the runoff is either being detained to allow for settling, being skimmed, baffled or even
chemically treated to ensure no potentially dangerous oils, greases, suspended solids or sediments are
reaching the surface waters. A concern of some States is the effect of roadway salts on water quality,
specifically on shallow groundwater, because surface waters usually have enough flow to dilute road
salts to acceptable levels. Research has shown that most roadway salt problems are a result of
improper storage techniques rather than the runoff of the material from the highway.
1.2.5.2 Fish and Wildlife
The transportation agency, and in particular the hydraulics engineer, must be aware of critical
fisheries and the needs for adequate fish passage. The hydraulics engineer should, during the
preliminary phase, work closely with the local fish and wildlife agency, to acquire data such as:
which streams are fisheries,
what kind of fish do they support,
when is the spawning season, and
what special actions need to be considered.
With this information and a working relationship with fisheries, acceptable plans can be developed so
that delays during design and construction will not occur.
Because of some inappropriate installations, some fish biologists often believe that culverts present a
barrier to fish passage. Wide boxes with flat bottoms create a shallow flow while culverts on steep
grades produce velocities too great for fish to swim against. The hydraulics engineer can minimize
these concerns with designs that concentrate flows to create deeper sections, flatten gradients through
the structure and create special basins at outlets. Hydrologic studies may have to include analyses of
low-flow periods or of spawning periods to demonstrate that depths and velocities during these
periods are at acceptable values for fish. In some cases, the lowering of a culvert below the streambed
will be sufficient. When multiple structures are used, one might be lower than the other so that low
flows will be concentrated in only one section, creating a deeper flow. This creates a natural channel
bed through the structure, slows the water and results in greater flow depth. The hydraulics engineer
will have to use the natural channel characteristic values, however, in the sizing of the pipe.
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Highway Drainage Guidelines
Channelization, overhead cover, erosion control and pollution are other areas in which the fish and
wildlife personnel will be interested. The hydraulics engineer will need to balance those concerns
with hydraulic capacity, considering both low- and high-flow conditions.
Wildlife migration patterns may also be affected by such things as the elimination of a water source
or crossing. The installation of game or deer passages, which also serve as drainage structures, can
sometimes solve this problem.
1.2.5.3 Other Environmental Considerations
There are other considerations that should be made during the planning and location phase that are
environmental in nature and are discussed elsewhere in the guidelines. Changes in flow patterns
influenced by the highway may affect the environment of the area in which it occurs. This is both a
legal and environmental consideration. Aesthetics are also part of the environment. Although this may
not be a primary concern, the hydraulics engineer should attempt to select hydraulic features that
blend with the surroundings. These features may include meandering channels, overhanging banks,
revegetating and landscaping stream banks, placing rocks in streams, and creating pools and riffles.
Although the hydraulics engineer is primarily concerned with peak flow conditions, low-flow
hydraulics may also need to be considered. During low-flow periods, streams may require low-flow
sections that create enough depth for fish and prevent undesirable mud flats. It is during low-flow
conditions that pollution becomes a major concern because there is so little flow available for mixing
or dilution. The identification of rare or endangered species of wildlife or fauna may require special
drainage considerations, to avoid or preserve them.
1.3 SPECIFIC CONSIDERATIONS
As the project progresses and becomes better defined, the decision-making considerations become
more specific and more detailed. This usually occurs as the project moves into the location phase.
Among the factors that must be considered are the interrelationship of the terrain and hydrologic
features and how they may affect the cost, construction, and operation of the highway.
The location phase is often the most critical and difficult of the entire project. Each highway
discipline has its own design requirements. Some of these requirements will be able to be fully
satisfied while others will have to be balanced within accepted design practices and good engineering
judgment. Some hydraulic requirements must be met within specific limits, and it is important for the
hydraulics engineer to convey this need. There will be areas where the alignment should conform to
the river or locations where a stream crossing is not practicable. It is recognized that resolution of
some of these considerations discussed will be made during the detailed design phase; these topics are
discussed in this chapter as items that may be more easily resolved through early location
adjustments. In either case, these determinations will require a knowledge of stream geomorphology
and river mechanics. A brief introduction to these topics will be made in the following sections.
Hydraulic Considerations in Highway Planning and Location
1-13
1.3.1 Stream Geomorphology
An understanding of river channels, how they are formed, how they react to natural or constructed
actions, and how they behave without any outside influences is necessary to evaluate a highway
project’s effect on a river reach.
Geomorphology is the science that deals with the land and submarine relief features of the earth’s
surface. Stream or fluvial geomorphology is that science that deals with those features of the earth’s
surface that are produced by the action of streams.
This section will explain the broader, more general aspects of stream geomorphology and discuss how
these features may affect or be affected by the highway location. For those not directly involved in
the specific hydraulic analyses of streams, it should be used as an introduction to the subject and aid
them in recognizing when the counsel of the hydraulics engineer is required. More detailed sections
on this subject appear in the Highway Drainage Guidelines’ Chapter 6, “Hydraulic Analysis and
Design of Open Channels,” and Chapter 7, “Hydraulic Analysis for the Location and Design of
Bridges” (2).
1.3.1.1 Types of Streams
Streams are generally classified as those that have floodplains and those that do not. A more common
classification, regardless of the presence of floodplains or not, is braided, straight, or meandering.
This is the classification normally used by transportation planners and engineers. Figure 1-1 shows
these stream channel patterns that will be discussed in more detail in the following sections.
Figure 1-1. River Channel Patterns
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1.3.1.1.1 Braided Streams
A braided stream consists of multiple and interconnected channels. Braided systems reflect the
relationship dynamics between sediment transport, the nature of the materials in the floodplain, and
seasonal variations in stream discharge. That portion of the bed load that exceeds the stream’s
transport capacity is deposited in the channel and decreases conveyance capacity. Subsequent higher
flows increase the velocity and multiple channels develop. This, in turn, causes the banks to erode,
thereby allowing the overall channel system to widen.
Braided streams cause serious location problems due to the unstable nature of their beds and banks,
rapid changes of alignment, wide and shallow flow, degradation and aggradation, and large quantities
of sediment carried and deposited. Highway crossings and even longitudinal highway encroachments
on braided streams should be avoided wherever possible because of their unstable nature. Alternative
sites should be selected when practicable.
If a crossing over a braided stream reach cannot be avoided, there are certain design features that
should be considered. These include spanning the entire channel, stabilizing banks around the
abutments, and designing substructures for anticipated scour. It is important to minimize any effect on
the stream’s sediment transport capacity. This could cause potential channel changes to occur
upstream or downstream of the highway.
1.3.1.1.2 Straight Streams
Streams are never really straight. Even if the banks are parallel to each other, the flow or thalweg, the
path of deepest flow, usually oscillates from one side of the channel to the other. For purposes of
definition, a stream reach is straight when the ratio of the length of thalweg to the length of the valley
is less than 1:5.
Natural straight reaches of alluvial channels are often only a temporary condition or a transitional
stage until such time as a meander moves into the area. Aerial photographs, maps or a field
investigation may reveal former channel locations and give an indication of future directions of
movement. Unless they are incised into rock or similarly constrained, natural straight reaches should
not be depended upon to remain as permanent, stable features.
Even though constructed channels are often designed and constructed straight, they may be unstable.
Unless straightened channels contain drop structures, they will have steeper gradients that cause
higher stream velocities, often causing or increasing upstream degradation and downstream
aggradation. Straightened channels must be considered from two possible viewpoints. First, if the
highway will be crossing an already straightened segment, consideration must be given to its stability.
Then, if a channel needs to be straightened to better accommodate the highway, the effects of the
straightening need to be assessed both upstream and downstream.
1.3.1.1.3 Meandering Streams
A meandering stream is defined as one in which its ratio of the length of thalweg to the length of
valley is greater than 1:5. A more common observation of a meandering stream, however, is that it
consists of alternating bends of an S-shape.
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Meanders, or bends, are formed by the erosion of the banks on the outside of the bends and the
deposition of this eroded material on the inside of the bends to form point bars. These point bars
constrict the channel, force the flow to the outside, and continue the erosive process on the bank
further downstream. In this manner, the meandering process continues both laterally and
longitudinally, usually in a downstream direction.
The meanders do not progress at an even rate or create uniform, equal bends. Rather, the stream takes
the path of least resistance where the banks will more easily erode. Therefore, bends can be of greatly
varying length as will the distance between them. Occasionally, bends will double back on
themselves or even cut through the loop. This isolated or cutoff portion of the bend is called an
oxbow lake, as the loops or bends themselves are known as oxbows. Aerial photographs, especially a
series taken over a period of years, are useful in detecting oxbow lakes, former meanders, and point
bars. Comparing those photos can also yield an approximation of the rate of movement. Alternative
crossing sites can be evaluated for risk, knowing the life of the structure and the historic stability of
the channel reaches.
Highway projects in or near a meandering stream will usually require bank protection to control
lateral erosion and meandering. This, however, usually entails some channel straightening and the
resulting problems described in the previous section. Even if these can be dealt with, the possibility
may still exist in which a meander may move into the protected area from an unexpected direction
and circumvent the channel armoring. In planning or locating a highway or bridge project within a
meandering reach, these potential channel shifts should be considered and the effects on the
transportation facility assessed. Finally, any channel work must be continued to a reach, stabilized
either naturally or by humans, beyond which meandering cannot continue. This should be done both
upstream and downstream of the project.
1.3.1.2 Islands
Islands may form in a river either from the deposition of material or from the erosion of fines that
leave the more resistant soils or rock as the island. Erosion resistant vegetation may also influence
island formation. Islands can be of any size and can be in any stage of formation. They can be
increasing in size, decreasing or eroding, or remaining relatively stable. The amount and size of
vegetation on an island may be an indicator of the age and stability of the formation.
Initially, it may appear feasible to construct a roadway section or pier on an island. Stream stability,
meander movement and flood levels must be investigated to determine whether protection may prove
to be more costly than the use of other methods, such as dumped rock fill.
On the other hand, it may appear desirable to remove the island. Here, too, the consequences must be
evaluated. If the island is made of alluvial deposits, it may be reformed elsewhere or even in the same
location. The redirection of flow caused by its removal may increase scour in an unprotected region
of the stream.
Sometimes, too, it may be best to not disturb the island and design the facility by taking into account
the hydraulic effects caused by it. These may include such things as varied cross sections and
velocities and, in general, characteristics that can change dramatically with changes in flow rates or
island size.
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1.3.1.3 Delta Formations and Alluvial Fans
Delta formations and alluvial fans are both formed by the deposition of alluvial material. These
formations should be avoided whenever possible because of the unpredictable stream behavior and
the large volume of material that can be deposited.
Deltas occur where streams enter bodies of water and the resultant velocity and turbulence decrease
cannot maintain the transport of material. As the material is deposited, a delta forms and this builds
outward from the mouth of the stream into the body of water. This increases the channel length,
reduces its slope and velocity and causes upstream deposition. A similar process occurs at stream
confluences, especially where steep, sediment-laden streams meet slower, larger rivers. Here, the
deposition extends out into the main stream, deflects the flow and often forces a change in the main
stream thalweg.
Alluvial fans occur at the mouths of canyons and similar geologic features where there is a sudden
change from a steep to a flatter gradient. Bed load material is deposited because of the change in
stream slope and velocity. In an alluvial fan, the material is spread fanlike, spilling over a wide
section of floodplain. Multiple temporary channels may form through a fan, and there is seldom any
indication as to which channel the stream may utilize during higher flows.
A structure may be built over what appears to be the prominent channel through an alluvial fan or
delta only to have the channel shift. The structure is then either filled in with deposition or left
spanning a dry channel. The highway may be overtopped by the river leaving its apparent channel, or
the channel may need to be continually cleaned to maintain an adequate waterway. Alluvial fans are
usually the more serious of the two types. A good example of the problems encountered in building
on an alluvial fan is given in Highway Drainage Guidelines’ Chapter 7, “Hydraulic Analysis for the
Location and Design of Bridges” (2).
If construction must take place on either of these formations, the crossing should be located as close
as possible to the upstream end of the fan or at the mouth of the valley with a stabilized, well-defined
channel formed with sufficient protection to ensure its permanency. In addition, maintenance
personnel should be consulted so that they realize the potential problems that may occur in the future,
such as the need for periodic cleaning of the channel.
1.3.1.4 Aggradation and Degradation
Many stream reaches are considered stable and should cause no problems to a highway facility. The
current state of stability or equilibrium, however, does not preclude significant long-term changes.
Degradation, or the natural removal of material from the streambed, results when the sediment
transport capacity of a stream is increased or the sediment supply is decreased. Any change to a
stream that increases its gradient, velocity, or flow rate increases the stream’s capacity to transport
sediment and cause degradation.
Aggradation, or the deposition of material along the streambed, occurs when the sediment transport
capacity of a stream is decreased. This happens when the velocity or flow rate decreases, such as the
flatter reach of an otherwise steep channel or just above a reservoir or lake or pond. Unwanted
deposition may occur upstream of a culvert where the flow velocity decreases before it enters the
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culvert. This deposition may eventually accumulate sufficient volume to decrease the effective
opening of the culvert.
One way of determining if a stream is aggrading or degrading is to compare channel cross sections
from old plans with present conditions. Borings may also reveal historical changes in material types,
and pier and abutment footings may display old material lines from which changes may be measured.
Once the possible effects of aggradation and degradation have been considered, locations or
alignments of bridge piers may need to be changed. Bridge decks or roadway grades may need to be
raised or roadway alignment may be changed because of aggradation.
Even though these effects of aggradation and degradation have been considered during the early
phases of project development, some will need reconsideration during design as plan details become
specific. Others, regardless of the planning considerations, will become construction considerations
and, more likely, maintenance considerations. A stream is dynamic; it is always changing.
The determination of how, when, and where a stream responds to change is a task that should be
addressed by the experienced hydraulics engineer trained in stream morphology. Highway Drainage
Guidelines’ Chapter 6, “Hydraulic Analysis and Design of Open Channels,” (2) presents a more
detailed introduction to this portion of stream morphology and references of other explanations of this
specialized topic.
1.3.2 Highway Alignment
The alignment of the highway and its relationship with the drainage systems is the foremost concern
of the hydraulics engineer during the location phase. This section will discuss these concerns for both
the horizontal and vertical alignments.
1.3.2.1 Horizontal Alignment
The horizontal alignment of a highway determines where stream crossings will occur and where there
will be transverse or longitudinal encroachments. Two aspects of the proposed alignment must be
considered. First, the hydraulics engineer must consider how the streams or storm drain systems may
affect the roadway and, second, how the roadway may affect the flow characteristics of such streams
or systems. Slight changes in alignment can sometimes alter the flooding characteristics significantly.
Whether or not changes to the horizontal alignment can be made often depends on whether the project
is an improvement to an existing highway or the construction of a highway in a new location.
1.3.2.1.1 Existing Alignment
There is often little opportunity to change horizontal alignments when the project is an improvement
to an existing highway. The alignment should still be reviewed, though, to identify locations where:
slopes need to be protected against scour,
abutments moved or skewed differently,
drainage structures protected against headcutting, and
meanders are endangering the highway.
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Highway Drainage Guidelines
Minor alignment improvements or roadway widening may cause slopes to encroach upon streams. If
unavoidable, the hydraulics engineer must be prepared to offer actions to accommodate these
encroachments.
Changes to the horizontal alignment of the highway at stream crossings can also result in hydraulic
consequences. Many older structures were constructed to cross the stream at a right angle to the flow.
This sometimes resulted in sharp curves in the roadway approaches to the bridges. Replacement
structures are often planned to correct this poor alignment by crossing the stream at a skew. Proper
abutment and pier alignment of the replacement structure must be ensured. If the existing
substructures are to be used as part of the replacement, their alignment with the channel must be
considered. If the substructures will not be used in the new alignment, it is generally preferable to
remove them.
1.3.2.1.2 New Location or Relocation
The construction of a highway on a new alignment affords the greatest opportunity for the hydraulics
engineer to influence the alignment during the location phase. During this phase, changes can be
recommended to locate the highway away from a stream or situate a bridge at a more stable channel
location. These recommendations should be made early in the development of a project to avoid
delays during the design or right-of-way acquisition phase when the horizontal alignment is difficult
to change.
During relocation there may also be constraints that control the alignment. Topographic and cultural
features may have to be avoided, resulting in the use of the river environment for the highway. In
these cases, the constraints noted in the previous section will often exist. Besides these constraints,
there may be other alternatives that should be studied because of other considerations, such as costeffective designs or land development plans.
1.3.2.2 Vertical Alignment
The effect of the vertical alignment, commonly called the profile, on highway drainage facilities is
significant and must be assessed in comparing alternative locations. Although the profile usually is of
greater interest to the hydraulics engineer than the horizontal alignment, it is normally easier to alter
and is not firmly set as early in the project development.
The profile is that feature, along with the hydraulic opening, that determines when, and where, the
highway will be overtopped. By raising or lowering the profile, the frequency of overtopping can be
either decreased or increased. Not only does the profile affect the frequency of overtopping, but it
also determines the level of upstream flooding.
Depressed roadways act as drainage interceptors and may require that upstream surface runoff be
accommodated in storm drains or diversion channels. Fills on wide, flat areas may intercept surface
flows and require special drainage treatments. These problems will be of special concern with large
urban expressways and deserve careful evaluation at the location phase. On streams where navigation
exists, clearances required for waterway vessels may become the factor controlling vertical
alignment.
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The profile not only affects the flow from streams either over the roadway or through the structure
opening, but it also affects the flow of the roadway runoff water. Sag vertical curves are critical
profile areas, because they can serve to trap highway drainage unless adequately sized and spaced
outlets or catch basins are provided. Steepness of the highway grade also determines the spacing of
inlets in areas where the roadway has curbs.
An extensive discussion on this topic has been included in Highway Drainage Guidelines’ Chapter 7,
“Hydraulic Analysis for the Location and Design of Bridges” (2).
1.3.3 Location of Stream Crossings
The location of the highway when crossing a stream is important for several reasons. Hydrologic and
hydraulic considerations are different when crossing near the confluence of two streams as compared
to a single stream. Higher backwaters may be better tolerated in rural areas than in urban locations.
Tidal areas present a list of entirely different hydraulic considerations. Whether the structure is a
bridge or a culvert can make a difference in the hydraulic study as well.
1.3.3.1 Physical Considerations
A highway crossing near the confluence of several streams will present a different set of
considerations than will one crossing a tidal area. This section will discuss some of those items that
should be considered during the planning and location phases relative to the type of physical area
being crossed.
1.3.3.1.1 Confluences
A highway crossing at or near to the confluence of two or more streams is a complex hydrologic and
hydraulic location that should be avoided. The hydrology design should consider several
combinations of storm events. Peak flows can occur simultaneously on confluent streams. This
probability is usually small if one watershed is much larger or hydrologically different from the
others. (See Table 9-2 in Chapter 9, “Storm Drain Systems” (2).) Large peaks on one watershed
should also be evaluated in combination with lesser events on the other streams because stream
velocities could be higher.
Such locations require an analysis involving the hydraulics of confluences. This includes an analysis
of the various combinations of flood events and how they may change flow distributions, hydraulic
gradients, headwaters, and velocities. Stream stability can also be more critical at confluences
because the middle and point bar formation can cause abrupt changes in flow directions. Pier location
and alignment and culvert alignment near confluences should be carefully analyzed for these effects.
Although these complexities do not have to be studied in detail during the early planning and location
stages, their effects on the location should be recognized and documented. The future potential
problems with such sites must be emphasized and the positive factors of avoiding these locations.
Minor alignment changes may eliminate the problems of a crossing near a confluence. Additional
information on confluences and their hydraulic and hydrologic effects can be found in Chapters 6 and
7 of the Highway Drainage Guidelines (2).
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1.3.3.1.2 Tidal Areas
Crossings of tidal waters present the hydraulics engineer with special considerations such as regular
changes in water level from astronomically induced tides, storm surges from wind and high waves, or
even seismic waves or tsunamis.
Tidal inlets and their related marshes may also be highly sensitive environmental areas because of the
different and often rare wildlife and biological systems they support. Crossings should not
significantly alter or restrict the flow, either into or out of these marshes.
The altering of flows can affect the ecological nature of the area and the area-wide hydraulics. A
possible reduction in interior tide heights because of the isolation of an inlet may cause increased
velocities, scour, or increased wave heights somewhere else, often along the highway. Salinity may
be changed, with stratified freshwaters and saltwaters flowing in different directions. This could
change the type and extent of vegetation that, in turn, could affect the wildlife of the marsh.
Again, although these problems might not be solved during the planning and location phase, they will
have been recognized and the need for special studies, if necessary, realized. Two-dimensional
models would be a study method applicable to tidal areas. In special cases, extensive studies and even
specialists in tidal hydraulics might be required to ensure that an acceptable design is provided.
1.3.3.2 Land Use Considerations
The use of land adjacent to the stream must be considered. In rural areas, the most significant
consideration is how the crossing may affect property, both upstream and downstream. Upstream, the
concern is usually with backwater effects and increased flood stages. The degree and duration of an
increased flood stage could affect the present and future land use. Even agricultural land has to be
evaluated for increased risks due to flooding. As an example, crops may be impacted by inundation.
Downstream, the hydraulic effects that are of usual concern are related to increased velocity through
the structure. This higher velocity may increase scour immediately below the crossing or increase
aggradation downstream. Potential downstream effects are usually more difficult to quantify than
upstream effects.
In urban areas, the effects of increased flood stages or increased velocities become important
considerations. In addition to the impact on future land use, the existing property may suffer extensive
physical damage from an increased flood stage. FEMA floodplain and floodway requirements must
be carefully evaluated.
The impact on traffic safety and operation may extend well beyond the stream crossing, as increased
flooding may occur on the adjacent street network, inhibiting or obstructing vehicular movement.
This may result in extensive delays, more frequent accidents and the inability to provide the necessary
fire, police, and rescue services.
Most urban areas will have stream or watershed management regulations or may be in the national
flood insurance program. These generally dictate the limits on the changes that can be made to the
flow characteristics of a watershed.
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1.3.3.3 Type of Structure
The location of a stream crossing may influence and limit the type of structure that can be used.
Decisions made during the preliminary phases of project development should not constrain the final
recommendations of the hydraulics engineer. Detailed surveys and comprehensive hydrologic and
hydraulic studies are needed to make conclusive recommendations. Even then, the hydraulics
engineer may recommend alternative types and shapes, depending on the site and State policy.
Sometimes there will be critical crossing sites, such as those within a designated flood insurance area
that may require detailed studies early in the project development. Permits acquisition in these areas
requires that plans be more specific than usual at earlier stages of the project. When this is the case, a
final structure type can be provided.
There are many considerations to be made before selecting a final design alternative. These include
hydrologic, hydraulic, environmental, economic, construction and maintenance factors. Other
chapters of these guidelines cover these considerations in detail. These include Chapter 4, Chapter 7,
and Chapter 8 (2).
1.3.4 Encroachments
There are two types of encroachments and each presents the need for unique considerations.
Longitudinal encroachments are where the highway is within the stream floodplain boundary and the
highway alignment is approximately parallel to the stream. These encroachments can be critical
features because of the long distance that may be exposed to the stream. Transverse encroachments
are stream crossings, either normal or skewed, where there is some encroachment on the floodplain.
1.3.4.1 Longitudinal Encroachments
Historically, transportation facilities have been located along streams to obtain flatter roadway grades.
There is less vegetation, soils are more conducive to construction, and it may provide the only
feasible route without a tunnel. Historically, transportation facilities with longitudinal encroachments
have suffered severe damage during flood events. The hazards associated with building within the
floodplain must be recognized during the planning and location stages so that either precautions
against damage can be taken or changes made to the location.
Longitudinal encroachments should be avoided wherever practicable and alternative routes outside
the floodplain are available.
Hazards associated with longitudinal or parallel locations are greatest in narrow or V-shaped valleys
with steep gradients. At flood stage, the stream covers all or most of the valley section. Locations in
U-shaped valleys with broad terraces above the channel may be secure from flooding except during
rare or infrequent floods. These latter valley locations usually involve streams in alluvium, and
problems may develop from the outward and downstream migration of bends, from aggrading or
degrading channels, and at confluences.
Figure 1-2 illustrates the three general types of longitudinal encroachments. These may be classified,
according to proximity of main and overflow channels, as:
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Highway Drainage Guidelines
floodplain encroachments (Figure 1-2A),
stream encroachment—fill section (Figure 1-2B), and
channel encroachment—cut and fill section (Figure 1-2C).
Figure 1-2. General Types of Longitudinal Encroachments
Roadway locations parallel to alluvial channels may be jeopardized by eroding stream banks and
velocity and flow concentrations adjacent to the roadway. All or portions of the highway alignment
may be fairly remote from the stream and would appear to be secure (Figure 1-2A), but lack of access
to the eroding stream bank may deter defensive measures until a meander cuts through private
property and attacks the highway. Possible advantages of a location that is relatively remote from the
stream bank are lower velocities, less expensive embankment protection, and less impact on flood
stages.
Channel encroachment locations (Figures 1-2B and 1-2C) are common where highways follow
mountain streams in narrow valleys or canyons. Much of the roadway may be on fill which
encroaches on some portion of the stream channel. When the interference with normal flow is not
substantial, the cost of embankment protection may be moderate except at points of impingement and
at bends. On the other hand, if the encroachment significantly constricts the natural stream and flood
conveyance section, the possible effects could be (1) acceleration of flow resulting in attack on the
highway embankment or, if the embankment is sufficiently armored, the erosive power can attack the
streambed or opposite bank; (2) potential flooding of upstream property due to backwater effects
from the construction; and (3) accumulation of drift and/or ice.
The obvious disadvantages of an encroaching location are the increased flood risk, potential for losing
the highway, cost to protect the facility and environmental impacts. It may be necessary to provide
additional waterway opening through the constricted section by widening along the opposite bank or
providing adequate transition sections into and away from the constriction and sufficient conveyance
modification to increase the channel capacity.
Channel encroachment locations may require channel modifications such as stream and bank
excavation, and replacement of tree and rock cover with riprap. Environmental impacts in the form of
silt and erosion, deterioration of fish and wildlife habitat, and loss of wetlands may result from
locations adjacent to channels.
Longitudinal encroachments crossing tributary streams near stream confluences should be avoided
due to probable aggradation or degradation resulting from the instability of the confluence location.
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Tributary channel crossings could be adversely affected by both low and high stages on the major
stream.
1.3.4.2 Transverse Encroachments
Stream crossings, whether normal or skewed, will usually involve some encroachment on the stream.
The exception to this general statement is the crossing of a narrow canyon or gorge where
topographic, geometric, or structural considerations require spanning the entire channel. This type of
crossing seldom imposes any measurable constriction of the stream and floodplain.
The more common types of crossings involve construction of an approach embankment across a
portion of the floodplain with a structure across the main stream. Occasionally, supplemental
structures are located on the floodplain to accommodate overbank flow during flood events. The
floodplain may be relatively narrow or sometimes several kilometers [miles] in width, clear or heavily
wooded, symmetrical about the stream channel or eccentric. Land use on floodplains may vary from
wetlands and swamps to commercial and residential use.
Localized channel modifications are sometimes necessary to accommodate the approach
embankments and structure. The extent of modifications required varies with the degree of
encroachment and should be a consideration in the study of alternative locations. Alternative
transverse encroachments should be evaluated in the location phase of planning to assure
consideration of hydraulic, economic, and environmental concerns.
Undesirable features of transverse encroachments are illustrated in Figure 1-3. These include (A)
reverse curvature of roadway; (B) reverse curvature in channel; (C) extreme skew; and (D) extreme
encroachment on the stream.
Figure 1-3. Geometric Features of Encroachments
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Highway Drainage Guidelines
Reference (5) presents several illustrations and comments on the common types of transverse
encroachments and the potential local, upstream and downstream effects that may result from a
particular crossing. The possible effect of the stream on the roadway and the potential effects of the
roadway on the stream are presented.
Additional detailed discussion and guidelines on transverse encroachments are presented in Chapter
7, “Hydraulic Analysis for the Location and Design of Bridges,” of the Highway Drainage
Guidelines (2).
1.3.5 Ice and Debris
The history and potential for ice and debris jams and the possible consequences from these events
should be noted during the planning and location phases of the project development. The most
common causes of both ice and debris jams is a sudden change in cross section geometry in either
width or depth or in stream gradient that changes the stream velocity. These changes tend to constrict
or slow the flow, giving the ice or debris the chance to accumulate, bind together and, thus, create a
barrier.
Highway fills and structures usually are not the cause of ice jamming unless they create a severe
stream constriction. The loss of a bridge or the flooding of a roadway caused by a large buildup of
water behind an ice jam is an occurrence that might be avoided by raising the facility above the water
level predicted to be reached during such an event. Transportation Research Record 995, Wastewater
Treatment and Hydraulics, contains a paper “Ice Jams at Highways and Bridges—Causes and
Remedial Measures” (9) that may be helpful in learning more about ice jams and highways.
Debris jams can be caused by a bridge or culvert opening being too small to efficiently pass objects
(e.g., logs, large trash). Unfortunately, it is not easy to predict the size or occurrence of debris.
Alternative measures include:
trapping debris upstream of the structure,
raising the roadway grade,
lengthening the bridge or increasing the culvert size, and
designing an overflow section so water can flow around the structure.
During the design phase, it may be determined that fins or walls may be needed at the entrance to a
pipe to direct or guide debris into alignment with the flow. Thus, the need for debris racks or
deflectors, often as suggested by the maintenance personnel familiar with the area, should be
considered during the planning or location phases. The need for these special items may require
additional right-of-way to accommodate them. Recommended references on the design and placement
of debris racks are the FHWA Hydraulic Engineering Circular No. 9, Debris-Control Structures
Evaluation and Countermeasures (7) and FHWA-RD-97-028, Potential Drift Accumulation at
Bridges (6).
1.3.6 Location of Storm Drainage Facilities
The location of storm drainage facilities is another item that should be considered during the early
phases of a project. Collection systems, the main pipes, pumping stations, and particularly, the outfall
alternatives should be tentatively located.
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Collection points should be located early in the project development, especially for large systems,
chiefly for right-of-way considerations. Another reason, however, is the possibility of combining the
collection of stormwater from several watersheds or for connecting to an existing system. The
capacity of existing systems to accept the flows from these collection points and water quality
considerations would be the main concerns at this point in the project.
If a project is an improvement to an existing highway, collection points will have been in existence
for several years. The possibility of altering, adding, or deleting points should not be overlooked,
however, as a more cost-effective and hydraulically efficient system may be possible.
Storm drain collection pipes are commonly located parallel to the highway. However, consideration
should be given to the terrain, and the possibility of construction problems with this generally
accepted solution. Sometimes, a route with less excavation or other advantages may be available.
The location of outfall alternatives is the most important consideration for storm drainage systems
made during the planning and location phases. Drainage must be discharged into natural or
constructed drainage features capable of conveying this flow in a safe and efficient manner. Sinkholes
or other low-lying areas without a natural outlet must be avoided. With constructed facilities, such as
irrigation canals, it is advisable to obtain written agreements for the discharge and assurance the
facility will remain in perpetuity.
Existing outfalls must be checked for present and future adequacy and whether or not downstream
problems such as erosion or flooding could occur. Proposed outlet locations should be checked for the
same considerations, and ensuring the legality of creating a flow where none, or very little, has
previously existed. Coordination with the local community will often be necessary when tying into
existing outfalls. New outfalls may also need to be coordinated because the community may have
plans in progress utilizing the outfall area for other purposes.
Highways on new locations in urban areas may significantly affect existing surface runoff patterns
and storm drainage systems. Depressed highways will most likely cut through existing storm drains
while highways on fills will isolate drainage areas. Early and careful attention to these types of
projects is needed or alternatives suggested to ensure a feasible system for accommodating disrupted
drainage patterns can be designed.
1.3.7 Location of Utilities
During the location phase, it is important for the hydraulics engineer to be aware of utility locations
and types and their relationship to the proposed highway project.
Locations of overhead power lines, underground and underwater water and sewer lines, and utility
facilities, such as pumping stations or access points to any appurtenant tunnel chambers, will be found
by others. The hydraulics engineer must then evaluate if and how these features may affect the
various hydraulic structures or conversely be affected by them.
If power lines have to be relocated on or buried within an encroachment, their relationship to the
projected flood levels must be considered. The placement of a telephone line on an embankment at a
level about that which ice flows could knock over the poles might be such a consideration required.
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Highway Drainage Guidelines
The reconstruction of a pumping station that could either be flooded or an obstacle to flood flows if
not placed at a proper level is another example of what may need to be considered.
Even the maintenance of utility facilities may entail hydraulic considerations. Excavating a utility for
repairs buried within an encroachment could affect the stability of the embankment or stream and thus
expose, even temporarily, the highway to increased erosion potential.
The construction of a storm drainage system or the improvement to an existing one can interfere with
utilities. Often, in older urban areas, types of utilities and their locations are not accurately
documented, if at all. In these cases, the hydraulics engineer should coordinate early with all
appropriate utility personnel to locate as many of the lines as possible to facilitate the later design
process and provide input to the location process.
1.3.8 Floodplain Development and Use
The floodplain has traditionally been an area of great activity and use by humans and by other
biological systems. Humans, plants, and animals all compete for its use. Transportation systems have
been located on floodplains to serve existing development and because of the ease and economy of
construction.
However, today there is much effort to reduce floodplain uses to those that can accommodate the
periodic flooding and ecological systems traditionally associated with these areas. The FHWA
regulation (4) emphasizes that practicable alternatives to actions within the floodplain must be
evaluated and considered. To retain floodplains for their natural use, these alternatives should be
adopted, whenever possible.
The practicability of locating outside of the floodplain must be determined during planning and
location. If these alternatives are not available, studies will have to be expanded on how best to
integrate the highway into the environment of the floodplain.
Land use plans are a good source of data on the present use of a floodplain. In those instances where
none are available, field surveys or aerial photographs can be used. It is helpful to superimpose these
on a floodplain map. Recognizing that the map itself may not be prepared this early in the project
development, initiating data collection necessary for this map should be started.
From study of such maps, critical areas such as those prone to regular flooding, environmentally
sensitive areas, wildlife refuges and urban areas should be evident. Overlays such as described in
Highway Drainage Guidelines’ Chapter 10, “Evaluating Highway Effects on Surface Water
Environments,” (2) could be prepared.
Future development and land use within a floodplain may not be readily known or even easily
projected. Talking to local officials, working with zoning and floodplain maps, and contacting other
resource and regulatory agencies may give indications about future development. Within the highway
agency itself, economists, planners, and right-of-way appraisers may already have obtained data on
future plans for lands in or near projected highway corridors.
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1.4 PRELIMINARY SURVEYS
Surveys can take many forms but, for purposes of this section, they shall be the collection of site
information from any source. This can include bridge inventories and inspections. Site information of
particular interest to the hydraulics engineer includes all topographic data, hydrologic and hydraulic
data of the region, and environmental information.
Some of the more common sources of this data include:
aerial and field surveys,
Water Resource Agencies,
Fish and Wildlife Agencies,
Planning Agencies,
floodplain insurance studies,
newspapers,
interviews,
photographs, and
field visits.
Data collection should be as complete as possible during the initial survey to avoid repeat visits, but
must also be tailored to satisfy the requirements of the specific location and magnitude of the project
for which the study is required. Coordination with all sections requiring survey data before the initial
field work is begun will help ensure the acquisition of sufficient, but not excessive, survey data.
The following sections detail the types of survey data that may be needed by the hydraulics engineer
during the planning and location phases of project development.
1.4.1 Topographic Data
Topographic data should be acquired at those sites requiring hydraulic studies. When needed, this
data is used to analyze existing flow conditions and future flow conditions caused by various design
alternatives. Significant physical and cultural features in the vicinity of the project should be located
by the survey to obtain their elevation. Such features as residences, commercial buildings, schools,
churches, farmlands, other roadways and bridges, and utilities can affect, and be affected by, the
design of any new hydraulic structures.
Often, recent topographic surveys will not be available at this early stage of project development.
Aerial photographs, photogrammetric maps, USGS quadrangle sheets, and old highway plans may be
used during the planning and location phases. When more complete survey data becomes available,
usually during the design phase, these early estimates may need to be upgraded to correspond with the
most recent field information.
Digital elevation models (DEMs) or digital terrain models (DTMs) are becoming increasingly popular
within the field of digital topographical data. They have become valuable in numerous hydraulic
engineering and hydrological applications due to the precision of a DEM in replicating true terrestrial
elevation, slope and land-use characteristics.
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1.4.2 Channel Characteristics
To perform a detailed hydraulic analysis, the stream profile, horizontal alignment, and cross sections
are required. This data usually is not available during the planning and location phases. The
hydraulics engineer must therefore make preliminary analyses based on data such as aerial
photographs, USGS maps, and old plans.
One method that is beneficial for determining channel characteristics is the taking of photographs.
They are useful for identifying material in the streambeds and banks, type and density of vegetation,
and evidence of drift, debris, or ice. Field visits made early in the project life include the
photographing of the channel, upstream, and downstream, and the adjoining floodplain. The photos
will be valuable aids for not only preliminary studies, but also for documentation of existing
conditions.
During these early phases of project development, the hydraulics engineer should be involved in
determining the detail of field survey required at the site for the design phase. This should include the
upstream and downstream limits of the survey, the number of or distance between cross sections, and
how far to either side of the channel the sections should extend. The number of cross sections needed
will vary with the study requirements and the particular stream characteristics. For some projects, the
topographic accuracy achieved by aerial photogrammetry will be sufficient for the level of hydraulic
study needed, while other sites will require a higher level of accuracy. With planning and location
studies, the level or accuracy of survey required should be a consideration when determining the
degree of hydraulic assessment or analysis needed. The USACE’s Hydrologic Engineering Center has
made a detailed study of survey requirements. The results of this study are available in Accuracy of
Computer Water Surface Profiles by M.W. Burnham and D.W. Davis, Technical Paper No. 114,
1986 (3).
1.4.3 Hydrologic Data
Information required by the hydraulics engineer for analysis and design includes not only the physical
characteristics of the land and channel, but all the features that can affect the magnitude and
frequency of the flood that will pass the site under study. This data may include climatological
characteristics, land runoff characteristics, stream gaging records, highwater marks, and the sizes and
past performances of existing structures in the vicinity. The exact data required will depend upon the
methods utilized to estimate flood discharges, frequencies and stages. This subject is discussed in
detail in Highway Drainage Guidelines’ Chapter 2, “Hydrology” (2). This section will therefore only
highlight some of the more important aspects that should be considered in the early stages of the
project development.
It should be noted that much of this data is not ordinarily used during the planning and location
phases but during the design phase. It is important to determine the need for the data during the
planning and location phase to minimize potential delays.
1.4.3.1 Basin Characteristics
The hydrologic characteristics of the basin or watershed of the stream under study are needed for any
predictive methods used to forecast flood flows. Although many of these characteristics can be
determined from office studies, some may require a field survey. The size and configuration of the
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watershed, the geometry of the stream network, storage volumes of ponds, lakes, reservoirs and
floodplains, and the general geology and soils of the basin can all be found from maps. Land use and
vegetal cover sometimes appear on maps and aerial photos but, with rapidly changing land uses, a
more accurate survey will probably be achieved by a field visit.
With these characteristics, runoff times, infiltration values, storage values, and runoff coefficients can
be estimated for use in calculating flood magnitudes.
1.4.3.2 Precipitation
A precipitation survey normally consists of the collection of rainfall records for the rainfall stations in
the vicinity of the study site. If necessary, rainfall records from outside the watershed can be utilized.
These records should contain several years of events, for every month and season, and should include
duration values for various length rainstorms. Snowfall accumulations may also be available and are
often helpful.
If rainfall records are lacking, the National Oceanic and Atmospheric Administration (formerly the
Weather Bureau) has publications that give general rainfall amounts for various duration storms that
can be used where local rainfall information is not available. Rainfall intensity maps and monthly
climatic summaries for various regions of the country are some of the publications available.
1.4.3.3 Flood Data
The collection of flood data must precede any hydraulic analysis. This data can be collected both in
the office and in the field. Office acquisition includes the collection of past flood records, stream
gaging records and newspaper accounts. Field collection will consist mainly of interviews with
residents, maintenance personnel, and local officials who may have recollections or photos of past
flood events in the area. Whenever possible, highwater marks should be associated with these
recollections as noted in the next section.
If a stream gaging station is on the stream under study, close to the crossing site, and has many years
of measurements, this may be the best hydrologic data available. The data should be analyzed to
ensure stream flows have not changed over the time of measurement due to the watershed alteration,
such as the construction of a large storage facility, diversion of flow to or from another watershed, or
development that has significantly altered the runoff characteristics. USGS Bulletin 17B, Guidelines
for Determining Flow Frequency (8), is helpful in presenting methods to make these studies. Local
USGS offices may also be available to assist.
1.4.3.4 Highwater Information
Highwater marks can be identified in several ways. Small debris (e.g., grass or twigs caught in tree
branches, hay or crops matted down, mud lines on buildings or bridges) are all highwater indicators.
Ice will often cut or gouge into the bark of trees, sometimes even tearing the bark off on the side of
the tree exposed to the flow.
Sometimes, highwater marks are the only data of past floods available. When collected, this data
should include the date and elevation of the flood event when possible. The cause of the highwater
mark should also be noted. The mark may be caused by an unusual debris or ice jam rather than an
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inadequate structure. Designing the roadway grade or structure opening for such an event could lead
to an unrealistic, uneconomical design.
1.4.3.5 Existing Structures
The size, location and condition of existing bridges and culverts on the stream under study can be a
valuable indicator when selecting the size for a new structure. Data to be obtained on existing
structures includes size, type, age, and condition. Scour holes, erosion around the abutments, or just
upstream or downstream, or abrupt changes in material gradation or type can all indicate a structure
too small for the site. Overflow sections, particularly at culvert crossings, must be included in the
evaluation of total waterway available. Given a history of floods at the site and information available
from the National Bridge Inventory or State Inventory, the hydraulics engineer may be able to
determine if the structure has been adequate.
If a structure is relatively new, information may still be available on the previous one, and why it was
replaced. Although crossings are normally replaced because of poor structural conditions, other
conditions, often hydraulic in nature, also enter into the decision to replace a relatively new structure.
Old plans may also contain highwater or other flood information that can be of use. Existing
structures upstream or downstream of the site under study should always be evaluated for the
factors just discussed. This includes highway and railroad structures and any private crossings that
might exist.
1.4.4 Environmental Data
An environmental team should identify the environmental data required to evaluate the highway
impacts on the surface water. A coordination meeting with representatives of the various
environmental disciplines concerned is often beneficial during the planning and location phase. Data
may need to be collected on such things as fish and wildlife, vegetation, and the quality of the water.
A judgment on the aesthetic values may also be necessary. The effects of the highway on the
environment are discussed in detail in Chapter 10, “Evaluating Highway Effects on Surface Water
Environments,” of the Highway Drainage Guidelines (2).
1.4.4.1 Fish and Wildlife
There are many sources available from which information on fish and wildlife can be gathered.
Probably the most beneficial are the State fish and wildlife agencies. Biologists can provide data on
fish and other aquatic animals, their spawning seasons, and critical stream areas. Maps may also be
available showing this information. Field visits including interviews with local residents can yield
information not found elsewhere.
1.4.4.2 Vegetation
The types and extent of vegetal cover can affect the rate of runoff, quantity, and quality of the water.
There are three primary sources from which information on vegetation may be found.
The first source is maps. Geological maps depict in general terms where the land is covered and
where it has been cleared. Often, this is sufficient, particularly during the preliminary stages of a
study. As the study progresses, more data may be needed (e.g., the type of cover, agricultural crop
land, pasture, evergreen forest). Some more detailed scale maps from State agricultural or extension
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units may show this. Many of these maps are available on the internet as shape files for Geographical
Information Systems (GIS) programs.
A second and more accurate source is aerial photographs. An experienced person can distinguish the
various types of vegetation. Should photos in color or infrared be available, the classification of
different types of cover may be even easier. Aerial photos must be up-to-date, of good quality, and to
scale to be of most value.
The third source is the field visit. It may not be possible to survey the entire watershed, so a sample
area may have to be studied. It is important to set out the exact field needs before the trip is made to
ensure that all information is collected and all important sites visited.
1.4.4.3 Water Quality
Water quality data can be the most expensive and time-consuming information to collect. Sometimes,
water quality records are available for or near the site under study but, even then, the information
most often required for highway studies has not been gathered. Sample collection is expensive
because of the equipment and laboratory facilities needed. If the highway agency itself chooses not to
do the testing, then the cost of having samples taken and analyzed by other laboratories must be
considered. Sampling techniques are specific and must be followed, or else other agencies may not
accept the results.
Sample collection can be time consuming because one sample or several taken at the same time is not
usually satisfactory. Water quality can reflect seasonal, monthly, or even daily variations depending
on such things as the weather, flow rate and traffic. Therefore, a sampling program must be of
sufficient duration to detect these variations. Usually a year is necessary to achieve this. (See
Reference (10).)
Water quality data collection and analysis must be conducted by an experienced person who has been
properly trained. This may be someone within the highway agency who has been trained in this field,
or it may be necessary to retain an outside firm to perform this portion of the environmental analysis.
1.4.5 Field Review
An actual visit to the site where the project will be constructed should be made before any detailed
hydraulic design is undertaken. Most likely, this will be combined with the visit by others, such as
the roadway and structural designers, environmental reviewers and even local officials. Often,
though, the hydraulics engineer will visit the site separately because of interests that are different
from the others.
There are several considerations that should be made before making the field visit. What kind of
equipment should be taken and, most important, what exactly are the critical items at this site?
Photographs should always be taken. Ideally, photos should be taken looking upstream and
downstream from the site and along the contemplated highway line in both directions. Details of the
streambed and banks should also be photographed along with other structures in the vicinity.
A printed form containing a list of the more important data to be obtained from the field visit should
be prepared so that no element is overlooked. This is a valuable and useful document when the actual
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design is performed, especially if the designer was not the one who made the field visit. An example
of such a form is shown after Section 1.6. Many hydraulic publications have similar examples.
1.5 PRELIMINARY HYDRAULIC REPORTS
All data considered and used in reaching conclusions and recommendations made during the
preliminary study should be included in a report. This should include hydrologic and hydraulic data,
pertinent field information, photographs, calculations, and structure sizes and locations. At this stage
of the study, several structure sizes and types may be given because the preliminary designer only
needs generalities to obtain a rough estimate of needs and costs. Often, specifics cannot be provided
until an accurate topographic survey of the area has been made and precise hydraulic computations
performed. Sometimes, however, the report will require detailed design studies to justify the extent of
mitigation required. In general, the more environmentally sensitive sites and those in highly urbanized
areas will necessitate more detail at earlier stages.
All this information, however, serves as documentation for decisions made at this time and excellent
reference material when the later, more detailed studies are performed. Therefore, it is important that
this material be as carefully collected, prepared, referenced, and put into an easily understood
preliminary report folder as would be done for the final study. It is important that this work be clearly
marked as preliminary. Otherwise, the preliminary work might be used as final data and no further
involvement of the hydraulics engineer requested.
The hydraulics engineer can provide information relating to surface waters for the Draft
Environmental Impact Statement (DEIS). This information may include the effects of the project on
water quality, flooding and general water resources values.
1.6 REFERENCES
(1)
AASHTO. A Policy on Geometric Design of Highways and Streets. American Association of
State Highway and Transportation Officials, Washington, DC, 2004.
(2)
AASHTO. Highway Drainage Guidelines. Task Force on Hydrology and Hydraulics, American
Association of State Highway and Transportation Officials, Washington, DC, 2007.
(3)
Burnham, M. W. and D. W. Davis. Accuracy of Computer Water Surface Profiles. Technical
Paper 114. Hydrologic Engineering Center, U.S. Army Corps of Engineers, Washington, DC,
1986.
(4)
FHWA. Location and Hydraulic Design of Encroachments on Floodplains. 23 CFR 650,
Subpart A. Federal Highway Administration, U.S. Department of Transportation, Washington,
DC, 1969.
(5)
FHWA. Highways in the River Environment, Hydraulic and Environmental Considerations,
Training and Design Manual. National Highway Institute, Federal Highway Administration,
1973, Revised 1990.
Hydraulic Considerations in Highway Planning and Location
1-33
(6)
FHWA. Potential Drift Accumulation at Bridges. FHWA-RD-97-028. Federal Highway
Administration, U.S. Department of Transportation, Washington, DC, 1997.
(7)
FHWA. Debris Control Structures Evaluation and Countermeasures, Third Edition. Hydraulic
Engineering Circular No. 9, FHWA-IF-04-016. Federal Highway Administration, U.S.
Department of Transportation, Washington, DC, September 2005.
(8)
Interagency Advisory Committee on Water Data. Guidelines for Determining Flood Flow
Frequency. Bulletin 17B. March 1982.
(9)
Shattuck, R. F. Ice Jams at Highways and Bridges—Causes and Remedial Measures. In
Transportation Research Record 995, Wastewater Treatment and Hydraulics. Transportation
Research Board, National Research Council, Washington, DC, 1984.
(10) USGS. National Field Manual for the Collection of Water-Quality Data. Chapter A, Handbooks
for Water-Resources Investigations. U.S. Geological Survey, Washington, DC, 1997.
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APPENDIX 1A
FIELD VISIT INVESTIGATION FORM
DATE __________________
PROJECT _______________
BY _____________________
TYPE _____________________________________
PIERS: TYPE _______________________
SPAN _____________________________________
SKEW _____________________________
NO. OF SPANS _____________________________
INLET _____________________________
CLEAR HT. ________________________________
OUTLET ___________________________
ABUT. TYPES _____________________________
GRADE OF ROAD ___________________
EXISTING WATERWAY COVER _____________
% GRADE OF STREAM ______________
OVERFLOW BEGINS @ EL. _________________
LENGTH OF OVERFLOW ____________
MAX AHW ________ M. REASON ____________
CHECK FOR DEBRIS ________________
UP OR DOWNSTREAM RESTRICTION ________
CHECK FOR ICE ____________________
OUTLET CHANNEL, BASE __________________
SIDE SLOPES _______________________
HEIGHT OF BANKS _________________
MANNING’S n VALUE
TYPE OF MATERIAL IN STREAM __________________________________________________
PONDING _______________________________________________________________________
CHECK BRIDGES UPSTREAM AND DOWNSTREAM _________________________________
CHECK LAND USE UPSTREAM AND DOWNSTREAM ________________________________
SURVEY REQUIRED:
REMARKS:
YES_______
NO________
CHAPTER 2
HYDROLOGY
Chapter 2
Hydrology
2.1 INTRODUCTION
Hydrology is the science that treats the waters of the earth, their occurrence, circulation and
distribution, their chemical and physical properties, and their reaction with their environment,
1
including their relation to living things (1). It is also defined as the science that deals with the
processes governing the depletion and replenishment of the water resources of the land areas of the
earth (84). It is concerned with the transportation of water through the air, over the ground surface,
and through the strata of the earth.
Although hydrology is a very broad science encompassing many disciplines relating to water, the
hydraulics engineer is more concerned with estimating runoff than any other hydrologic problem. The
scope of this chapter will be primarily limited to surface hydrology.
Hydrologic analysis is the most important step prior to the hydraulic design of a highway drainage
structure regardless of its size or cost. Such an analysis is necessary to determine the discharge (rate
of runoff) and volume of runoff that the drainage facility will be required to convey or control.
Although some hydrologic analysis is necessary for all highway drainage facilities, the extent of such
studies should be commensurate with the hazard associated with the facilities and with other
economic, engineering, social, and environmental concerns. While performing the hydrologic
analysis and hydraulic design of highway drainage facilities, the hydraulics engineer should be
cognizant of potential environmental problems that would impact the specific design of a structure.
This area should be evaluated before spending a large amount of time in detailed design.
Highway drainage facilities are designed to convey predetermined discharges to avoid a significant
flood hazard. Provision is also made to convey floods in excess of these discharges in a manner that
minimizes the damage and hazard to the extent practicable. These discharges are often referred to as
peak discharges because they occur at the peak of the stream’s flood hydrograph (discharge over
time). These flood discharge magnitudes are a function of their expected frequency of occurrence that
in turn relates to the magnitude of the potential damage and hazard.
Also of interest is the performance of highway drainage facilities during the frequently occurring lowflood flow periods. Because low-flood flows do occur frequently, the potential exists for lesser
amounts of flood damage to occur more frequently. It is entirely possible to design a drainage facility
to convey a large, infrequently occurring flood with an acceptable amount of floodplain damage only
to find that the aggregate of the lesser damage from frequently occurring floods is intolerable.
1
Italicized numbers in parentheses refer to publications in “References” (Section 2.11).
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Besides the peak discharges, the hydraulics engineer is sometimes interested in the flood volume
associated with a flood hydrograph. Flood hydrographs can be used to route floods through culverts,
flood storage structures, and other highway facilities. By considering the stored flood volume, the
hydraulics engineer can often design a storage structure to decrease the flood peak discharge and thus
the size of the drainage facility. Flood hydrographs are also useful in environmental and land use
analyses.
Hydrology is considered an interdisciplinary science because it borrows heavily from many other
branches of science and integrates them for its own interpretation and uses. The supporting sciences
required for hydrologic investigations include such things as physics, chemistry, biology, geology,
fluid mechanics, mathematics, statistics, and the related research. Because hydrologic science is not
exact, it is possible that different hydrologic methods developed for determining flood runoff may
produce different results for a particular situation. To this end, sound engineering judgment must be
exercised to select the proper method or methods to be applied. Reference (61) is useful when
comparing hydrologic methods. In some instances, certain Federal, State, or local agencies may
require that a specific hydrologic method(s) be used for computing the runoff.
In this chapter, key aspects of hydrologic information relevant to highway engineering are discussed.
The chapter is not intended to be all inclusive, but an effort has been made to cover as broad a
spectrum of the subject as deemed appropriate, and references are cited for more detailed information.
2.2 FACTORS AFFECTING FLOOD RUNOFF
The hydraulics engineer should become familiar with the many factors or characteristics that affect
flood runoff before making a hydrologic analysis. The peak discharge and volume of runoff are
considered to be affected by similar factors, although the degree of influence by any given factor may
be different between these two runoff categories. Factors affecting flood runoff can be broadly
classified as physiographic, site specific, and meteorological; however, the three classes are
interrelated in their flood-producing effects. In addition, components within such classes are so
interrelated that experience and judgment are necessary to properly evaluate the various factors that
apply to a particular situation.
There have been numerous studies that establish that some factors are more important than others in
affecting peak discharge or volume of runoff. The dominant factors may vary with each individual
site and hydrologic method.
Some of the major factors related to runoff are discussed in the following sections. Those factors
responsible for floods attributed to dam failures, tidal action and similar events are not presented. The
physiographic, site-specific, and meteorological characteristics that may be used for flood runoff
analysis are detailed in References (35), (36), (53), and (68).
2.2.1 Physiographic Characteristics
Physiographic factors may be grouped into two categories: basin characteristics and channel
characteristics (22). Basin characteristics include such factors as size, shape, and slope of drainage
area, soil permeability, and capacity of groundwater formations, presence of lakes and swamps, and
land use. Channel characteristics are related mostly to the hydraulic properties of the channel that
Hydrology
2-3
govern the movement of stream flows and determine channel storage capacity. In evaluating the
importance of various hydrologic characteristics for determining flood runoff (Section 2.7), it is often
necessary to compare drainage basins; therefore, the hydraulics engineer should be familiar with
drainage basin characteristics and how they affect flood runoff. Surface and subsurface runoff are
collected and conveyed through the stream channels. The natural or altered condition of these
channels can materially affect the volume and rate of runoff, so these conditions should be considered
in the hydrologic and hydraulic analyses.
The relative importance of physiographic characteristics varies between different hydrologic areas
and geologic and geographic regions.
2.2.1.1 Drainage Area
A drainage basin is commonly surrounded by a readily discernible topographic divide, which is a line
of separation that divides the precipitation that falls on two adjoining basins so that the ensuing runoff
is directed into one or the other channel system. The size or area of the drainage basin is considered to
be the area that contributes the surface runoff and is bounded by all or portions of the topographic
divide.
The size of a drainage basin is an important parameter with respect to the response of the basin to
rainfall. Flood runoff in the same geographical area can be generally shown to be proportional to
some power of the drainage area (Section 2.7.1.3). However, the effect of other basin characteristics
often obscures the effect of drainage basin size alone.
Determining the size of the drainage area that contributes to flow at the drainage structure site is a
2
basic step in a hydrologic analysis. The drainage area, usually expressed in hectares [acres], or km
2
[mi ], is determined from field surveys, topographic maps, aerial photographs, or a combination of
these items.
Topographic maps are valuable aids in obtaining the size of drainage areas. The most commonly used
topographic maps are those of the USGS. Information concerning these can be obtained from the
USGS Information Center, Box 25286, Federal Center, Denver, CO 80225 or over the counter at
various USGS Earth Science Information Centers (see www.usgs.gov).
Field inspection of the drainage area, especially for small basins, is very desirable because
topographic maps are not always current. Although the contour maps may show many areas as
contributing to the runoff, a field inspection may show natural or man-made depressions such as
gravel pits, playa lakes, or natural sinks, which may intercept a portion of the runoff from the
drainage area. There may also be subtle topographic features that divert runoff from one watershed
into another or indistinct divides not apparent on topographic maps. Once the boundaries of the
contributing areas have been established, they should be delineated on a base map and the areas
determined. For urban areas, a local agency’s sewer maps may be a valuable source of drainage
boundary information.
Accurate aerial photography supplemented by vertical and horizontal control surveys provides a
means of measuring the size of a drainage area. Although uncontrolled aerial photographs aid the
engineer and are of generally acceptable accuracy for large areas, the determination of the boundary
of a drainage area by the photographs should be supplemented with field verification.
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Digital elevation models (DEMs) or digital terrain models (DTMs) are becoming increasing popular
within the field of digital topographical data. They have become valuable in numerous hydraulic
engineering and hydrological applications due to the precision of a DEM in replicating true terrestrial
elevation, slope, and land-use characteristics. Although availability is a concern because DEMs are
not available in many regions, it is anticipated that DEMs will eventually replace standard mapping
technology.
2.2.1.2 Shape Factor
The shape, or outline form, of a drainage basin mainly affects the rate at which water is supplied to
the main stream as it proceeds along its course from the runoff source to the site of interest.
Long, narrow watersheds have generally been considered to give lower peaks than fan- or pearshaped watersheds, other characteristics being equal. It has been observed that while long, narrow
watersheds may have lower runoff rates where storm direction is across the watershed, rates would be
higher if a storm moves longitudinally down the basin axis.
In regard to the shape factor, the distance from the basin outlet to the centroid of the basin can be an
important element in some locations for determination of the quantity of flood runoff, especially peak
discharge. Other physiographic characteristics being equal, a watershed having a longer length to the
centroid has been considered to produce lower peak discharges than the watershed with a shorter
length to the centroid. This length is, generally, highly correlated with the shape of a basin.
2.2.1.3 Slope
The slope of a drainage basin has an important, but rather complex relation to infiltration, surface
runoff, soil moisture, and groundwater contribution to stream flow. It is one of the major factors
controlling the time of overland flow and concentration of rainfall in stream channels. It is of direct
importance in relation to flood magnitude. Basin slopes are usually estimated from contours on
topographic maps or may be determined by a field survey. This parameter is important in that steeper
basins yield a quicker response time whereas flat basins reflect a slower response time. Long response
time will lower flood peaks while a short response time will increase the peak discharges.
2.2.1.4 Land Use
Because human activities can change basin runoff characteristics, land use studies are necessary to
define present and future conditions, particularly with regard to the degree of urbanization or other
changes that may take place within the drainage basin during the expected service life of a project that
might affect runoff. Information concerning land use trends may be obtained from various local,
State, and Federal agencies and planning studies ((60), Chapters 8 and 12).
There are several interrelated, but separable effects of land use changes on the hydrology of an area.
Among these are changes in peak discharge characteristics, changes in volume of runoff, changes in
quality of water, and changes in other hydrologic characteristics. Of land use changes affecting the
hydrology of an area, urbanization appears to be a dominant factor.
The effect of urbanization on peak discharges depends upon such factors as the percent of the area
made impervious, the changes made in the drainage pattern through the installation of storm drains,
the modification of surface channels and, with frequently occurring storms, depression storage.
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Alteration of the land use of a watershed changes its response to precipitation. The most common
effects are reduced infiltration and decreased travel time, which can result in higher peak discharges.
Infiltration and depression effects are normally most apparent on the more frequent storms—up to the
15- to 20-year recurrence interval. Above this threshold, the amount of infiltration is generally small
compared to the total amount of precipitation. Although urbanization tends to increase peak
discharges and volume of runoff, there are some instances where these may be reduced by the
application of stormwater management techniques such as the installation of impoundment facilities.
However, such techniques, applied at various sites within a watershed, may not achieve the intended
reduction in runoff without a coordinated basinwide management plan. The potential effects of
stormwater management should not be overlooked.
Urbanization and rural watershed practices have a significant effect on the hydrology of small
watersheds, but they do not generally have a great effect on large watersheds because the percentage
of the total watershed that is changed is likely to be small; this is particularly important in showing
that the relative effect of highways (81) is likely to be small.
In References (3), (19), (40), (50), (51), (70), (79), and (83), the effect of urbanization on flood runoff
is discussed. To obtain a true picture of the relative effects of urbanization at a particular location, the
peak discharge should be calculated and compared with the drainage area in its natural state and after
urbanization has taken place. Such measurements are seldom practical and require several years of
investigation. It often becomes necessary to estimate the magnitude and frequency of peak discharges
through modeling of runoff using measurable watershed characteristics.
2.2.1.5 Soil and Geology
Soil type generally has an important effect on flood runoff, principally in its effect on infiltration
((60), Chapter 7, (68), Chapter 3). The effect of soil type often varies with the magnitude and
intensity of rainfall. As with effects of urbanization, the effect of soil type decreases as flood
recurrence interval increases. The condition of soil at the time of precipitation can change the amount
of runoff, especially the flood peaks. If the ground is frozen or saturated with moisture, most of the
precipitation will result in runoff.
The basic make-up of underlying rock formations and other geophysical factors such as glacial and
river deposits, faults, limestone sinks, and playa lakes can be quite significant in affecting runoff in
some areas. Stream flow records are an integrated effect of many factors, and the study of such
records often indicates the effect of surface soils and geology of the area on floods.
Regions underlain by soluble rock formations, especially limestone, often have characteristics of
“karst” topography, which produce little surface runoff ((58), Chapters 14 and 15; (84), Chapter 3). In
these areas, the runoff usually enters the ground through sinkholes and pursues its course to an outlet
through a system of underground passages. In determining the runoff from basins containing karst
topography, it may be appropriate to exclude all karst areas, for they do not always contribute to flood
runoff, especially for low-intensity events.
2.2.1.6 Storage Area—Volume
Storage within a drainage basin may be interception storage, which is the rainfall intercepted by
vegetation that consequently never becomes runoff; depression storage, which is the rainfall lost in
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filling small depressions in the ground surface; storage in transit in overland or channel flow; or
storage in ponds, lakes, or swamps. Storage may also occur in flood-control or other reservoirs and in
surface mining areas. The effect of storage on the quantity and rate of flood runoff can be quite
significant in some instances.
In some areas, interception and depression storage may not be important in highway engineering and
may conservatively be ignored in rural design. However, depression storage can be important in
urban drainage design. Because of the complex parameters involved in the determination of the
storage for overland or channel flow and its limited applicability, this type of storage is not usually
considered as a reduction factor in the flood runoff computations relative to highway drainage
structures. It is more commonly considered in the design of urban storm drains.
In a study for the Delaware River Basin (77), flood storage in lakes and swamps was found to be an
important factor in New York, northeastern Pennsylvania, and New Jersey. Fletcher (37) found that
the effect on flood peaks would be negligible for storages smaller than one percent of the watershed
area. To obtain the proper 10-year peak flow corrected for storage (lakes, ponds, swamps, and
playas), he suggested that the unadjusted 10-year peak flow be multiplied by a multiplier that ranges
from 1.0 for one percent of the storage area to 0.28 for 100 percent of the storage area. The Natural
Resources Conservation Service has some simple methods for accounting for storage in its Technical
Release No. 55 (79).
The effect of flood-control reservoirs in changing downstream conditions should be considered in
evaluating flood peaks and river stages for design of highway structures. Often, helpful data can be
obtained from the controlling public agency or the owner of the reservoir project. Before
consideration of these effects, the flood-control project should exist or be under construction. Many
flood-control projects are authorized but never constructed due to a lack of funds.
2.2.1.7 Elevation
The variations in elevation and also the mean elevation of a drainage basin are important factors in
relation to temperature and to precipitation, particularly as the fraction of the total amount that falls as
snow. Elevation is an important factor in determining the extent to which the available water supply
in winter is impounded as a frozen resource in the form of snow storage, ice in lakes and rivers, and
soil moisture within the zone of frost penetration, which may eventually become flood runoff in
spring or summer. It is representative of the anticipated effects of solar radiation, temperature, wind,
vegetation, and basin ruggedness.
The effect of elevation on flood runoff varies from area to area. The studies by Benson in the U.S.
Southwest region (10) showed that, for the rain-flood area, elevation was not an important factor in
contributing to flood runoff but that, for the snowmelt-flood area, it was one of six significant factors
attributed to peak flows. Thomas and Benson (76) investigated the simple correlation coefficients for
independent variables used in the four U.S. regions and presented a detailed statistical analysis. They
found that mean basin elevation was a significant basin characteristic for high flow in the western
region, but was unimportant in other areas. Elevation is an important variable in mixed population
floods that are presented later in Section 2.2.3.4.
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2.2.1.8 Orientation of the Basin
Although slope affects the rainfall-runoff relationship principally because of an increase in the
velocity of overland flow, thereby shortening the period of infiltration and producing a greater
concentration of surface runoff in the stream channels, a secondary influence resulting from the
general direction of the resultant slope, or orientation of basin, should be recognized. This factor
affects the transpiration and evaporation losses because of its influence on the amount of heat
received from the sun. Also, the direction of the resultant slope to the north or the south affects the
time of melting of accumulated snows. If the general slope is to the south, each successive snowfall
may soon melt and either infiltrate into the ground or produce surface runoff. On the other hand, if the
slope is to the north, these snows may accumulate throughout the winter and remain on the ground
until late spring when they may be removed by a heavy rain, thus producing a potential for a highflood peak.
The amount of flood runoff can be affected by the orientation of the basin with respect to the
direction of storm movement. A storm traversing a drainage basin in the direction of stream flow
would produce a higher flood peak and a shorter period of surface runoff than would otherwise
occur. On the other hand, a storm traversing the outlet first and traveling upstream would have the
opposite effect.
2.2.1.9 Configuration of Channel and Floodplain Geometry
Surface and subsurface runoff are collected and conveyed by stream channels. The natural or altered
condition of these channels and floodplains can materially affect the volume and rate of runoff;
therefore, these conditions are sometimes considered in the hydrologic analysis.
Some streams have well-defined channels; others have relatively small, low-flow channels and wide
floodplains. Some streams have numerous tributaries, while others have one main watercourse
receiving runoff from overland flow. The sinuosity of channels affects channel storage and the
progression of peak discharges. The effect of the stream network often varies with flood magnitude.
Channel cross section can affect flood discharges. Channel storage, especially in channels with
extremely wide vegetated floodplains, can be very significant and can reduce discharges
considerably. This effect is an integral, although transparent, component in some flood forecasting
methods that have a statistical base such as the various practices of the USGS. Where floodplain
storage is not integral with a flood forecasting method, it would be necessary to use a flood routing
model having a dynamic component such as the USACE HEC-1 or HEC-HMS hydrology programs
and HEC-2 or HEC-RAS water surface profiles computer programs. The flood would be predicted at
a point where floodplain storage was not significant, and then routed to the point of interest.
2.2.1.10 Stream and Drainage Densities
The stream density or stream frequency of a drainage basin may be expressed by relating the number
of streams to the area drained. The stream density may be expressed as the number of streams per unit
area of the drainage area. The inverse form, namely the area per stream, might also be used as a
measure of stream density. In some cases, the stream density does not provide a true measure of
drainage efficiency. However, it does usually reflect the potential of the magnitude of flood runoff.
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Generally, the larger the value of the stream density, the higher the peak and total volume of runoff
will be.
Drainage density is expressed as the length of stream per unit of the drainage area. Drainage density
varies inversely as the length of overland flow and therefore provides at least an indication of the
drainage efficiency of the basin which, in turn, affects the quantity of flood runoff.
2.2.2 Site-Specific Characteristics
Site-specific characteristics include both natural and artificial controls that determine the relation of
stage to discharge and regulate the flow.
Natural control of stream flow may occur at channel constrictions, gravel bars, rock outcrops,
aggradation and degradation, and ice and debris jams. Tidal fluctuation also determines the relation of
stage to discharge. Sometimes, channel roughness is a control. Artificial controls include dams,
floodwater-retarding structures, diversion dams, grade-control structures, irrigation distribution
systems, and recreational and water-use facilities. Channel modification may also affect the stagedischarge relationship. Usually information concerning these structures or facilities can be obtained
from the agency responsible for the operation and maintenance.
The hydrologic analysis should determine the degree or effect of such controls upon flood flow.
2.2.2.1 Aggradation and Degradation
The water surface profile of a stream or river will be affected through a reach where deposition or
scour occurs. This also affects the validity of using historical highwater marks to define present
conditions. Aggradation (the deposition of sediment) may lessen the channel capacity, increase flood
heights, and cause overflow at a lower discharge while degradation (the erosion of streambed
material) may increase channel capacity thereby reducing the effect of floodplain attenuation and
result in a higher flood peak downstream. Although difficult to determine quantitatively, the effect of
present and future aggradation or degradation should be assessed when designing a highway at or
near a stream so that a design can be provided to accommodate this phenomenon.
Although channel aggradation or degradation may occur naturally in the river system, this
phenomenon happens frequently as a result of man-made activities. Activities that will induce the
aggradation or degradation may include, for example, water diversions from the river system, water
diversions to the river system, construction of reservoirs, flood control works, cutoffs, levees,
channelization, navigation works, the mining of sand and gravel, and changes in land use.
2.2.2.2 Ice and Debris
The quantity and size of ice and debris carried by a stream should be considered in the design of
drainage structures. The times of occurrence of ice or debris in relation to the occurrence of flood
peaks should be determined. The effect of backwater from ice or debris jams on recorded flood
heights should be considered in using streamflow records. The location of the constriction or other
obstacle causing jams, whether at the site of the structure under study or downstream, should be
investigated and the feasibility of correcting the problem considered. Ice or debris jams may form
below the control, backing the water up, shifting the control, and completely or partially destroying
the stage-discharge relationship. Ice may also form at the control, entirely changing the relationship
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between gage height and discharge. A false measurement may be obtained in these cases for rating a
highwater mark to estimate a historical flood.
2.2.2.3 Seasonal and Progressive Changes in Vegetation
A realistic evaluation of the conveyance or carrying capacity of a floodplain requires consideration of
both seasonal and progressive changes in vegetation. A reach of floodplain may have an appreciably
lower stage for a given discharge in late winter or early spring than for the same discharge during the
height of a growing season. The difference between a row crop such as corn being planted normal or
parallel with the flood flow direction can, during the later part of the growing season, have a
considerable effect in the floodplain conveyance. Such differences must be considered in selecting the
friction or roughness factor in the conveyance equation.
Aside from a marked effect on conveyance, summer vegetation including weeds, leaves on trees and
crops increases temporary floodplain storage and infiltration, which tends to change the basin
response time and, as a result, alter the quantity of flood runoff.
References for estimating friction or roughness factors are Open Channel Hydraulics (23), Roughness
Characteristics of Natural Channels (USGS Water Supply Paper 1849) (8), and Guide for Selecting
Manning’s Roughness Coefficients for Natural Channels and Flood Plains (FHWA-TS-84-204) (34).
2.2.2.4 Channel Modifications
Channel modifications may range from small alterations, such as localized dredging or minor
channel-straightening, to large-scale channel improvements or major installation of flood control
levees. Channel improvements include any type of work designed to improve the carrying capacity of
the stream—for example, changes in alignment, dredging, cutoffs, overbank clearing, and removal of
obstructions.
By lowering the stage corresponding to a given flow, channel improvements will modify the storage
relationship downward in the reach adjacent to and upstream from the improvements. This reduces
the natural attenuation and thus increases flood peaks downstream. Likewise, one effect of a levee
system is to impede normal attenuation and thus tend to make flood peaks downstream from the
system higher than they were before its construction.
It is to be noted that short channel modifications, such as those commonly caused by highway
constrictions are usually considered not to affect flood flows.
Storm drainage systems generally reduce infiltration and decrease travel time, which results in
significantly higher peak rates of runoff.
2.2.3 Meteorological Characteristics
Among the many elements of meteorological phenomena, rainfall, snow, temperature, wind, hail, and
evaporation are considered as the most important factors that could affect the quantity of flood runoff.
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2.2.3.1 Rainfall
Rainfall amounts occur as a function of time and can be graphically shown as a hyetograph. The
hyetograph is usually plotted with the time element indicated on the horizontal axis and the rainfall
intensity on the vertical axis. Although the relationship between rainfall and runoff is not well
defined, to a point, runoff usually increases in proportion to the rainfall on a drainage basin. Basin
characteristics and antecedent conditions have a major effect on the proportion of rainfall that
becomes runoff. For example, most of the rain falling on frozen or saturated ground runs off rapidly,
while most of the rain falling on dry, porous soil infiltrates. There is little correlation between the
recurrence interval of rainfall and the recurrence interval of the corresponding peak discharge (43,
44). However, studies (46, 71) have shown that when peak discharge and rainfall intensity were
considered separately, the ratio of peak discharge of a given frequency to rainfall intensity for the
same frequency remained reasonably constant for the various frequencies. This indicates that rainfall
can be used to estimate flood runoff, although a rainfall of a given frequency will seldom produce a
peak runoff of the same frequency for any one storm.
The proportion of rainfall that becomes flood runoff depends on such things as the rainfall intensity
and duration, distribution of rainfall on the basin, direction of storm movement, antecedent
precipitation, and soil moisture. Rainfall is generally the most significant of all the meteorological
factors that affect and determine the magnitude of flood runoff in non-mountainous regions. In
mountainous regions, the snowfall as manifested in the snow-pack appears to be the more significant
meteorological factor in concert with temperature and in some instances with rainfall depending on
the elevation.
When the rainfall intensity is less than the infiltration capacity of a soil, no surface runoff is produced.
After the infiltration capacity is exceeded, surface runoff will increase rapidly with an increase in
rainfall intensity. However, the increase in stream flow is not at the same rate as the increase in
rainfall excess because of the lag effect resulting from storage.
One effect of rainfall duration is that the infiltration capacity decreases during a rain. As a
consequence, rains of long duration may produce considerable surface runoff, even though the
intensity is relatively low. If rains continue over an extended period, the watertable may reach the
surface of the ground in low-lying areas, thus reducing their infiltration capacity to zero.
Quantitative parameters of the rainfall characteristics that are often considered for determining flood
runoff may include:
mean annual precipitation,
mean seasonal precipitation,
t-hour (t-h) rainfall intensity of T years of return period,
mean annual number of thunderstorm days,
mean seasonal number of thunderstorm days,
direction of storms,
antecedent precipitation index,
storm duration, and
total storm rainfall.
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Rainfall is one of the most common factors that have been used in many equations developed to
predict flood runoff, especially peak discharges.
2.2.3.2 Snow
Snow generally delays runoff. If the snow melts slowly, low-peak runoff results. In areas of diverse
terrain (i.e., mountain and valley topography), the snowpack serves as a storage mechanism. During
periods of normal spring runoff, a particular watershed will have primary and secondary peaks.
During the early period of runoff, the lowlands contribute most, causing the primary peaks. Later, the
highlands begin contributing, creating secondary peaks.
After an accumulation of snow, a rain, particularly with increasing ambient temperatures, can cause
runoff peaks much greater than would occur from the rainfall alone. The relationship of millimeters
[inches] of rainfall to millimeters [inches] of snow, or the water content of snow, varies over the
country and from year to year.
The extent of snowmelt floods is directly proportional to the drainage area. Major floods on a large
river in snowmelt areas are almost exclusively due to snowmelt.
The parameters of snow that may be considered for quantifying flood runoff include “mean annual
snowfall,” “water equivalent of snow before the flood season,” and “t-h snowmelt rate of T years of
return period” ((68), Chapter 19). Snow measurements can provide useful indices for estimating flood
runoff, but their usefulness varies from region to region.
The studies by Benson in 1964 in the U.S. Southwest region showed that snow was not a significant
factor in affecting flood runoff. However, a subsequent study by Thomas and Benson in 1970 (76)
indicated that the snow index was quite significant in the Central and Eastern regions and slightly
significant in the Western region, while it was of no significance in the Southern region. Fletcher and
Reynolds (38) showed that percent normal annual runoff peaks were closely associated with the
percent normal annual 1 April snowwater equivalent. The 1 April snowwater equivalent was thus
introduced to supplement the other precipitation factors and to take advantage of the valuable data by
the NRCS Cooperative Snow Surveys in the western United States.
2.2.3.3 Temperature, Wind, Evaporation, and Transpiration
Temperature may not directly affect the quantity of flood runoff, but it has an indirect effect because
weather changes are associated with variations of atmospheric temperature. Because solar energy is
the principal source of heat for the surface of the earth, the rate at which heat is received is an
important factor in meteorological processes. It has been recognized that most meteorological factors
which affect flood runoff and, to some degree, certain physiographic characteristics, are interrelated
to temperature.
Wind is an important agent in the hydrologic cycle, because there could be no significant moisture
transport without air movement. Precipitation rates, snowmelt, reservoir evaporation, and many other
hydrologic phenomena are directly affected by wind. Winds are mainly the result of horizontal
differences in pressure. In the absence of other factors influencing wind, it should be expected that its
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direction would be from high-to-low pressures and that its speed would vary with the pressure
gradient.
Evaporation is the process by which the precipitation reaching the earth’s surface is returned to the
atmosphere as vapor. The combined evaporation from water, snow and soil surfaces, including
evaporation of intercepted precipitation and transpiration from vegetation, is termed total evaporation
or evapotranspiration.
The moisture conditions of the watershed at the onset of a storm may be a factor in determining the
quantity of runoff from the storm. Because the rate of evapotranspiration influences the moisture
conditions of the watershed, it is naturally correlated with the amount of flood runoff. The elements
of evaporation or evapotranspiration that can be considered in a multiple regression analysis of flood
characteristics may include “mean annual evaporation or evapotranspiration rate” and “mean
evaporation or evapotranspiration rate during the flood season.”
Evapotranspiration usually is not an important parameter in correlating flood runoff. This is because it
tends to be overshadowed by other interrelated physiographic and meteorological characteristics. An
index of average annual lake and reservoir evaporation was incorporated in the Thomas and Benson’s
1970 flood studies (76). This index was found to be marginally significant to the stream flow in the
Central region of the United States, but was determined to be insignificant in other regions.
Evapotranspiration in the short term is minimal; however, it can be significant in the long term.
It should be noted that the factors discussed in this section are rarely considered in flood hydrology
for most highway drainage design purposes.
2.2.3.4 Mixed Population Floods
Mixed population floods are caused by a variety of climatological events and cannot be attributed to
one sole cause such as snowmelt or rainfall. One example would be where rainfall from a foothills
portion of a watershed joins with alpine runoff from the snowpack located in another portion of the
watershed. Although not well understood at this time, guidance can be obtained from References (47)
and (48).
2.3 DATA SOURCES
To design hydraulic structures or other facilities to convey runoff, the hydraulics engineer must obtain
and analyze relevant hydrologic information prior to undertaking the hydraulic design. In the
acquisition and analysis of this hydrologic information, it is advisable to make use of the data
developed by others, whenever available and applicable.
There are two conflicting problems with respect to hydrologic data. First, seldom is sufficient data
available at the right location and in the right form; second, large quantities of data make storage and
retrieval difficult. Besides the raw data such as stream gage records and rainfall records, there are
much secondary calculated data which have previously been developed and that could be useful to the
hydraulics engineer. Nevertheless, the engineer should recognize the specific information needed for
a particular project and collect and retrieve this data from the sources identified later.
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2.3.1 Categories of Hydrologic Data
Generally, there are eight basic categories of hydrologic data (68). These categories of data as
described below are either directly or indirectly related to the highway discipline:
Surface water runoff data. This includes such items as average runoff, peak discharge,
hydrographs, instantaneous values, and highwater marks. Average runoff can refer to annual
average, monthly average, daily average, or any other division of time.
Rainfall and other climatic data such as temperature, wind, and relative humidity.
Drainage basin characteristics.
Sediment transport data including bed loads, suspended load, wash load, and water quality.
Snow pack variations.
Levels and quality of groundwater.
Biological, chemical, and physical water quality data.
Special purpose data collected by local agencies for the purpose of pinpointing specific
phenomena.
For the purposes of highway hydrology, the three primary data types of interest are runoff, rainfall
and drainage basin characteristics. The information on drainage basin characteristics is usually not
readily available. However, they can generally be estimated or measured in the field or obtained from
maps. The items and sources of data available for runoff and rainfall are described in the following
sections.
2.3.2 Sources of Hydrologic Data
Federal water resources agencies can be contacted to obtain hydrologic data for large streams. These
agencies include USACE, U.S. Bureau of Reclamation (USBR), NRCS, U.S. FWS, U.S. Geological
Survey (USGS), FEMA, Tennessee Valley Authority (TVA), River Basin Commission, Boundary
Water Commissions, and Bureau of Land Management (BLM). State agencies, including the agency
for floodplain management, can often supply information on completed work and studies underway.
Local entities such as cities, counties, flood-control districts, or local improvement districts often
have studies. Records of water-using industries or utilities are often also valuable. Local consultants
frequently have hydrologic data.
2.3.2.1 Runoff Data
Stream flow data are usually available as mean daily flow or peak flow. Mean daily flow is a
measurement of the mean flow in volume per unit time for the 24-hour period from midnight to
midnight. Another type of runoff data, rate of flow with respect to time, is not normally published or
readily available. Commonly referred to as a hydrograph, it is the result of data accumulated by a
continuous-recording stream gage. Mean daily flows may be sufficient to describe the hydrograph of
a large stream, but increments as short as 10 minutes may be necessary for small basins.
Stream flow and flood-related data are commonly divided into two types: historical data and recorded
data. Historical data are characteristically noncontinuous and consist of indirect stream flow
measurements based on observed highwater marks. Historical data can be useful in extending stream
gage records (47). Another type of historical data that has been found to be useful in extending stream
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gage records is paleoflood (ancient flood) data (6, 7, 65, 74). The reliability of historical and
paleoflood data could be questionable, especially when estimated discharges are associated with
unsupported observations or questionable ancient channel geometries and changing conditions.
Recorded data are those observed at recording gage stations. The reliability of data observed at wellmaintained gaging stations is generally good because these records are based on detailed information
about the stream channel cross section. Flow rates or velocities in the stream have also been measured
by current meters and accurately reflect the transverse velocities in a cross section.
The primary source for runoff data is the USGS. Normally, rate of flow is estimated from the stream
stage using correlations in the form of stage-discharge diagrams established using a current meter or
other measurements. Sources of streamflow records available from the USGS are variable. Usually,
one finds the records of interest in water supply papers but, occasionally, the needed records will be
available from other references; an example is the publication entitled, Manual for Estimating Flood
Characteristics of Natural-Flow Streams in Colorado (56), which was essentially compiled and
analyzed from the available USGS records. Presently, much of the data appearing in the water supply
papers can be retrieved electronically.
The USACE and USBR also have substantial amounts of surface water data. Like the USGS, the
USACE prepares reports for each flood resulting in loss of life or significant property damage. Flood
data and analyses have been presented in a large number of Preliminary Examination and Survey
Reports in connection with investigations of the feasibility of erecting flood control works. Some of
these reports are published as House or Senate documents, while others are available for inspection in
USACE’s division or district offices.
The USGS, USACE and USBR together collect approximately 90 percent of all surface water data
available in the United States. Many State agencies, local organizations, and universities may also
have extremely valuable data that should not be overlooked. Often this data, and analyses of this data,
can be retrieved electronically.
Railroad maintenance files often contain accurate information regarding flood stages that have been
experienced at railway structures or along tracks bordering a stream. Newspaper accounts and
magazine articles should not be overlooked as sources of documentation of unusual floods.
All of these sources may provide valuable supplementary information that can be used
advantageously; however, discrepancies sometimes are revealed when these data are compared. This
indicates the need for verification and evaluation of flood data, regardless of the source. The effect on
runoff from development within the watershed should be carefully evaluated before using flood data
predating the development. Inconsistencies can also occur when channels widen, aggrade or degrade,
thereby providing false estimates of flood discharges associated with historical or paleoflood data.
The USGS serves as a central clearinghouse and management center for the many different sources
of runoff data. Some private companies sell the USGS runoff data in compact disc form. State
universities should not be overlooked as sources of electronically processed flood and
climatological data.
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2.3.2.2 Rainfall Data
In storm generated flood runoff, rainfall is the primary form of precipitation. Under certain
circumstances, the melting of snow can contribute significantly to runoff but such instances are
unique so precipitation in this chapter is considered primarily as rainfall in flood runoff analyses.
Combinations of rainfall and snowmelt that are known as “mixed population” events are presented in
Section 2.2.3.4.
The storm rainfall data generally used are daily total amounts or storm totals as measured at rain
gages, or total amounts for specified durations as found in statistical studies made by the National
Weather Service (NWS), NOAA of the U.S. Department of Commerce (formerly U.S. Weather
Bureau). Rainfall data are collected by various instruments including a vertical cylindrical rain gage.
The rainfall collected in this manner is usually designated as “point rainfall.”
The NWS is the principal source of precipitation data. In addition to its own gage network, it
publishes data from gages maintained by others. Daily and hourly amounts are published regularly in
Climatological Data, and original records of both published and unpublished data may be examined at
its headquarters or branch offices. Certain rainfall data are also available from the offices of the State
Climatologist and State universities. This precipitation data is often available electronically from
these same sources. Some local and regional sewer authorities also collect rainfall data.
Data are available from various sources. The National Weather Service Technical Paper Nos. 40, 43,
and 47 give rainfall for durations of 30 minutes, 1, 2, 3, 6, 12, and 24 hours for frequencies of
recurrence of 2, 5, 10, 25, 50, and 100 years. Since 1973, the 11 volume NOAA Atlas No. 2 replaces
any information given in NWS TP No. 40 for 11 western States. Technical Memorandum NWS
HYDRO-35 Five to Sixty-Minute Precipitation and Frequency for Eastern and Central United States
was published in June 1977. For the eastern and central regions of the United States, information in
TP 40 should be used only for durations greater than two hours. Between one and two hours, the data
in TP 40 and HYDRO-35 must be extrapolated and adjusted to merge the rainfall data.
Frequently, in the design of drainage facilities, a rainfall intensity-duration-frequency (IDF) curve is
needed to determine the expected amount of rainfall for a specified time period and recurrence
interval. The rainfall intensity-duration-frequency curves for certain durations, recurrence intervals
and locations are included in some NWS publications. If a specific curve is desired for design
purposes, it may be developed from readily available NWS data. Examples of how to develop this
curve are illustrated in Reference (49), Chapter 2, and Reference (33), Appendix A. The IDF curves
for any location in the contiguous United States are included in the transportation agencies’ software
system HYDRAIN: the “HYDRO” subroutine. The time distribution of rainfall, which is also an
important characteristic to highway drainage, is presented in References (35), (36), and (38).
The Probable Maximum Precipitation (PMP) for a particular area is determined from an envelope of
the depth-duration-area rainfall relations for all storm types affecting that area. These are adjusted
meteorologically to maximum conditions (18). The PMP is used to check large detention or other
storage impoundments where breaching might result in loss of life and significant property damage.
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Highway Drainage Guidelines
2.3.2.3 Flood History and Historical Floods
Flood hazards must be evaluated whenever highway locations cross or encroach upon floodplains.
The history of past floods and their effect on existing structures is important in flood hazard
evaluation studies and provides information for estimating structure sizes. The information on floodcontrol works and land use planning data is also necessary in making a flood hazard evaluation.
Floods that occurred before the start of records are often called historical floods, although the USACE
considers any flood event that happened in the past including last year or last week to be a historical
flood. In describing these events, it is necessary to determine the date of occurrence and the
magnitude to fully utilize the information. Some information on past floods might be available from
old newspaper accounts, long-term residents and other similar sources. Often, the USGS and other
agencies make flood estimations using flood marks or other evidence showing the height of historical
floods. Changes in channel and watershed conditions occurring since the time of the flood, or at the
time of the flood, must be established to accurately relate historical floods to the present. Historical
floods of unusual magnitude are valuable data in flood-frequency analysis to extend short-term
gaging station records (47).
2.3.2.4 Flood History of Existing Structures
An existing structure may have been subjected to unusual floods, and thus indicate historical flood
heights and damage. Interviews with local witnesses and the examination of maintenance records may
be helpful in evaluating past floods at a structure.
Highwater evaluations indicated by such things as deposits of debris, seed or mud lines on tree trunks
and bridge abutments, washlines or fine-debris lines on banks, wisps of grass or hay lodged in tree
limbs or fences, and erosion and scour may provide information for estimating flood discharges and
reliable flood stages for use in designing a proposed structure. More obvious items of flood evidence
such as large deposits of debris or prominent washlines do not necessarily indicate the true peak
stage. Usually, the actual peak is somewhat higher than would be indicated by the rather obvious
marks. Highwater marks in bridge and culvert openings can be particularly misleading because this is
where rapidly varied flow often occurs. Interviews with highway maintenance foremen and the longtime residents in the area can be helpful.
A performance record for drainage structures during floods, including photographs, is valuable for
use in designing future structures and for determining modifications to structures that might reduce
maintenance or increase safety. The routine collection of this data may be helpful in defending the
State against damage claims. These records may include:
Maximum flood height upstream and downstream from a structure; observed differences in water
surface elevations on the upstream and downstream sides of the embankments at several points
well away from each abutment. (Highwater marks in a structure opening are often misleading due
to a draw-down condition of the water surface);
Distribution of flow and approximate velocities in different reaches of the stream and relative
quantity of overbank flow and how it returns to the channel;
Direction of flow with respect to the piers and the low-water channel;
Observed drift size and concentration; remarks on clearance or freeboard;
Duration of flooding;
Hydrology
Magnitude of flood and its relation to other notable floods;
Headwater and tailwater at culverts (away from the region of rapidly varied flow);
Scour, erosion, and sediment or gravel deposits; and
Damage to structure and adjacent property.
2-17
All of these observations may not necessarily be available for every structure. The size of the
structure, magnitude of the design flood, extent of potential damage, and probability of legal action
may determine the extent of necessary data collection.
2.3.2.5 Paleoflood Data
Paleoflood (ancient flood) data is an often overlooked source of valuable information—particularly in
western States. This data is obtained from field studies and requires the expertise of a geologist and a
hydrologist. On gaged streams, the geologist locates the remains of ancient floods from observations
of such things as eroded stream banks, vegetal remnants, terrace formations, and the sides of
excavated trenches (“digs”). These “digs” are commonly located in backwash areas. The remains of
ancient drift or artifacts are dated by carbon dating or other paleodating techniques to date the flood.
The hydrologist must then devise a means of estimating a discharge for any paleofloods discovered
by the geologist; as noted below, conventional hydrologic accuracy is not important.
Paleoflood data is used to extend gaged records in time; the Water Resources Council has published
(47) techniques for accomplishing this extension. Quite often, short-term stream gage records will
reflect an outlier(s) that tends to distort the computed flood frequency relationship; paleoflood data is
valuable in removing this distortion.
Although paleoflood data may prove useful in resolving the return period of an outlier(s) at a
particular gage, its real value is in removing the outlier distortion when analyzing stream gage data.
Attempts have been made to further increase the reliability and accuracy of regional flood frequency
analyses that use stream gage data by removing this outlier distortion (14).
As might be surmised, the date and discharge estimates for paleoflood data are not nearly as good as
that for recent indirect measurements or gaged data; nor need it be. Paleoflood data, while relatively
crude and inaccurate, will usually result in minimal error and increased reliability in the range of
floods normally used in highway drainage design (2-to-500-year range). This is because paleofloods
are ancient and plot outside the normal limits; i.e., they can clearly identify the long range trend for a
flood-frequency plot based on stream gage record. References (6), (7), (14), (65), and (74) provide
more detail on paleoflood data and their use.
2.4 ELEMENTS OF RUNOFF PROCESS
Precipitation falling to the earth’s surface is either retained where it falls, passes through the soil
surface as infiltration, or finds its way into the surface channel system of the basin. A portion of the
rain at the beginning of a storm is stored in the vegetal cover as interception and in surface puddles as
depression storage. As rain continues, the soil surface becomes covered with a film of water, known
as surface detention, and flow begins down the slope toward an established surface channel.
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Highway Drainage Guidelines
Overland flow, a major portion of surface waters, is that water that travels over the ground surface to
a small channel and then to a stream. Such small channels are numerous, and the distance water must
travel as overland flow is relatively short. Therefore, overland flow soon reaches a small channel, and
if this occurs quickly and in sufficient quantity, it can be an important element in the formation of
flood peaks.
Some of the water that infiltrates the soil surface may move laterally through the upper soil layers
until it enters a stream channel. This water, called “interflow or subsurface flow,” moves more slowly
than the overland flow and reaches the streams later. The proportion of total runoff that occurs as
interflow depends on the geology of the basin. A thin soil cover overlying rock, or hardpan a short
distance below the soil surface, favors substantial quantities of interflow, whereas uniformly
permeable soil encourages downward percolation to the watertable. Although travelling more slowly
than overland flow, interflow may be much larger in quantity, especially in storms of moderate
intensity, and hence may be the principal factor in the smaller rises of streamflow.
Some precipitation may percolate downward until it reaches the watertable. This groundwater
accretion may eventually discharge into the streams if the watertable intersects the stream channels of
the basin. The groundwater contribution to streamflow cannot fluctuate rapidly because of its lowflow velocity.
The distinctions drawn between the above types of flow are arbitrary. Water may start out as surface
runoff, infiltrate from the sheet of overland flow, and complete its trip to the stream as interflow. On
the other hand, interflow may surface (i.e., springs) where a relatively impervious stratum intersects a
hillside and finish its journey to the stream as overland flow. For the purpose of highway hydrology
where flood peaks and volumes are the primary interest, all groundwater flows remaining beneath the
surface of the earth are considered to be of little significance.
Once the water enters into a stream, it becomes streamflow. A considerable portion of water in the
hydrologic cycle is returned as the streamflow in which the water is moved under the force of gravity
through well-defined, semi-permanent surface channels. The measurement, analysis and
interpretation of stream flow data is an important component of hydrology. Streamflow is the only
portion of the hydrologic cycle in which moisture or water is so confined as to make possible
reasonably accurate measurements of the discharges or volumes involved. All other measurements in
the hydrologic cycle are, at best, only estimates of the whole.
Some important elements associated with the runoff process are discussed in the following sections.
2.4.1 Infiltration
Quantitatively, the most significant abstraction from rainfall before it becomes runoff is infiltration.
The term infiltration has been used with diverse meanings, sometimes as a synonym of “percolation.”
However, for the purpose of highway hydrology, it may be termed as the phenomenon of water
penetration from the surface of the ground into the subjacent soil. Actual infiltration (the passage of
water through the soil surface into the soil) and percolation (the movement of water within the soil)
are closely related with the lesser of the two governing the abstraction of rainfall through infiltration.
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Hydrology
Infiltration often begins at a high rate and decreases, often exponentially, to a much lower and more
or less constant rate as the rain continues. The maximum rate at which a soil, in a given condition, can
absorb water is called its “infiltration capacity.”
Relative minimum infiltration capacities for three broad soil groups are (2) (provided for illustration
purposes only):
Soil Group
Infiltration Capacity
mm/h
in./h
Sandy, Open-Structured
13.0–25.0
0.50–1.00
Loam
2.5–13.0
0.10–0.50
Clay, Dense-Structured
0.25–2.5
0.01–0.10
Infiltration capacity is influenced by many factors including soil type, moisture content, organic
matter, vegetal cover, and the time of the year. Antecedent precipitation such as high-intensity rains
of short duration coming after a dry period significantly affects soil infiltration capacity. It is
noteworthy that for most soils, the infiltration capacity curve ultimately reaches a substantially
constant infiltration capacity rate after a relatively short period, 30–45 minutes ordinarily.
By definition, the surface runoff produced by a given storm is equal to that portion of the rainfall that
is not disposed of through (1) interception and depression storage, (2) evaporation, and
(3) infiltration. Therefore, assuming that an estimate can be made of the first two items, which may or
may not be significant, then concern for only the rainfall, infiltration, and runoff needs to be
determined. If the rainfall intensity is at all times greater than the infiltration capacity, then the
surface runoff can be computed, provided that the duration and amount of rainfall are known, and
provided that an applicable curve of infiltration capacity is available.
The infiltration concept can be applied to the rational computation of surface runoff only when the
following factors are essentially uniform throughout the area under consideration: (1) amount,
intensity, and duration of rainfall; (2) infiltration characteristics; and (3) surface storage
characteristics. These severe limitations preclude direct application of the infiltration approach to a
large watershed area.
Reference (68) describes and refers to various methods and procedures to estimate infiltration. These
methods and procedures are grouped into two cases: (1) when data of rainfall and streamflow are
available for the watershed of interest, and (2) where no data of streamflows are available for the
watershed of interest. A detailed description given for case one includes “the Phi-Index Method,”
“Horton’s Equation,” “Green and Ampt Equation,” and other infiltration capacity formulae such as
Kostiakov’s formula, Philip’s corresponding formulae and Holtan’s formula. For the second case, the
Natural Resources Conservation Service Method is exclusively cited in Reference (68).
At times, it may be necessary to actually measure the infiltration capacity of the soil(s) in the field.
Basically, two major types of infiltrometers are used: “Flooding-Type Infiltrometers” and “Rainfall
Simulation Infiltrometers.” These infiltrometers are also described in detail in Reference (68).
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Highway Drainage Guidelines
The infiltration concept has been frequently used in mathematical models for predicting flood runoff
from a relatively small drainage area, especially in a developed watershed.
2.4.2 Detention and Depression Storage
Some of the water ponds on the soil surface, but only up to a certain depth. Where the ponded water
accumulates in a low point with no possibility for escape as runoff, it constitutes depression storage.
The accumulation of depression storage may be lost to the atmosphere by evaporation or to the soil by
infiltration. The volume of water in motion over the land constitutes the detention storage or the
detention depth for a unit area. The detained water as opposed to being retained will contribute to
runoff and to infiltration. The retention storage is the combination of depression and interception
storage.
The Denver Regional Council of Governments’ (DRCOG) Urban Storm Drainage Manual provides
orders of magnitude for depression and detention depths for various land covers as shown in
Table 2-1:
TABLE 2-1. Typical Depression and Detention for Various Land Covers
Type of Land Cover
Large Paved Areas
Roofs—Flat
Roofs—Sloped
Lawn Grass
Wooded Areas and Open Fields
Depression
and Detention
mm
in.
2–4
3–8
2–3
5–13
5–15
0.05–0.15
0.10–0.3
0.05–0.1
0.20–0.5
0.20–0.6
Recommended (by
the DRCOG)
mm
in.
3
3
2
8
10
0.1
0.1
0.05
0.3
0.4
2.4.3 Stream Flow and Flood Hydrograph
A hydrograph is a plot of discharge or stage against time (Figure 2-1). A discharge hydrograph may
show either mean daily stream flows or instantaneous flow discharges. A great deal of graphical
analysis is performed directly on the hydrograph, and a judicious selection of scale and care in
plotting are essential in obtaining satisfactory results. Hydrographs of mean daily flow plotted on a
relatively condensed time scale provide an effective visual reference for selection of time periods for
analysis. Where volume of flow is an important element in the analysis, instantaneous flow discharge
hydrographs are also suitable. The time period used in hydrographs can be minutes, hours, or days,
and it should be selected to be representative of the basin response.
Hydrology
2-21
Figure 2-1. Typical Hydrograph Segmented into Component
Parts: Rising, Limb, Peak, and Falling Limb
The apparent shape of the hydrograph is established by the selection of scales. It is possible to easily
distort the visual impression of a stream’s characteristics by a poor selection of scales. Most
hydrographs are plotted on arithmetic paper; occasionally, a logarithmic discharge scale is useful.
Stage hydrographs are frequently used in hydrologic problems where discharge is not a factor. The
chart from a water stage recorder is itself a gage height hydrograph.
Hydrographs can be either natural or synthetic. Natural hydrographs are those obtained directly from
the flow records of a gaged stream channel or conduit. Synthetic hydrographs are obtained through
the use of watershed parameters and storm characteristics to simulate a natural hydrograph.
Numerous natural hydrographs can also be synthesized into a statistically representative hydrograph
for a gage or a hydrologic region.
An ordinary or simple hydrograph assumes an isolated streamflow event without subsequent rainfall
until after direct runoff has left the basin. This type of event is easier to analyze than the complex
(composite) hydrographs resulting from two or more closely spaced bursts of rainfall.
The base flow of the stream, which is assumed to be unrelated to storm runoff, must be eliminated if a
direct runoff hydrograph is to be determined. References (35), (36), and (68) describe the technique
for separating the base flow.
References (35) and (36) provide detailed information on determination of flood hydrographs.
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Highway Drainage Guidelines
2.4.4 Hydrograph Parameters
For a given storm of uniform intensity and areal distribution having a specified duration, the shape of
the runoff or discharge hydrograph and the location of the inflection points in the hydrograph and the
detention volume will be functions of the watershed characteristics. The precise relationships between
hydrograph shape and watershed shape, slope, and soil type are not known. However, there are
parameters and generalized response function characteristics that are unique for a specific watershed.
The parameters are defined in terms of basin response times and recession coefficients.
There are several basin response time definitions that have been used in the analysis of hydrographs.
These identify a time interval between the rainfall and some point on the resulting hydrograph.
Several examples of these time definitions are described in the following sections.
2.4.4.1 Time of Concentration
A common assumption need in the formulation of runoff prediction equations is that the maximum
rate of flow results from a uniform rainfall intensity over the entire watershed area where the rainfall
has a duration equal to the time of the concentration period, so all points in the watershed are
contributing flow. The term “time of concentration” is generally defined as “the time required for
runoff to travel from the most remote point in the contributing watershed (point from which the travel
time of flow is greatest) to the point of interest.”
The principal need for the time of concentration is to select the average rainfall intensity for a selected
frequency of recurrence. This average rainfall intensity is required in using certain empirical
hydrologic methods, such as the Rational Method.
The time of concentration is the sum of (1) overland flow, (2) swale, ditch or stream channel flow,
and (3) storm drain or culvert flow times, where such systems will influence the concentration time.
The overland flow time is normally slower than the other flow time components. Overland flow by
definition is non-channelized flow. The length of overland flow, particularly in a natural watershed, is
easily overestimated. When definitive information is lacking for the determination of overland flow
length, a shorter rather than longer length should be assumed. The practice of using a single equation
for the entire concentration time is usually too simplistic. The following are presented as various
methods and procedures for estimating each component of the time of concentration (4).
2.4.4.1.1 Overland Flow
Equations frequently used for overland flow are presented below. At this time, the Kinematic Wave
Equation is recognized as being more definitive albeit a more complex method to use.
Metric
U.S. Customary
NRCS Equation: NRCS, 1986 (79)
to =
5.48(nL)0.8
( P2 )0.5 S 0.4
to =
0.42(nL)0.8
( P2 )0.5 S 0.4
2-23
Hydrology
Kinematic Wave Equation: Ragan, 1971 (62)
to =
6.92 L0.6 n0.6
I 0.4 S 0.3
to =
where:
to
L
S
n
=
=
=
=
I
P2
=
=
0.93L0.6 n 0.6
I 0.4 S 0.3
where:
Overland flow travel time, minutes
Overland flow path length, m
Slope of overland flow path, m/m
Manning’s roughness factor for
shallow depth (overland) flows
(0.2 to 0.4)
Design rainfall intensity, mm/h
2-year—24-hour precipitation, mm
to
L
S
n
=
=
=
=
I
P2
=
=
Overland flow travel time, minutes
Overland flow path length, ft
Slope of overland flow path, ft/ft
Manning’s roughness factor for
shallow depth (overland) flows
(0.2 to 0.4)
Design rainfall intensity, in./h
2-year—24-hour precipitation, in.
2.4.4.1.2 Swale, Ditch, or Stream Channel Flow
Storm drainage flow along swales, ditches, or stream channels should be treated separately from
overland flow because flow depths, and thus velocities, tend to be much larger in these concentrated
flow paths. The Manning’s equation as indicated below can be used for determining the velocity, and
thus travel time:
Metric
(R ) (S )
§1·
V =¨ ¸
©n¹
0.67
0.5
where:
V
n
R
S
=
=
=
=
U.S. Customary
§ 1.49 · 0.67
0.5
V =¨
¸ ( R )( S )
© n ¹
where:
Mean velocity, m/s
Manning’s roughness coefficient
Hydraulic radius, m
Slope of energy grade line, m/m
V
n
R
S
=
=
=
=
Mean velocity, ft/s
Manning’s roughness coefficient
Hydraulic radius, ft
Slope of energy grade line, ft/ft
It is emphasized that the Manning’s “n” roughness value in this case applies to open channel flow and
should be taken from appropriate tables as provided in most hydrology or hydraulics text or reference
books. In general, the channel roughness factors will be much lower than the values for overland flow
with similar surface appearance due to a higher ratio of flow area to wetted perimeter in the channel.
To obtain the travel time, the velocity in Manning’s equation can be computed for bankfull conditions
at the mid-point and divided into the flow path length. The travel time can be calculated with an
equation:
tt =
L
60 V
where:
2-24
tt =
L =
V =
Highway Drainage Guidelines
Travel time, minutes
Flow path length, m (ft)
Velocity, m/s (ft/s)
2.4.4.1.3 Storm Drain or Culvert Flow
Runoff usually travels faster through storm drains or culverts than along ditches. A preliminary travel
time estimate using Manning’s equation will provide an estimate for use in designing the storm
drains. This preliminary travel time can be verified upon completion of the design. Notably, the
variation of culvert or storm drain travel time is not very sensitive to the change of culvert or storm
drain diameter.
2.4.4.2 Lag Time, Rise Time, and Time to Peak
Lag time is the length of time from the mid-point of the rainfall hyetograph to the centroid of the
runoff hydrograph, and basin lag is the length of time from the centroid of rainfall hyetograph to
hydrograph peak.
Rise time refers to the length of time from beginning of rainfall excess (rainfall that is direct runoff)
to peak discharge.
Time to peak is the same as “basin lag” except time is measured from centroid of rainfall excess to
hydrograph peak.
The above hydrograph parameters (68) have been used as variables for estimating flood runoff in
some hydrologic methods. The schematic definition of these parameters is indicated in Figure 2-2. It
should be noted that separate definitions may be adopted among different agencies.
Figure 2-2. Hydrograph Variables
Hydrology
2-25
2.4.5 Unit Hydrographs
For many drainage design problems, a runoff hydrograph, or hyetograph for the design storm is not
available. When this occurs, the unit hydrograph method developed by Sherman (72) can be applied if
several criteria are met and several assumptions can be shown to reasonably apply.
A unit hydrograph (35, 36, 73) is the direct runoff response function for a specific watershed
subjected to a volume of one millimeter [one inch] of excess rainfall (that portion of the rainfall
contributing to runoff) for a specified duration of time. Unit hydrographs, sometimes called “unit
graphs,” have the descriptor “unit” because of the unit of time for which the hydrograph applies, not
because of the one-millimeter [one-inch] volume of runoff.
A watershed can have several unit hydrographs, each one caused by rainfalls of different durations.
For example, a two-hour unit hydrograph for a specific watershed is the direct runoff hydrograph
resulting from a storm lasting two hours and having one millimeter [one inch] of excess rainfall. It is
assumed that the rainfall excess is uniform in time and space and, for these assumptions to have some
2
2
foundation, the watershed area should not be greater than 5,000 km (2000 mi ), according to Linsley
2
2
et al. (52). Information reveals that, for watershed areas exceeding even 500 km (200 mi ) where
rainfall is prevalent, the assumption of uniform areal distribution is difficult to achieve, particularly in
arid and semi-arid regions. Research for flood flow characteristics of Wyoming streams has shown
that, in arid and semi-arid regions, uniform rainfall is limited to small watersheds. This same research
generally determined that arid and semi-arid watersheds having a drainage area greater than 30 to
2
2
40 km (12 to 15 mi ) will provide complex and inconsistent hydrograph geometrics comprised of
multiple peaks and unusual shapes.
Additional information on unit hydrographs is presented in Section 2.7.4.
2.5 MEASUREMENTS OF FLOOD MAGNITUDES
The accurate measurement and determination of flood magnitudes requires a background in openchannel hydraulics and a knowledge of floodwater behavioral patterns; however, knowledge must be
coupled with experience if the measurements are to be correctly interpreted.
Many hydrologic and hydraulic textbooks and other references, including USGS publications (8, 11,
12, 15, 16, 20, 21, 27, and 55), outline procedures for making such measurements. Although these
publications provide technical procedures for measuring flood flow, only by using the methods in the
company of experienced hydraulics engineers can proficiency in their use be gained.
2.5.1 Direct Measurements of Flood Magnitudes
The direct measurement of flood flow consists of measurements that are made during a flood (16).
Discharge is determined by simultaneously measuring the flow depth and velocity at a sufficient
number of points in a cross section to define significant changes in either depth or velocity. From
these measurements, the area and average velocity can be determined and the discharge calculated.
Discharge measurements at various stages at a site or gaging station provide data for developing a
rating curve (20) or a plot of stage versus discharge. Continuous records of stage gaging stations
provide discharge data for studying the recurrence interval or frequency of floods (15, 21).
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Highway Drainage Guidelines
2.5.2 Indirect Measurements of Flood Magnitudes
Indirect measurements are made when it is impractical to measure flood flows directly. Generally,
these measurements are made after the flood subsides (11). Such measurements include highwater
marks, channel geometry, channel or water surface slopes, and an estimate of roughness coefficients
(8). From these data, the flood magnitude is calculated using basic hydraulic equations such as
Manning’s equation and the equations of continuity and energy. Indirect methods for determining the
magnitudes of actual floods include measurement of the discharge by the slope area method (27),
flow through culverts (12), contracted opening (55) and flow over dams. This tool in measuring flood
flows is most valuable to the hydraulics engineer, and a thorough understanding of the methods used
in the listed publications is recommended.
2.5.3 Ordinary Highwater and Mean Annual Flood
The term “ordinary highwater” or “ordinary highwater mark” often referred to by regulatory agencies
does not appear to have major hydrologic significance. However, it has gained significant importance
in recent years since the USACE has used this term in establishing their jurisdiction on Federal
Section 404 water quality permits. As defined by the USACE, the term “ordinary highwater mark”
means the line on the shore established by the fluctuation of water and indicated by physical
characteristics such as a clear, natural line impressed on the bank, shelving, changes in the character
of soil, destruction of terrestrial vegetation, the presence of litter and debris or other appropriate
means that consider the characteristics of the surrounding areas. Some USACE Districts have
developed guidelines for estimating the ordinary highwater mark to be used when preparing Section
404 permit applications. The appropriate USACE Office should be contacted for this information.
The USGS has determined an empirical relationship for estimating the ordinary highwater for some
geographical areas. The formula is presented as Q = a coefficient multiplied by the drainage area
2
2
(sometimes to a power) in km (mi ). The ordinary highwater elevation is established by conventional
hydraulic calculations using this discharge. A water surface elevation established for the average
bankfull discharge, sometimes termed the “dominate discharge,” has also been used by some agencies
to determine the ordinary highwater mark. If the magnitude of the bankfull discharge is desired for
the purpose of designing certain drainage structures such as the temporary pipes under a haul road, it
can be estimated by means of the indirect measurement method(s) described in the preceding section.
Also, research findings indicate that bankfull discharge often corresponds to a flood with a return
period of approximately 1.5 years (68).
In arid and semi-arid regions the mean annual flood, which is defined as the discharge with a
recurrence interval of 2.33 years based on the Gumbel Extreme Value, has been frequently used as an
index flood for estimating design floods and other flood magnitudes of ungaged watersheds. Some
methods of significance that use this flood parameter include the USGS Index-Flood method and the
PSU III method (64). In the arid and semi-arid regions, this flood has been used frequently to identify
the bankfull discharge, even though a 1.5-year return period has been suggested in research findings
as previously mentioned.
Hydrology
2-27
2.6 FLOOD PROBABILITY AND FREQUENCY AS APPLIED TO HIGHWAY
HYDROLOGY
Because the hydrology of drainage structures is concerned with future events whose time or
magnitude cannot be forecast precisely, hydraulics engineers must resort to statements of probability
or frequency with which a specified rate or volume of flow will be equalled or exceeded. Selection of
the appropriate level of probability for determining the risk that will be acceptable rests on such
things as economics, policy, custom or regulatory requirements. Designating a flood of a specific
recurrence interval in the analysis for determination of a structure size involves a calculated risk. If
the analysis is correct, the system will occasionally be overtaxed. The alternative of accommodating
the worst possible event that could happen is usually so costly that it may be justified only where
consequences of failure are especially grave.
In the past, it was customary that a particular flood frequency be selected for each class of highway in
determining the design discharge for sizing drainage structures. Sometimes, this empirical practice
was enhanced by also considering traffic densities, structure size and the value of any adjacent
property. However, contemporary designs employ a range of discharges with a range of flood
frequencies in the determination of structure size.
2.6.1 Concepts of Probability and Frequency Analysis
Available streamflow records may be analyzed to express in terms of frequency the maximum flood
that may be expected to occur at the site in an average interval of years. Frequency analysis defines
the “N-year flood”; that is, the flood that will, over a long period of time, be equalled or exceeded on
the average of once every N years. Probability analysis, which constitutes a similar meaning as
frequency analysis, seeks to define the flood flow with a probability p of being equalled or exceeded
in any given year. Return period N (that is synonymous with recurrence interval, flood frequency or
the N-year flood) and probability are reciprocals, i.e., p = 1/N. Sometimes, flood recurrence is
expressed in terms of probability, as a percentage, rather than in terms of frequency. As an example, a
flood having a 50-year frequency can be expressed as a two percent flood or a flood with the
occurrence probability of 0.02. This means a 50-year flood has a two percent chance of being
equalled or exceeded in any given year. By expressing recurrence intervals in terms of a percentage, it
is possible to avoid the misinterpretation associated with using a frequency in terms of years.
If the highest floods from each year of record for a particular drainage basin are listed in order of
magnitude, they constitute a statistical array from which a flood frequency curve can be developed.
Because the length of record is usually short, the frequency distribution is somewhat irregular.
However, an infinitely long record would define a relatively smooth frequency curve. If the
characteristics of this curve were known, one could predict with considerable assurance the number of
floods within a specified range of magnitude that could be expected to occur during any long period
of time. Because of the limited samples available, it is not easy to determine the functions that best
describe the actual frequency distributions. Numerous techniques for fitting the observed data to
smooth frequency curves are available, but the length of record must be multiplied several folds
before one can ascertain which of the proposed techniques best fits actual events. These fitting
techniques will be discussed further in Section 2.7.1.
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2.6.2 Floods Considered in Hydrologic and Hydraulic Analysis
Several types of floods are usually considered in the hydrologic and hydraulic analysis of bridges,
culverts, or other highway floodplain encroachments. Although the definition of a certain type flood
is generally applicable to most engineering applications, there is, at times, a slight variation in its
interpretation, depending on the intended purpose and function of a particular engineering
application. As an example, the term “design flood” used in designing a highway drainage structure
could be different from the one applicable to the design of a dam. In addition, there are certain type
floods that are included exclusively in highway terminologies, but are generally unfamiliar to other
engineering disciplines. The following paragraphs describe those types of floods that should be
considered in the hydrologic and hydraulic analysis of highway encroachments in floodplains.
2.6.2.1 Base Flood and Super Flood
‘“Base Flood” is defined as the flood (storm or tide) having a one percent chance of being equalled or
exceeded in any given year. It is also known as the 100-year flood and can be expected to be equalled
or exceeded on the average, and over an infinite period of time, once every 100 years.
The base flood is commonly used as the standard flood in FEMA’s flood insurance studies and has
been adopted for flood hazard analysis by many agencies to comply with regulatory requirements.
The flood insurance studies and work by other agencies compute a flood with 0.2 percent (or 500year) exceedance probability. This event is used to define the possible consequences of a flood
occurrence significantly greater than the one percent event. Although it is seldom possible to compute
a 0.2 percent discharge with the same accuracy as the one percent discharge, it nonetheless serves to
draw attention to the fact that floods greater than the one percent event can occur. This 500-year flood
can be classified as one of the super floods. The term “Super Flood” may be defined as a flood
exceeding the base flood, which magnitude is subject to the limitation of the state-of-the-art practices.
2.6.2.2 Overtopping Flood
The term “overtopping flood” means the flood described by the probability of exceedance and water
surface elevation at which incipient flow occurs over the highway, over the watershed divide, or
through structure(s) provided for emergency relief.
The information on this flood is of particular interest to highway engineers because it will indicate
when a highway will be inundated and perhaps the threshold where the highway may act as a flood
relief structure for the purpose of minimizing upstream backwater damages.
2.6.2.3 Design Flood
The term “design flood” means the peak discharge, volume if appropriate, stage or wave crest
elevation of the flood associated with the probability of exceedance selected for the design of a
highway encroachment. By definition, the highway will not be inundated from the stage of the design
flood. However, an exception to this definition applies to a low-water crossing, where a 100 mm to
150 mm (4 in. to 6 in.) water depth is usually permitted over the crossing during the design flood. A
separate definition can also be given for the floods in ephemeral streams crossing highways in desert
areas where no bridges or culverts are used, but where the flow is expected to cross the roadway.
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Because the highway will not generally be inundated at the design flood, this flood should be equal to
or less than the incipient “overtopping flood.” As such, the water surface elevation created by the
design flood should be set at or below the overtopping flood elevation, depending on the degree of
risks involved or on the individual practice of the highway agencies.
Another type of the design flood commonly considered is the discharge used for designing storm
drains or appurtenant drainage structures, such as energy dissipators, riprap revetments, and scour
prevention facilities. In these cases, the highway overtopping may not be a factor in the consideration
of determining the design flood used for designing drainage facilities.
Still another definition for the design flood that is sometimes used relates to upstream property. A
design flood is selected not so much to avoid inundation of the highway, but to minimize additional
inundation of upstream property. This property design flood can differ from the highway design
flood, and the one resulting in the larger waterway opening is sometimes referred to as the “design
flood.”
2.6.2.4 Maximum Historical Flood
The term “maximum historical flood” is the maximum flood that has been recorded or known to have
occurred at or near a highway location. This information is desired because it is an indication that the
flood of this magnitude can be repeated at the project site or within a hydrologic region. Even if the
hydrologic analysis at a particular location suggests that the chance for recurrence of the maximum
historical flood is very small, the possibility for designing the waterway opening to convey this flood
still should not be overlooked. It should also be recognized that the maximum flood known to have
occurred may not have been of as great a magnitude as those normally considered by the design
analysis. This is particularly true if the period of historical record is short.
2.6.2.5 Probable Maximum Flood
The term “probable maximum flood” is the greatest flood that may reasonably be expected, taking
into collective account the most adverse flood-related conditions based on geographic location,
meteorology, and terrain. The effects of this flood should be considered if the highway embankment
is designed to serve as a dam or other critical flood control facility where failure may result in
catastrophic consequences.
Pertinent information for determining the probable maximum flood may be obtained from the
USACE, Bureau of Reclamation, USGS, and State water resource agencies.
Although the probable maximum flood can be considered as one of the super floods, it is generally of
a greater magnitude than those super floods used in hydrologic or hydraulic analysis.
2.6.3 Design Flood Frequency
As discussed in Section 2.6.2.3, the design flood may be equal to the incipient overtopping flood.
Therefore, any reference to the design flood frequency in this section may also be applied to the
incipient overtopping flood frequency.
Two alternatives can be used in highway drainage designs to establish the design flood frequency at a
specific site. These practices are referred to in this chapter as the Policy Alternative and the Economic
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Assessment Alternative. These alternatives can be applied exclusively or jointly at a given site. The
design flood frequency criteria presented in the following sections are mostly qualitative and
applicable to the policy alternative whereas the economic assessment alternative is a more
quantitative practice.
An emerging term in the practice of hydrology is “joint probabilities.” Current practice is to estimate
an independent flood-frequency relationship for each drainage site as if there were no other sites
along a particular highway route. Where only general rainstorms occur over a wide area or in
exclusively snowmelt regions, this may be an acceptable practice. However, the frequency of traffic
interruptions due to the failure of different drainage structures each year along a transportation route
has, under certain hydrologic circumstances, been shown to be a function of the statistical
interdependence of mutually exclusive flood events in adjacent catchments and the total number of
crossings (39, 66). The hydraulics engineer is alerted to this possibility so that the ramifications may
better be understood while selecting a design flood frequency relationship at a specific site. Specific
guidance beyond the foregoing references is presently beyond the scope of these guidelines.
2.6.3.1 Policy Alternative
Although modern design concepts recommend that a range of flood magnitudes be considered and
included in the hydraulic analysis of a highway drainage facility, a specific design flood frequency
may, nevertheless, be designated for design by policy. As indicated in Section 2.6.2.3, by definition,
the highway will not be inundated from the stage of the design flood. The policy of a highway agency
may require that a certain percent chance or flood frequency be adopted as the design flood for a
certain highway that must not be inundated during the occurrence of this flood. As an example, 23
CFR 650, Subpart A, specifies that the design flood for encroachment of through lanes of Interstate
highways shall not be less than the flood with a two percent chance of being equalled or exceeded in
any given year.
It should be noted that the accommodation of a desired design flood frequency is generally
practicable only to a new highway location. For those projects involving replacement of existing
facilities, the existing right-of-way, and terrain controls often dictate highway profiles and, therefore,
the freedom to select the prescribed design flood frequency may not exist.
The selection of a particular design flood frequency is a complex problem that involves consideration
of many factors. The factors that should be considered in selecting the design flood frequency are
described in detail in the following sections.
2.6.3.2 Economic Assessment Alternative
There are circumstances that sometimes warrant a more quantitative practice for establishing a design
flood frequency. In addition to capital costs, the design of highway drainage structures, and other
floodplain encroachments should include an evaluation of the inherent flood-related hazards to the
highway facility and to the surrounding property. When this evaluation indicates that a potential flood
hazard warrants additional studies, a detailed analysis of alternative designs should be considered to
determine the design providing the greatest flood hazard avoidance at the least total expected cost
(LTEC) to the public.
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The LTEC design practice philosophy of flood hazard avoidance is not new in engineering, but it is
relatively new to the hydraulic design concept for floodplain encroachments. The LTEC design
process is one of optimization, where capital cost and flood hazard analyses of alternative designs
provide the basis for decision making.
An essential ingredient in the LTEC design concept for drainage design is flood hazard analysis.
Flood hazard analysis provides the vehicle for analyzing the losses incurred for the various design
strategies due to possible flood events. All losses that can be quantified in a monetary term are
included in the quantitative portion of flood hazard analysis. These may involve estimates of damage
to structures, embankments, surrounding property, traffic-related losses and scour of stream channel
damage. The product of the quantitative portion of the flood hazard analysis is the annual economic
risk associated with each design strategy; the lowest risk is usually associated with the most costeffective strategy.
It is important to recognize that the quantitative results provided by this practice assume that all
variables have been considered, and that their predicted impacts are reliable. Because this is not the
case, a prudent hydraulics engineer may wish to consider using a factor of safety in the LTEC
analysis to include such intangibles as agency public relations, the public’s accustomed level of
service, loss of life, and potential litigation costs.
Detailed discussion of the economic and flood hazard analysis is outside the scope of this chapter. For
additional information on this subject, one may refer to Reference (32). The application of this
analysis varies depending on the needs and budgetary concerns of each highway agency.
2.6.3.3 Highway Classification
Historically, the selection of a design flood frequency has been oversimplified. Over the years, a
range of frequencies or recurrence intervals was used for the design of various highway drainage
facilities dependent primarily on the class of highway. Roads with a minor classification were
designed using a high frequency of occurrence (i.e., small floods), and important major highways
were designed using a low frequency of occurrence (i.e., large floods). Often, too little regard was
given to the factors likely to cause damage or loss of life at the individual location.
In the past, availability of funds and the lack of hydrologic data played a major role in adopting the
concept of associating only the highway classification with a specified design frequency. However,
with better hydrologic data, improved methods of analysis, and an increasing public awareness of the
potential hazards associated with highways encroaching upon floodplains, the hydraulics engineer
now should consider both risk and economics involved in selecting the design frequency for each
highway hydraulic facility. Classification of highways should be considered as one of the important
factors, but certainly not the sole factor, in determining the design flood frequency for a drainage site.
2.6.3.4 Flood Hazard Criteria
Flood hazard is the consequence associated with the probability of flooding attributable to an
encroachment, and it includes the potential for property loss, hazard to life and other inconveniences
during the service life of the highway.
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In determining the design flood frequency, various elements of flood hazard criteria should be
carefully assessed before arriving at a decision. These might include such factors as the cost of the
property and its contents, whether the property is occupied by humans or livestock, whether
occupancy occurs at night (such as a residence) or just during the day (business), and the expected
traffic density and the nature of the traffic (commercial as opposed to non-commercial traffic).
Careful attention must be given to existing flood hazards at the site of a proposed highway
improvement, particularly in urban areas, so that flood hazards are not significantly increased by the
improvement.
2.6.3.4.1 Sensitivity to Increased Flood Magnitude
There always exists a probability that any flood greater than the design flood will occur in a given
year. The design flood can be exceeded at one location with minimal damage, but at another location
such an occurrence might approach a disaster. One way to account for this sensitivity by policy is to
establish different design flood frequency requirements as to whether the project location is rural,
suburban or urban.
Depending on the sensitivity of response to an increased flood magnitude, the design flood frequency
of a particular hydraulic facility may be increased or reduced accordingly. Consideration must be
given to locating any highway overflow sections so as to avoid redirecting floodwaters in areas
subject to flood damage. If the area is more sensitive to upstream flooding caused by backwater,
consideration should be given to providing a low highway profile to act as a flood relief structure.
Sensitivity considerations should be included in any policy relating to design flood frequency to a
particular highway location. Nevertheless, the possibility of using an economic assessment alternative
discussed earlier under Section 2.6.3.2, particularly in costly urban locations, should not be
overlooked.
2.6.3.4.2 Loss of Life
Loss of life associated with highway flooding can occur when a vehicle and its occupants are washed
away from an inundated highway, vehicles fall into a stream or river because of the failure of a
highway structure, highway embankments are destroyed and cause flooding in downstream areas, or
when the highway diverts floodwaters or causes upstream inundations.
Because it is difficult to place a credible value on human life, potential flooding involving possible
loss of life related to a highway must be given careful consideration. Conversely, it is clearly
recognized that it is not generally economically feasible to provide for all flood eventualities; but this
does not relieve the hydraulics engineer of the obligation to weigh all factors before making a
decision. Factors to be considered in potential loss of life situations should include such things as the
probability of future flood occurrences and loss of structure, duration, depth, location, and velocity of
hazardous floodwaters, the dependability of adequate warning systems or devices, roadway approach
grades, sight distances and the availability of escape routes.
2.6.3.4.3 Property Damages
Property, as used here, denotes any property, whether private or public, involved with potentially
damaging floodwaters as related to the highway or its drainage facilities. Damages to such property
from floodwaters can include such things as eroded highway embankments, loss of highway
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structures, damage to adjacent property, and loss of animal life. Damage to highway property causes
increased maintenance costs and sometimes involves the cost of replacing a structure. Such costs
should be evaluated and considered in selecting the design frequency. The USACE has cost
information that can be used by a highway agency for estimating adjacent property damage from
increased flooding.
2.6.3.4.4 Traffic Interruption
When a portion of a highway is closed due to flooding, the travelling public’s journey may be
interrupted or delayed. Traffic interruptions are always a serious occurrence, particularly with
commercial vehicles. The seriousness of the situation may be evaluated by considering, for a given
highway site, the traffic volume, the traffic delay incurred, the availability of alternative routes, the
traffic composition, and the overall importance of the route, including the provisions of emergency
supply and rescue.
Interruptions and short delays due to floods sometimes can be tolerated. For instance, short duration
flooding of a low-volume highway might be acceptable or, if the duration of flooding is long and
there is a nearby, good quality, alternative route, then the flooding of a higher volume highway might
also be acceptable.
The importance of the highway route to national defense, to interstate commerce and to the economic
well-being of a community plays a major role in evaluating the flood hazard of traffic interruptions.
2.6.3.4.5 Economics and Budgetary Constraints
The cost for constructing a highway drainage structure to accommodate a rare flood is invariably
higher than that to convey a flood of frequent occurrence. Therefore, the economy and capital costs of
the facility may play an important role in the selection of the design frequency. Sometimes, the
availability of funds by various government agencies at the time of design is a serious problem and,
as such, any determination given to the selection of a flood magnitude for designing the facility
should afford a careful consideration to budgetary constraints.
2.7 METHODS FOR ESTIMATING FLOOD PEAKS, DURATIONS, AND
VOLUMES
Early flood estimate formulas were simple, generalized, largely empirical and with unknown margins
of error. Sometimes, they provided only an estimate of the maximum flood to be expected. Rainfall
data was used in many of these formulas to estimate discharge, as more information was available on
rainfall than on discharge. Statistical methods and increased data availability, particularly stream gage
records, have improved the analysis of flood events. These statistical methods coupled with increased
data have shown that the most cost-effective design requires a lesser discharge than the maximum
flood to be expected. When statistical analysis is applied to the stream gage records at a single point,
a large sampling error can be involved. Knowledge of this has led to the present analytical methods,
in which data from a wide region are combined to establish generalized relationships with known
margins of error that may be reliably applied anywhere within the region, both to gaged or
ungaged sites.
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Analyzing floods on a frequency basis allows economic considerations to influence decisions made in
relation to the planning and design of structures. Over the years, many flood frequency prediction
methods have been introduced in the United States and throughout many foreign countries. While
some methods have been developed solely for the purpose of estimating flood peak discharges, others
have been devised to also include the determination of flood durations and volumes as typified in a
hydrograph. A survey in 1980 by the American Public Works Association revealed that more than 45
different methods had been identified and approximately 40 different computer models had been
reported in use for predicting inflow hydrographs.
Various classification systems have been suggested to group the hydrologic procedures that could be
used to estimate flood peaks, durations, and volumes. Only systems that are applicable to highway
engineering for various flood prediction methods are provided in this chapter. References (17) and
(57) provide a detailed discussion on the accuracy and consistency of various methods.
2.7.1 Individual Station Flood Frequency Analysis
Annual peak discharge information at an individual station is systematically collected and recorded
by many Federal and State agencies and private enterprises. Most annual peak records are obtained
either from a continuous trace of river stages or from periodic observations of a crest-stage gage.
A statistical analysis of these data is the primary basis for the determination of the flood-frequency
curve, which illustrates the peak discharge at various recurrence intervals for each station. This
analysis is not applicable to those locations where there are controls regulating a stream.
2.7.1.1 Development of Flood-Frequency Curve
A flood frequency analysis of recorded data requires developing a flood-frequency curve. A study of
selected References (2, 9, 26, 47, 50, 63, 67, and 75) gives procedures for preparing and interpreting a
flood-frequency curve. A flood frequency curve is prepared from recorded stream flow data at a
single gaging station. This data may be obtained from the files or publications of the agency operating
the gaging station, usually the USGS. The availability of the stream flow data has been presented in
Section 2.3.2.1. When using gaged data, precautions should be taken to recognize the effects from
a) reservoirs, b) discontinuous records, c) changes in the watershed during the period of record, d) the
possibility of false flows (i.e., “ice jam floods”), and e) ponding and swamp areas. On regulated
rivers, special techniques are required to analyze gaged data. These techniques are beyond the scope
of these guidelines; Reference (17) should be consulted.
A flood-frequency curve may be developed by graphically fitting (visual-fit) a curve to points plotted
on special graph paper or by mathematically determining peak floods for various recurrence intervals.
Reference (47) provides guidance on the application of an appropriate method that has been accepted
by Federal agencies.
2.7.1.1.1 Graphical Method
The graphical method, based on Gumbel’s extreme value distribution, Powell’s special plotting paper
and Weibull’s plotting position formula, is a commonly used procedure for constructing a frequency
curve (26).
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When peak discharge data are plotted on probability paper, a plotting position formula is used to
locate the data points on the probability (or frequency) scale. The data are ranked and numbered in
order of magnitude, plotting positions are computed, and the data are plotted at their computed
positions.
Various plotting position formulas, such as Hazen, California, Weibull and Beard, have been used.
One of the commonly used formulae is the Weibull formula that is indicated below:
RI = (n+1)/m
where:
RI = recurrence Interval or frequency, years
N = number of years of record
M = the order of descending magnitude of the annual flood peaks with the largest flood
as number one
2.7.1.1.2 Mathematical Method
Several mathematical methods that have been used for frequency computations are (1) Log-Pearson
Type III, (2) Log Normal, (3) Gumbel (G), (4) Log Gumbel, (5) Two-parameter Gamma, (6) Threeparameter Gamma, (7) Regional Log-Pearson Type III, and (8) Best Linear Invariant Gumbel.
Bulletin 17B (47) prepared by the Interagency Advisory Committee on Water Data (IACWD)
provides valuable guidance and recommends the Log-Pearson Type III method. References for the
other seven methods are also cited in the Bulletin. The equation for the Log-Pearson Type III method
is expressed as:
_
Log Q = X + KS
_
In this formula, Q is the annual maximum discharge at selected exceedance probability, X the mean
of the logarithm of flow, S is the standard deviation of logarithms, and K is a function of the skew
coefficient and selected exceedance
probability. Values of K can be obtained from Appendix 3 of
_
Bulletin 17B (47). The values of X and S may be computed using the equations indicated in
Bulletin 17B.
This method and other mathematical methods can be computed by desktop calculators or computers.
Bulletin 17B also recommends procedures to be used in the flood-frequency analysis for treating
unusual events such as discontinuous records, incomplete records, zero flood years, mixed
populations, and outliers.
2.7.1.2 Extrapolating Flood-Frequency Curves
Because the records for most gaging stations are short term, frequency curves often must be
extrapolated beyond the recorded data to estimate the larger floods required for the design of highway
structures. Such extrapolations can be subject to considerable error, especially employing the
Graphical Method. Care must be taken in evaluating extrapolated results. Depending upon the record
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length, it may be more appropriate to use a regionalized flood-frequency analysis rather than sitespecific gage data.
2.7.1.3 Transfer of Data
If the site being studied is on the same stream and near a gaging station, peak discharges at the gaging
station can be adjusted to the site by a drainage area ratio that uses the drainage area to some power
appropriate to each specific hydrologic region; the USGS can usually suggest appropriate powers to
be used. Gaging station records of similar streams in the region should be used as a guide in making
this adjustment. If the bridge site is between two gaging stations on the same stream, the peak
discharge at the bridge site can be estimated by logarithmic interpolation of the peak discharges at the
two stations on the basis of drainage area.
The USGS suggests that no transfer of data be done if the drainage area varies more than 50 percent.
A method for this transfer is also discussed in Reference (60).
2.7.2 Regional Flood-Frequency Analysis
Whenever runoff data is unavailable or the historic record is very short, there are several
methodologies available for flood frequency studies. Two methods of significance are the USGS
Index-Flood Method and a method based on multiple regression analysis. There are two multiple
regression techniques, namely, the watershed parameter technique and the channel geometry
technique. The Index-Flood methodology is based upon recurrence intervals of floods from gaged
watersheds located within a homogeneous region. The gaged data are combined so as to establish a
regional relationship between flood events and various watershed and channel variables. Regression
equations used in the multiple regression analysis method relate flood relationships for various
recurrence intervals to watershed or channel parameters.
2.7.2.1 Index-Flood Method
The Index-Flood methodology (26) extrapolates statistical information of runoff events for flood
frequency analysis from gaged watersheds to ungaged watersheds in the region having similar
hydrologic and basin or channel characteristics.
There are two major parts to such an analysis. The first is the development of basic dimensionless
frequency curves representing the ratio of the flood of any frequency to an index flood (often the
mean annual flood). The second is the development of relations between hydrologic characteristics of
drainage areas or channel geometries and the mean annual flood, for predicting the mean annual flood
at any point within the region. Combining the mean annual flood with the regional frequency curve in
terms of the mean annual flood provides a frequency curve for any gaged or ungaged location within
the hydrologic region.
Many regional frequency reports based on the index-flood method have been prepared by the USGS
for individual States.
2.7.2.2 Multiple Regression Analysis—Watershed Characteristics
The multiple regression technique is one of the most important methods for estimating the flood
frequency relationship at sites where no stream gage data are available. This technique is based on
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relating flood characteristics at points of available stream gage data, either in the form of flood
magnitude of specified return periods or in the form of parameters of given flood distributions, with
the corresponding physiographic and meteorologic characteristics of the watersheds. The relationship
is in the form of a multiple regression model with the selected flood characteristics as the dependent
variable and the selected physiographic and meteorologic characteristics as the independent variables.
This method requires many years of record from gaging stations within a hydrologic region on
drainage areas ranging from small to large. Difficulty is sometimes encountered in defining
hydrologic boundaries due to mixed population events. References (14), (25), and (48) provide
guidance in either resolving or accommodating these problems.
The most popular and common mathematical linear regression model used for highway hydrology has
the form shown below. The choice of which parameters to use in the analysis will depend upon their
availability and statistical sensitivity:
b c d
n
QT = aB C D . . . N
where:
peak discharge of desired flood frequency, T
independent variables characterizing the basin and the
hydrologic conditions
a, b, c, d . . . n = constants of the regression equation
=
QT
B, C, D, . . . N =
Some States have encountered difficulty in obtaining reliable results through a wide range of drainage
areas using linear regression equations. Curvilinear regression equations have resolved this difficulty
(14). With curvilinear regression equations, the exponent (a, b, c, d . . . n) is no longer a constant but
includes a variable.
At this writing, numerous States are developing flood frequency practices based on increasingly
reliable methods of regression analysis. These practices should be considered as they become
available. Some hydrologic methods that have been developed in the past on the basis of multiple
regression analyses that have attracted nationwide attention are described in the following sections.
2.7.2.2.1 USGS-FHWA Urban Method
The USGS–FHWA urban method is presented in the USGS Water-Supply Paper 2207 titled, Flood
Characteristics of Urban Watersheds in the United States (70). Because this method was developed
from an extensive database, it is preferred for most urban situations.
This study investigated the effect of urbanization on peak discharges with recurrence intervals
varying from 2 to 500 years and developed a statistical method for estimating this effect that could be
used on a nationwide basis.
A database was established, consisting of topographic, climatic, land use, urbanization, and flood
frequency parameters for 269 watersheds in 56 cities or metropolitan areas located in 32 States from
the East Coast to the West Coast and Hawaii. This database was used to develop statistical
relationships between urban peak discharge and basin parameters.
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Multiple regression analysis was used to define a three parameter set, a seven parameter set, and a
seven parameter alternative set of equations that would relate the urban peak discharge to an
equivalent rural peak discharge and basin, urban and climatic parameters. Each set of equations
essentially adjusted the equivalent rural peak discharge to an urban condition. The basin development
factor, BDF, which is an index of the drainage improvements, storm drains, and curb-and-gutter
streets within the urban basin, was found to be the most important adjustment factor. Impervious area,
although significant, played a much lesser role. Other parameters defined the effects of drainage area
size, rainfall intensity, permanent basin storage, lag time, and channel slope. Tests indicated that the
equations are not geographically biased. Standard errors of regression for the seven parameter
equation vary from approximately 37 percent at the 5-year level to approximately 44 percent at the
100-year level.
Estimates of magnitude and frequency of urban peak discharges at ungaged sites throughout the
United States can be made by using the seven parameter or the three parameter regression equations.
Standard errors of prediction for either set of equations will vary from approximately 44 percent at
frequent recurrence intervals to approximately 50 percent at the 100-year recurrence interval.
2.7.2.2.2 USGS Regional or Local Rural Methods
Many regional or local methods utilizing the multiple regression technique have been developed
throughout the United States. The USGS has developed various flood prediction methods on the basis
of this technique for many States. These studies are cited in References (35), (36), and (68). This
would be the preferred method to be used where it is locally developed by the USGS or other
institutions. An important feature of this method is the identification of the margin of error to be
expected. Knowing the margin of error greatly enhances the engineer’s ability to apply judgment in
selecting a design discharge.
2.7.2.3 Multiple Regression Analysis—Channel/Characteristics Method
The Channel Characteristics Method is also a method developed using multiple regression techniques
where selected river channel cross-section factors are used as the independent variables rather than
the watershed characteristics.
Flows in a river establish the geometric characteristics of the river. Researchers have traced the
previous flow history of rivers by analyzing the river channel characteristics. These investigations
have provided engineers with extremely useful flood frequency relationships for use in the design of
hydraulic structures in ungaged rivers and streams. They also help verify the flood magnitudes for
gaged rivers and streams. Important river channel geometric characteristics related to previous flood
histories are such things as the shape of river junctions; width, depth and slope of rivers; alignment of
river; bed forms; and sediment transport conveyance.
Estimating flood discharges by means of the river channel geometric characteristics is frequently
typified by equations of the form:
a d
QT = AW D
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Where QT is the discharge for a desired frequency (T), W is uniquely measured channel width, D is
the depth associated with W, and A, a and d are constants to be determined. Several channel
characteristics studies are found in References (14), (25), (42), and (82).
2.7.3 Empirical Hydrologic Methods
Hydrologic methods are frequently based on empirical methods. This is a result of the great
complexity in forecasting the amount and movement of floodwater and lack of physical data related
to such occurrences. Chow identified one of the periods of development of hydrology as the “Period
of Empiricism (1900–1930)” (22). Actually, much empirical work had preceded that period. Chow
found that hundreds of empirical formulas had been used in hydrology.
Empirical methods in hydrology have great usefulness when they can be used correctly by those
knowledgeable in the method and its idiosyncrasies. The user is cautioned to know the limits of each
method applied and to always test the results against as much observed data and comparative
information as possible.
Some of the most commonly used empirical methods for predicting flood runoff are described in the
following subsections. Most of these methods are available for use on microcomputers.
2.7.3.1 Rational Method
The Rational Method is an empirical formula relating rainfall intensity to runoff (71, 78, Chapter 2).
Its use in America dates back to approximately 1889. The formula in itself is simple to use, and this
simplicity has helped to maintain its popularity. This apparent simplicity frequently leads to its
misuse.
The Rational Method (or Formula) is:
Metric
U.S. Customary
Q = CIA/360
where:
Q =
C
=
I
=
A
=
Q = CIA
3
Peak discharge, m /s
Runoff coefficient
Average rainfall intensity, mm/h,
for the selected frequency and for
duration equal to the time of
concentration
Drainage area, hectares
where:
Q =
C
=
I
=
A
=
3
Peak discharge, ft /s
Runoff coefficient
Average rainfall intensity, in./h,
for the selected frequency and for
duration equal to the time of
concentration
Drainage area, acres
Discharge, as computed by this method, is related to rainfall by assuming that the discharge has the
same frequency as the selected rainfall intensity. Because of the assumption that the rainfall is of
equal intensity over the entire watershed and because its frequency is not truly related to flood
frequency, it is recommended that this formula be used only for estimating runoff from small areas.
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Highway Drainage Guidelines
Some agencies use 80 ha (200 acres) as the upper limit of the drainage area for application of this
method.
The use of the Rational Method as described above has been frequently modified in an attempt to
achieve a more accurate estimate of peak discharges. Most modifications involve revisions to the
method for estimating the Runoff Coefficient C.
The use of average coefficients for different kinds of surfaces that are assumed to not vary throughout
the storm duration is common practice in the application of the traditional Rational Method. It is
generally agreed, however, that the coefficient of runoff for any particular surface varies with respect
to the length of time for prior wetting. Horner (45) suggested variations with time in two curves, one
for completely impervious surfaces and the other for completely pervious surfaces of dense soils, for
the runoff coefficient. Mitci (59) has developed a general formula that substantially reproduces the
Horner curves and intermediate ones for other percentage of imperviousness.
The rainfall intensity used in the Rational Method is determined by the time of concentration that has
been discussed in detail in Section 2.4.4.1. As noted, there are a number of methods available to aid in
the computations. The hydraulics engineer must exercise considerable care and judgment in selecting
methods for computing the time of concentration.
2.7.3.2 British Method
The concept of routing a time-area curve is very useful in describing the discharge hydrograph for
urban storm runoff. An application of time-area curves in urban drainage design is called the Road
Research Laboratory Method (1960–1980). It uses storm rainfall on an urban area as input and
provides the storm runoff hydrograph as output. This method includes several features that are
desirable for storm drainage design. It can be used to analyze existing systems or to design new ones.
It is capable of providing the entire runoff hydrograph for a simple or complex storm and the data
needed to apply the model would be needed for any comprehensive drainage design study. The
detailed principal steps and examples for this method are described and included in Reference (68),
Chapter 6. Since 1981, this method has been superseded by the Wallingford Procedure (29).
2.7.3.3 NRCS T.R. 55 Method
NRCS Technical Release No. 55 (79) analyzes the effects of urbanization in a watershed on
hydrologic and hydraulic parameters and presents methods of estimating runoff volume and peak
rates of discharge. Parameters considered in this methodology are the soil-cover complex numbers,
24-hour rainfall, time of concentration, percent of impervious area, and size of drainage area. This
method is available for use on a microcomputer.
2.7.4 Unit Hydrograph Methods
The unit hydrograph as defined in Section 2.4.5 can be used to derive the hydrograph of runoff due to
any amount of effective rainfall. The definition of the unit hydrograph and the following basic
assumptions constitute the unit hydrograph theory:
The effective rainfall is uniformly distributed within a specified period of time.
The effective rainfall is uniformly distributed throughout the entire drainage basin area.
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Hydrology
The base or time duration of the hydrograph of direct runoff due to an effective rainfall of unit
duration is constant.
Direct runoff hydrograph ordinates having a common time base are directly proportional to the
total amount of direct runoff represented by each hydrograph.
The runoff hydrograph for a given period of rainfall on a drainage basin reflects all the combined
physical characteristics of the basin.
Under natural rainfall and basin conditions, the above assumptions cannot be fully satisfied.
However, when the hydrologic data used for unit hydrograph analysis are carefully selected to closely
approximate the above assumptions, the unit hydrograph results have been found to be acceptable for
practical purposes.
Derivation of hydrographs based on various unit hydrograph methods is included in many text and
referenced books (22, 49, 52, 68). These methods may be classified into two broad categories: Finite
Time Unit Hydrograph and Synthetic Unit Hydrograph.
2.7.4.1 Finite Time Unit Hydrograph
Unit hydrographs are developed from available rainfall and runoff records (68). The procedure to
generate a unit hydrograph utilizing runoff data from a storm (or rainfall) excess of known duration is
as follows: (1) base flow is subtracted from the runoff, (2) total volume of direct runoff is determined
by estimating the area under the direct runoff hydrograph, (3) total volume of runoff is divided by the
watershed area to estimate runoff in millimeters [inches], and (4) each runoff hydrograph ordinate is
divided by the amount of runoff in millimeters [inches]. A schematic presentation of the direct runoff
and unit hydrographs is given in Figure 2-3 (35, 36).
where:
q
=
qp
=
t
=
Tp
=
discharge at time (t)
peak discharge
selected time
time to peak
Figure 2-3. Direct Runoff and Unit Hydrographs
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Highway Drainage Guidelines
When runoff hydrographs are available for several storms of equal rainfall excess duration, then an
average unit hydrograph can be constructed. Whenever unit hydrographs are available, unit
hydrographs resulting from storm excess having a greater duration can be determined from existing
unit hydrographs by application of the linearity and superposition principles (35, 36, and 68). Unit
hydrographs having rainfall excess duration shorter than the duration of available unit hydrographs
can also be estimated using the S-curve (35, 68) which is the summation of an infinite number of unit
hydrographs, each being lagged from the hydrograph preceding it by the rainfall excess duration.
Having available unit hydrographs for various storm excess durations, the runoff from a single or
complex storm event (a storm having various excess rainfall intensities and durations) can be
estimated and a hydrograph for the event can be derived.
2.7.4.2 Synthetic Unit Hydrograph
While it is preferable to derive unit hydrographs from actual rainfall runoff measurements, the lack of
useable data from many areas makes it necessary to use formulas relating the physical geometry and
characteristics of a watershed to the hydrographs resulting from known or assumed rainfall. The
synthetic unit hydrograph that is derived on the basis of this principle is a reasonable approach to the
determination of runoff.
2.7.4.2.1 Ten-Minute Unit Hydrographs
Espey (31) found that unit hydrographs resulting from rainfall durations of 10 minutes could
adequately describe most urban watersheds, subject to local validation with observed data. He
developed the generalized equations to derive a unit hydrograph based on various watershed
parameters which include drainage area, channel distance, main channel slope, percent of impervious
area, and a dimensionless watershed conveyance factor.
2.7.4.2.2 Dimensionless Hydrograph
A method of obtaining a satisfactory unit hydrograph is based upon the NRCS dimensionless unit
hydrograph (49, 60). The dimensionless hydrograph is essentially a unit hydrograph for which the
discharge is expressed by the ratio of discharge to the peak discharge as related to the ratio of time to
the lag time. The peak rate of flow, the time to peak, and the time from beginning of the unit rainfall
to peak are computed. Then the time and discharge ratios of the NRCS dimensionless hydrograph are
applied to the appropriate factors to obtain the coordinates of the unit hydrograph.
2.7.5 Regional Hydrographs
As with regional flood peak versus frequency studies such as those conducted by the USGS, similar
studies can be made to provide estimates of the shape and volume for hydrographs. Studies (14, 25)
2
of this type have been found to provide good results for drainage areas less than 30 to 40 km (12 to
2
15 mi ). With larger drainage areas, the gaged hydrographs can become randomly distorted by several
flood peaks reaching the gage site at different times: multipeak hydrographs result. This is often the
point above which such studies have questionable reliability. A possible solution to extrapolate
hydrograph forecasts into the multipeak range would be to compare the volume of a predicted single
peak hydrograph obtained by extrapolating the study to larger drainage areas to the volumes from the
distorted hydrographs obtained from specific gages on the larger areas. This comparison may show
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2-43
that such a synthesized single peak hydrograph would result in a conservative estimate of volume.
Further, by routing both this extrapolated single peak hydrograph and the multipeak hydrograph
through a drainage facility as described in Reference (25), it may be found that use of the extrapolated
single peak hydrograph will consistently result in a conservative design.
Although the peak frequency relationship may vary over a wide geographic region, there appears to
be a common hydrograph shape for similar watershed terrain and channel geometry regardless of how
widely separated the watersheds are. This is logical in that a change in the response of similar terrain
to different storm magnitudes would be principally in the peak discharge. To illustrate, studies in the
northern Great Plains (25) have shown the hydrographs on small watersheds to be similar in shape to
those from the Southwest arid regions.
2.7.6 Mathematical Models
A model is a simulation of some form of reality. In hydrologic design, one normally speaks of
mathematical models that are commonly programmed for computer application. In this sense,
emphasis has been directed at the category of models called mathematical algorithms that characterize
the prototype system and that give relationships between variables enabling description, analysis, and
prediction under the conditions to be modeled.
A mathematical model can be a simple or a complicated one. An example of a simple mathematical
model is the Rational Method equation Q = CIA/360 (metric) or Q = CIA (U.S. Customary) for
calculating the runoff expected from a small watershed subjected to thunderstorm rainfall. In this
equation, the model predicts the watershed discharge or yield as a function of the runoff coefficient,
rainfall intensity and watershed area. It is a very simplified version of a complicated physical
phenomenon. This simplified example illustrates that a hydrologic model is really a method to predict
the outcome of some physical process of interest. Complicated mathematical models, such as those
that require large computer capabilities and sophisticated algorithms and programming, are generally
more precise representations of this physical phenomena than are the simple models such as the
Rational Method.
Engineers have available a large number of hydrologic models for the purpose of calculating runoff
and hydraulic transport of water pollutants and other hydrologic phenomena. The models in
hydrology and water resources fall into the following basic categories: (1) runoff models that take
rainfall as an input or stimulus and calculate the response of a given watershed to the rainfall input,
(2) hydraulic routing models that are really related to hydrologic models as they are necessary to
determine the time variation of runoff at some downstream point in a channel or to model the
reservoir routing, and (3) other types of models, such as sediment transport routing models, and
general non-point pollution prediction models that frequently are only indirectly related to hydrologic
design.
Each of the previously described model types may be either physical, stochastic, or empirical.
Physical models attempt to predict events based on physically simulated (or modeled) features,
usually under controlled conditions in a laboratory. Stochastic (statistically based) models attempt to
forecast hydrologic events based on stochastically derived algorithms. Empirical models rely on
observed relationships to devise empirical algorithms that have no direct relevance to physical laws.
The use of Manning’s equation does, at least in part, make a mathematical model empirical. Most
models are, in truth, a combination of physical, stochastic and empirical relationships.
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Highway Drainage Guidelines
Some mathematical models related to the prediction of runoff volume and peak rate of runoff are
discussed in the following Sections. It is to be noted that most of these models may be analyzed with
the aid of a computer program, and some may utilize the FHWA Watershed Modeling System
(WMS). WMS allows users to visualize spatial data, document watershed characteristics, perform
spatial analysis, delineate subbasins and streams, construct inputs to hydrologic models, and assist
with report preparation.
2.7.6.1 HYDRAIN Computer System
HYDRAIN was developed jointly by the FHWA and numerous State transportation agencies. The
system has modules for estimating rural flood frequency relationships (HYDRO) and urban
relationships (HYDRA).
HYDRO is comprised of four different discharge volume forecasting methods: Log-Pearson Type III
Distribution, Modified Rational Method, NRCS Method, and USGS multiple regression equations.
The system was developed to facilitate both simple and complex rural drainage design analysis for
those problems commonly encountered by transportation agencies. As such, the survey data is often
more readily available and the methods easier to apply than with other models.
HYDRA is a sophisticated urban storm drain model. The model provides storm simulation using
synthetic or actual storm data to generate the hyetograph. A variable intensity and size storm cell can
be routed across the watershed. Precipitation is infiltrated until the soils are saturated at which time
runoff is routed overland to the gutter in such a way as to satisfy depression storage. Upon reaching
the gutter, the flow is directed to predetermined inlet locations, partially or fully intercepted as
directed by the hydraulics engineer, and then routed through a storm drain. HYDRA will size all lines
and inlets, evaluate an existing system or combination thereof. Runoff can be routed through channels
and detention ponds to determine storage effects. The system generates hydrographs at each inlet,
junction and line and at the outfall. Inlets are sized based on specified interception rates. The model
does not include dynamic flood routing algorithms. Notably, the model can also generate hydrographs
and peak discharges from small rural catchments through the selection of input parameters; a
procedure sometimes used to calibrate the model against known rural peak discharges obtained from
other sources or prediction methods.
2.7.6.2 HEC-1/HEC-HMS Models
Models developed by the USACE Hydrologic Engineering Center (HEC) in Davis, California,
include HEC-1 and HEC-HMS, HEC—Hydrologic Modeling System; HEC-HMS is an updated
version of HEC-1 (68).
These models are designed to simulate the surface runoff response of a basin. Runoff simulations are
limited to a single storm event due to the fact that no provision is made for soil moisture recovery
during periods of no precipitation.
The basin may be represented as an interconnected system of hydrologic and hydraulic components.
Hydrographs, which may be inputted by the user or may be generated by the models from userdefined parameters, may be routed through channels and reservoirs to represent the basin response.
The models provide a dam break simulation that defines the consequences of dam overtopping and
structural failure. The models are also capable of economic assessment of flood damages.
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2-45
The models can be calibrated to known rainfall and runoff events through the use of an optimization
feature. Hydrograph data may be inputted using Clark, Snyder, or NRCS Soil Cover Complex
parameters. Flood routing computations can be developed using any of five different methods.
HEC-1 and HEC-HMS require extensive survey data. Their sophistication usually limits their use to
more complex analysis requirements. However, as with any program, inputs can be as simple or
complex as desired thereby providing flexibility and latitude in using the programs.
2.7.6.3 NRCS TR-20 Method
The Natural Resources Conservation Service TR-20 model (80) computes surface runoff resulting
from any synthetic or natural rainstorm. It will take into account conditions having bearing on runoff
and will route the flow through stream channels and reservoirs. It will combine the routed hydrograph
with those from other tributaries and print out the peak discharges, their time of occurrence, and the
water surface elevation for each at any desired cross section or structure. In addition, it will print out
the coordinates of the routed hydrograph together with the corresponding elevation of each if
requested. The program provides for the analysis of nine non-continuous or single storm events over a
watershed under present conditions, and with various combinations of land treatment, floodwaterretarding structures and channel improvement. It will perform these routings through as many as 120
reaches and 60 structures in any one continuous run.
This model is based on the NRCS Soil Cover Complex method of hydrology and does not provide an
option to calibrate the computations to known rainfall and runoff events. This is a single-storm event
model in that no provision is made for soil moisture recovery during periods of no rainfall.
2.7.6.4 The Stormwater Management Model (SWMM)
The Stormwater Management Model (SWMM) (13, 68) is a detailed, mathematical, computer-based
model for urban watersheds that can be used to determine the amount of runoff from a storm, route
the runoff through a combined (or separate) storm drain and sanitary sewer system with userspecified storage and treatment facilities and operating policies, and finally into the receiving waters.
The model also has the capability of determining the amounts and location of local flooding, and
determining the water quality at various locations both in the system and in the receiving waters. This
model lends itself to analyzing more complex urban watersheds.
2.7.6.5 The Stanford Watershed or Hydrocomp (HSP) Model
The HSP is an outgrowth of the Stanford Watershed Model (68). The program requires rainfall and
evapotranspiration as inputs for flow quantity simulation, and temperature, radiation, wind and
humidity for water quality simulation. This sophisticated program must be calibrated against observed
or assumed values of the factor under study (e.g., stream flow, sediment, dissolved oxygen) and
parameters for the study area are derived. Once calibrated, HSP can simulate continuous flow and
quality factors for periods limited only by the available input data. Typically, 20 to 50 years are
simulated. These data may be used for analysis of probability of occurrence for the factors of interest.
River stage, reservoir levels, and flow diversions can be included in the output. Outputs can be
obtained for any desired point within the watershed. For urban drainage, HSP is used to simulate flow
within an assumed drainage system, flow frequency is defined at all critical points in the system and
pipes or channels sized accordingly. Water quality data at outfalls or other points can also be output.
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Highway Drainage Guidelines
Use of artificial storage to modify urban runoff can be simulated. The model is best used in complex
situations or for watershed planning studies.
2.7.6.6 Penn State Urban Runoff Model
The Penn State Urban Runoff Model (5) was developed as an alternative to the traditional Rational
Method and other semi-empirical procedures for urban drainage design. The objectives adopted for
the development of the Penn State Urban Runoff Model were:
to produce an urban runoff simulation model that would provide acceptable hydraulic accuracy;
to keep the model as simple and concise as possible to ensure its convenient use; and
to allow for the analysis of the timing of subarea flow contributions to peak rates at various points
in a watershed.
2.7.6.7 The Massachusetts Institute of Technology Catchment (The MITCAT) Model
The MITCAT Model (41, 68) is a general purpose basin simulation model. Two methods of flood
routing within the individual elements have been implemented in the current operational model, a
kinematic wave model, and a complex linear solution to the full equation of motion.
The MITCAT Model simulates the physical movement of water over the catchment surface and
through the channel network. The effects of urbanization can be investigated experimentally with the
model by changing the overland flow parameters, infiltration parameters, stream flow parameters, and
the arrangement of drainage component segments.
2.7.6.8 USACE STORM Model
The STORM Model, as developed by the USACE, is a method of analysis used to estimate the
quantity and quality of runoff from small, primarily urban watersheds. Land surface erosion for urban
and rural areas is computed, in addition to the basic water quality parameters of suspended and
settleable solids, biochemical oxygen demand (BOD), total nitrogen (N), and orthophosphate (PO4).
The methodology used by this program includes computations on runoff, runoff quality, treatment,
storage, overflow, and land surface erosion. This is also a sophisticated model that may be more
applicable to complex urban watershed problems and watershed planning.
2.7.6.9 ILLUDAS Model
The Illinois Urban Drainage Area Simulator (ILLUDAS) was developed by the Illinois State Water
Survey’s Hydrology Section. ILLUDAS can be used to design a new storm drainage system or to
evaluate an existing one. It is based in part on a design method developed by the British Road
Research Laboratory (BRRL) and has been used successfully in Great Britain.
Primary input to ILLUDAS is an observed or specific temporal rainfall pattern uniformly distributed
over the drainage basin. This basin is then divided into sub-basins, one for each design point. Paved
and grass area hydrographs are produced for each sub-basin by applying the rainfall pattern to the
contributing areas. These hydrographs are then combined and routed downstream to the outlet. At
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2-47
each design point, ILLUDAS determines pipe sizes. Detention storage can be included as part of the
design in any sub-basin.
2.7.6.10 USGS “Dawdy” Model
A stochastic model was developed in 1972 to simulate the volume and peak runoff rate for small
watersheds (28, 68). This stochastic model is referred to as a parametric model, meaning that drainage
basin characteristics are described by model parameters. In the model, the following parameters are
included: infiltration, soil-moisture storages, percolation to groundwater, evapotranspiration, and
surface and subsurface flow routing. This model is principally a research tool used to extend and
improve the quality of stream gage records in regional analyses. It is not used routinely to establish
site-specific flood frequency relationships for highway projects.
2.7.7 Accuracy of Methods for Estimating Peak Discharges
The relative accuracy and ease of application of the many flood predicting methods for ungaged rural
watersheds is important. A pilot test was conducted to determine what are likely to be the most
accurate, reproducible, and simplest procedures for determining peak discharge frequencies for rural
ungaged watersheds. Data obtained from the Midwest and Northwest regions of the United States
were selected as being suitable for this study (61). Methods evaluated were the USGS regression
analysis (State equations), USGS Index Flood Method, Fletcher Method (FHWA’s regression
analysis), Reich Method, Rational Method, NRCS TR-55 (Charts Method), NRCS TR-55 (Graphic
Method), NRCS TR-20 (Computer Method), USACE’ Snowmelt Method, and the USACE HEC-1
computer method. The findings, which are summarized in Reference (61), suggested that the
foregoing two USGS methods were the most accurate, reproducible and simplest to employ by
different users in the foregoing regions. These USGS methods are based on actual gaged records from
which the regression equations were derived, and do not rely primarily on rainfall frequencies as do
several of the other methods. It should be noted, however, that in other areas or regions the most
accurate methods may not necessarily be the USGS methods.
While the USGS methods were found to be more accurate in the foregoing rural regions where they
were tested, it must be recognized that in general their accuracy of prediction throughout the United
States is not great except in some snowmelt regions or possibly areas subject only to widespread,
general rainfall storms. This is attributed largely to the lack of station records and the short length of
gaged record. As an example, a standard error approaching 100 percent is not uncommon for
relatively uncontrolled watersheds in the semi-arid and arid regions of the western States. However,
where snowmelt is the predominant cause of floods, the accuracy is much better with standard errors
frequently being less than 30 percent. Paleoflood data has shown (24, 30, 54), however, that a
significant reduction in the standard error of a gaged station record can be realized in that such data
greatly extends in time the relatively short record.
Regression equations are also available for estimating flood-frequency relationships from ungaged,
urban watersheds (69, 70). With these urban regression equations, the margin of error in discharge
predictions can be determined by the engineer. These equations were found to compare favorably
with measured urban runoff data.
Any hydrologic method for determining peak discharge is subject to uncertainties. However, as
inferred earlier with statistical methods, this uncertainty can be quantified with the standard error. The
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Highway Drainage Guidelines
standard error envelope identifies the upper and lower boundaries inside which the true peak
discharge for a particular site falls. Different standard error envelopes relate to a particular probability
of occurrence. Reference (47) discusses the standard errors and their related envelopes for flood
discharges that have specified exceedance probabilities.
The hydraulics engineer should recognize that the peak discharges estimated from the hydrologic
methods that are supported by gaged data generally reflect the average value of the data. In most
instances, it is common engineering practice to use the average predicted value of the peak discharge
for the design of drainage facilities and to evaluate flood hazards; although, because there is always a
chance that the true value of the discharge for a particular frequency of flood event may be greater
than the average predicted value (or, for that matter, less than), it may on occasion be desirable to
select a peak discharge larger than this average predicted value. Employing such a safety factor
might, as an example, be desirable for sensitive, high-risk locations. For these locations, the selected
peak discharge value used for a flood hazard analysis should be consistent with the desired level of
certainty as reflected in the standard error.
2.8 CHARACTERISTICS AND ANALYSIS OF LOW FLOWS
Planning, design, construction, and maintenance of highways may require knowledge of low-flow
discharge properties such as discharges, flow stages, flow durations, and related flow variables. For
example, the construction of a highway abutting a stream reach or the construction of a bridge may
require knowledge of the time frame at which flows are below a certain level or below a certain
magnitude. This knowledge might be useful in scheduling construction or in designing temporary
construction facilities. Similar information may be required for those periods when highway or bridge
maintenance is contemplated. With land use facilities, it is often necessary to avoid long periods
where the facilities are unavailable to the user due to prolonged or frequent low flows.
The USGS annually publishes Water Resources Data for gaged streams in each State, listing mean
daily discharge. Based on these daily records, a low-flow analysis may determine an acceptable
discharge for the hydraulic design of temporary construction facilities. A rigorous flood frequency
analysis is not generally required for these low-flow studies. Low-flow discharges may be cursorily
determined on the basis of a visual examination of monthly mean discharge data as determined from
the mean daily discharge values for all years of record, and with consideration given to construction
timing and degree of risk. Data for the monthly mean discharge may be obtained from the USGS field
offices. These data may be transferred to other locations by using the procedure suggested in Section
2.7.1.3. If a detailed analysis is desired, one may consult the procedure suggested in Chapter 14 of
Reference (68). Reference (68) identifies additional references for the analysis of low flows. It is to
be noted that the USGS has developed some regression equations for low flows.
On ungaged streams, the low-flow discharges may be estimated from an indirect measurement
technique (discussed in Section 2.5.2) using the low-flow water surface elevations and stream cross
sections surveyed at the project sites.
Low flows may affect fisheries. A sufficient flow depth is required for fish migration, survival and
reproduction. As such, it may be important that highway facilities permit fish passage during lowflow periods.
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2.9 STORAGE AND FLOOD ROUTING FOR STORMWATER MANAGEMENT
The primary objective of stormwater management is to mitigate the changes in the runoff quality and
quantity brought about by changed land uses. Increased urbanization has caused (1) larger peak flows,
(2) shorter concentration times, (3) higher stages, (4) increased runoff volume, (5) increased flow
velocities, (6) increased soil erosion and sedimentation, (7) deterioration of water quality, and
(8) reduced recharge capacity.
Highways replace varying amounts of permeable areas with hard surfaces that lessen the depression
storage and infiltration rates. The paved surface combined with efficient drainage systems speed up
the conveyance of runoff. These changes can result in greater quantities of runoff at higher rates than
would occur under pre-highway conditions. Generally, this occurs only in urban areas where hard
surfaced areas occupy considerable land area and have closed drainage systems. A stormwater
management system can minimize or eliminate entirely these increases in runoff. In many areas,
urban or rural grassed right-of-way can offset more than the effects of the pavement and provide
some mitigation of water quality.
Various means are available to mitigate the increased flood peaks and quantities due to highway
construction. These may include such things as porous pavement, cisterns, contoured landscape,
groundwater recharge, vegetated depressions, providing a runoff retarding grassed or gravel surface,
flood control channels, increasing the length of runoff travel distance, using diversions and various
storage facilities. Storage facilities normally provide the most effective, practical and viable method
to control excess flood runoff.
Storage facility design requires the use of flood routing procedures that are presented later.
2.9.1 Storage Characteristics
When used properly, temporary storage of excess storm runoff is one of the most effective structural
methods to lessen the impact of development. There are four basic types of stormwater storage,
namely retention, detention, recharge, and conveyance storages.
A retention facility is characterized by a long-term storage period. Such storage has a permanent pool
and may be multipurpose; i.e., recreational, aesthetic. The flood storage volume is superimposed
above the permanent pool and may accommodate the entire runoff from a certain design rainfall
event. For very rare events, a manually controlled release gate is utilized to protect the impounding
facility. Because retention inherently involves a large impoundment volume, its use in stormwater
management may be limited.
Detention storage usually reduces outflow to a rate less than that of the peak inflow. Frequently, the
goal is to limit the peak outflow rate for a wide range of floods to that which existed from the same
watershed before development or to a level that is acceptable to downstream conditions. Normally,
the detention site drains completely within a short period of time. Consequently, the usually dry
detention storage facilities can often be used for short-term car parking and sport fields.
Recharge storage is provided by installation of recharge basins where stormwater is diverted into
these underground facilities constructed with porous materials. Storm runoff is temporarily stored in
the basins until such time that the water is dissipated into strata and then elsewhere through seepage.
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Highway Drainage Guidelines
Contaminated underground water is a major environmental concern; therefore, stormwater containing
contaminants or harmful constituents should not be permitted to discharge into the recharge basins. If
recharge storage is considered, the hydraulics engineer must be certain it meets all necessary health
requirements.
Conveyance storage is inherent to overland flow and in swales, channels, and conduits. The volume
required to sustain the movement of water is stored in a transient form. Consequently, it is
advantageous in the management of stormwater to increase such transient storage. Overland flow
storage can be increased by discharging flows from pavements onto turf covered surfaces. The greater
the extent or the longer the flow path across the turf, the greater is the overland flow storage (and the
longer the opportunity for infiltration in the underlying soils). If concentrated, storm runoff can be
routed via large cross section channels or conduits (oversized storm drains), significant conveyance
storage can be designed into the system.
Sometimes, special on-site storage provisions are also designed on certain highways for emergency
control of possible highway spills of environmentally damaging materials.
2.9.2 Storage Size and Location
Any one or more of the four basic types of storage as discussed in Section 2.9.1 can be designed in a
stormwater management system in a wide range of sizes at a variety of locations in the watershed.
The size of a storage facility is directly related to the objectives of the flow management scheme for a
particular watershed or subwatershed. The more frequent purpose is the reduction of increased rate of
runoff from development to that which prevailed prior to the construction. Controlling the outlet
discharge to a rate less than the maximum inflow rate involves a specific volume of detention storage
for chosen quantities and rates of inflow and established maximum outflows.
Storage can be classified by location as on-site, off-site, upstream, downstream, on stream and off
stream. Based on function, storage facilities may be for single or multipurpose use, temporary
(detention) or permanent (retention), and open or closed (surface or subsurface).
Detention storage of roadway runoff may be possible within ample right-of-way and large
interchanges on rural and suburban highways. However, acquisition of special land parcels for on-site
retention, detention or recharge storage may be necessary for urban highways.
All effects of storage should be considered during the design of a storage facility. For example, the
reduction in peak discharges within the upper reaches by a storage facility may serve to increase the
peak discharge at some point downstream. Because tributaries contribute floodwaters at different time
intervals, it is possible to cause a greater downstream peak discharge when the discharges are delayed
or altered from natural conditions. The hydraulics engineer may find it necessary to employ one of the
sophisticated computer models to investigate this possibility in complex drainage systems. Close
coordination with local regulatory agencies responsible for controlling drainage development is
strongly recommended.
Especially in urban areas, storage facility design must preclude nuisance aspects. Landscaping and
low-flow concrete drainage channels are examples of enhancing the quality of a storm runoff
storage facility.
Hydrology
2-51
2.9.3 Determination of Storage Volume and Flood Routing Procedures
The permissible discharge rate from a stormwater management facility must be known to establish
the required volume in the impoundment. The most common requirement is that the discharge rates
from the impoundment shall not exceed that which would occur under the same assumed design
conditions of rainfall and soil conditions either before development or with natural watershed
conditions.
This requirement applies not only to the maximum or design discharge, but for other lesser runoff
frequencies as well. Occasionally, the flow capacity of storm drainage facilities immediately
downstream from the proposed development will determine the permissible discharge from the
storage facility.
The required storage depends on the:
time distribution and volume of various inflow rates,
maximum allowable discharge versus inflow rates and the variation of discharge with pond depth,
detention facility configuration, and
design and construction costs versus benefits.
The required volume of storage will be the maximum difference between the cumulative distribution
of inflow and the cumulative distribution of outflow where the maximum allowable discharges for a
selected range of frequencies are not exceeded. Inflow hydrographs of various durations and
frequencies, and the reservoir stage volume and stage-discharge curves for the storage structure, are
essential elements in determining required storage. Storage is then determined by routing the inflow
hydrograph through the proposed storage facility. Several iterations may be required before an
acceptable design for the storage facility can be determined.
References (18), (22), and (60) provide detailed procedures for performing the flood routing analysis.
2.10 DOCUMENTATION
Experience indicates that the design of highway drainage facilities including hydrologic data should
be adequately documented. Frequently, it is necessary to refer to plans, specifications and hydrologic
analyses long after the actual construction has been completed. One of the important reasons for
documentation is to provide factual information for use in preparing a defense against legal action.
The lack of documentation can be a detriment to a legal defense in that the defendant can not show
that reasonable and prudent actions were taken in light of the circumstances at the time of the design
or construction. Evaluation of the hydraulic performance of structures after large floods to determine
if the structures performed as anticipated or to establish the cause of unexpected behavior is another
important reason for documentation. In the event of failure, it is essential that documentation be
available to aid in the identification of contributing factors so that recurring damage can be avoided.
The documentation of the hydrologic portion of the hydraulic design is the compilation and
preservation of all pertinent information on which the hydrologic decisions were based. Such
documentation should include drainage area, maps, field survey information, source references, aerial
and ground level photographs, hydrologic calculations, flood frequency analyses, stage-discharge
2-52
Highway Drainage Guidelines
data, and flood history including narratives from the highway maintenance personnel and local
residents who witnessed or had knowledge of unusual runoff events. Although this list is not all
inclusive, it does contain those items that should be retained in the design files. The intent is to be
able to show the conditions in existence at the time of design, and that reasonable and prudent actions
were taken by a knowledgeable hydraulics engineer. The documentation should be stored as a part of
the permanent records of the highway agency.
Those hydrologic and hydraulic data that should be documented in the highway agency’s permanent
files or as-built plans could include size of drainage area, magnitude, and frequency of the design
flood, overtopping flood, base flood, the corresponding water surface elevations at critical location,
and the elevation, discharge and date of the maximum flood when available. Other drainage data to be
documented in the files and on the plans that is not directly related to the hydrologic findings is
discussed in other chapters of the Highway Drainage Guidelines.
2.11 REFERENCES
(1)
Ad Hoc Panel on Hydrology, U.S. Federal Council for Science and Technology. Scientific
Hydrology. U.S. Federal Council for Science and Technology, Washington, DC, June 1962.
(2)
American Society of Civil Engineers. Hydrology Handbook, Manuals of Engineering Practice,
No. 28. Prepared by the Hydrology Committee of the Hydraulics Division, American Society
of Civil Engineers, 1949.
(3)
Anderson, D. G. Effects of Urban Development on Floods in Northern Virginia. U.S.
Geological Survey Water-Supply Paper 2001-C. U.S. Government Printing Office,
Washington, DC, 1970.
(4)
Aron, G. and D. F. Kibler. Procedure PSU-IV for Estimating Design Flood Peaks on Ungaged
Pennsylvania Watersheds. Pennsylvania State University, April 1981.
(5)
Aron, G. and D. Lakatos. Penn State Urban Runoff Model. User’s Manual. Pennsylvania State
University, January 1980.
(6)
Baker, V. R., Paleoflood Hydrologic Techniques for the Extension of Streamflow Records. In
Transportation Research Record 922, TRB, National Research Council, Washington, DC,
1983.
(7)
Baker, V. R., et al. Long Term Flood Frequency Analysis Using Geological Data. Proc.,
Canberra Symposium, IAHS-AISH Publication No. 128, Canberra, Australia, December 1979.
(8)
Barnes, H. H., Jr. Roughness Characteristics of Natural Channels. U.S. Geological Survey
Water-Supply Paper 1849. U.S. Government Printing Office, Washington, DC, 1967.
(9)
Benson, M. A. Evolution of Methods for Evaluating the Occurrence of Floods. U.S. Geological
Survey Water-Supply Page, 1580-A. U.S. Government Printing Office, Washington, DC, 1962.
Hydrology
2-53
(10)
Benson, M. A. Factors Affecting the Occurrence of Floods in the Southwest. U.S. Geological
Survey Water-Supply Paper 1580-D. U.S. Government Printing Office, Washington, DC,
1964.
(11)
Benson, M.A. and T. Dalrymple. General Field and Office Procedures for Indirect Discharge
Measurement. U.S. Geological Survey Techniques of Water Resources Investigations, Book 3,
Chapter A1. U.S. Government Printing Office, Washington, DC, 1968.
(12)
Bodhaine, G. L. Measurement of Peak Discharge at Culverts by Indirect Methods. U.S.
Geological Survey Techniques of Water Supply Resources Investigations, Book 3, Chapter A3.
U.S. Government Printing Office, Washington, DC, 1968.
(13)
Brown, J. W., et al. Models and Methods Applicable to USACE Urban Studies. U.S. Army
Corps of Engineers, Washington, DC, 1974.
(14)
Bruce, S. A., et al. Flood Flow Characteristics of Wyoming Streams—A Compilation of
Previous Investigations. Joint USGS and Wyoming Highway Department Report, December
1988.
(15)
Buchanan, T. J. and W. P. Somers. Stage Measurements of Gaging Stations. U.S. Geological
Survey Techniques of Water Resources Investigations, Book 3, Chapter A7. U.S. Government
Printing Office, Washington, DC, 1968, 28 p.
(16)
Buchanan, T. J. and W. P. Somers. Discharge Measurements at Gaging Stations. U.S.
Geological Survey Techniques Water Resources Investigations, Book 3, Chapter AB. U.S.
Government Printing Office, Washington, DC, 1969.
(17)
Buchberger, S. G. Flood Frequency Analysis for Regulated Rivers. In Transportation Research
Record 832. TRB, National Research Council, Washington, DC, 1981.
(18)
Bureau of Reclamation, U.S. Department of the Interior. Design of Small Dams, 2nd edition.
1973.
(19)
Carter, R. W. Magnitude and Frequency of Floods in Suburban Areas. U.S. Geological Survey
Professional Paper 424-B. U.S. Government Printing Office, Washington, DC, 1961.
(20)
Carter, R. W. and J. Davidian. Discharge Ratings at Gaging Station. U.S. Geological Survey
Surface Water Techniques, Book 1, Chapter 12. U.S. Government Printing Office,
Washington, DC, 1965.
(21)
Carter, R. W. and J. Davidian. General Procedure for Gaging Streams. U.S. Geological Survey
Techniques of Water Resources Investigations, Book 3, Chapter A6. U.S. Government Printing
Office, Washington, DC, 1968.
(22)
Chow, V. T. Handbook of Applied Hydrology. McGraw-Hill Book Company, New York, NY,
1964.
(23)
Chow, V. T. Open Channel Hydraulics. McGraw-Hill, New York, NY, 1970.
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Highway Drainage Guidelines
(24)
Cooney, M. E. Use of Paleoflood Investigations to Improve Flood—Frequency Analyses of
Plains Streams in Wyoming. U.S. Geological Survey Water-Resources Investigations Report,
1989.
(25)
Craig, G. and J. Rankl. Analysis of Runoff from Small Drainage Basins in Wyoming. USGS
Water Supply Paper 2056, 1978.
(26)
Dalrymple, T. Flood Frequency Analysis. U.S. Geological Survey Water-Supply Paper 1543A. U.S. Government Printing Office, Washington, DC, 1960.
(27)
Dalrymple, T. and M. A. Benson. Measurement of Peak Discharge by the Slope-Area Method.
U.S. Geological Survey Techniques of Water-Resources Investigations, Book 3, Chapter A2.
U.S. Government Printing Office, Washington, DC, 1967.
(28)
Dawdy, D. R. A Rainfall-Runoff Simulation Model for Estimation of Flood Peaks for Small
Drainage Basins. U.S. Geological Survey Professional Paper 506-B. U.S. Government Printing
Office, Washington, DC, 1972.
(29)
Department of Environment, National Water Council, London, Great Britain. Design and
Analysis of Urban Storm Drainage—The Wallingford Procedure. Standing Technical
Committee Reports, Volumes 1 to 5, September 1981.
(30)
Druse, S. A. Flood Flow Characteristics of Wyoming Streams—A Compilation of Previous
Investigations. U.S. Geological Survey, Wyoming District, Surface Water Branch, December
1988.
(31)
Espey, W. H. and D. G. Altman. Nomographs for Ten-minute Unit Hydrographs for Small
Urban Watersheds. ASCE UVRRP TM 32. American Society of Civil Engineers, 1977.
(32)
Federal Highway Administration. The Design of Encroachments on Flood Plains Using Risk
Analysis. Hydraulic Engineering Circular No. 17. Federal Highway Administration, U.S.
Department of Transportation, Washington, DC, July 1980.
(33)
Federal Highway Administration. Drainage of Highway Pavements. Hydraulic Engineering
Circular No. 12. Federal Highway Administration, U.S. Department of Transportation,
Washington, DC, March 1984.
(34)
Federal Highway Administration. Guide for Selecting Manning’s Roughness Coefficients for
Natural Channels and Flood Plains. FHWA-TS-84-204. Federal Highway Administration,
U.S. Department of Transportation, Washington, DC, April 1984.
(35)
Federal Highway Administration. Hydrology. Hydraulic Engineering Circular No. 19. Federal
Highway Administration, U.S. Department of Transportation, Washington, DC, October 1984.
(36)
Federal Highway Administration. Highway Hydrology. Hydraulic Design Series No. 2,
FHWA-02-001. Federal Highway Administration, U.S. Department of Transportation,
Washington, DC, 2002.
Hydrology
2-55
(37)
Fletcher, J. E., A. L. Huber, F. W. Haws, and C. G. Clyde. Runoff Estimates for Small Rural
Watersheds and Development of a Sound Design Method. Research report prepared for the
Federal Highway Administration by Utah State University. 1977.
(38)
Fletcher, J. E. and G. W. Reynolds. Snowmelt Peak Flows and Antecedent Precipitation from
Watersheds in Transition. American Water Resources Association and Colorado State
University, 1972.
(39)
Fricke, T. J., M. R. Kennedy, and N. B. Wellington. The Use of Rainfall Correlation in
Determining Design Storms for Waterways on a Long Railway Line. Proc., Hydrology and
Water Resources Symposium, Hobart, Australia, November 8–10, 1983.
(40)
Harley, B. M. Research on the Effects of Urbanization and Small Stream Flow Quantity.
Report No. FHWA-RD-78-88. Prepared for the Federal Highway Administration, U.S.
Department of Transportation, Washington, DC, 1978.
(41)
Harley, B. M., F. E. Perkins, and P. S. Eagleson. A Modular Distributed Model of Catchment
Dynamics. Massachusetts Institute of Technology, Ralph M. Parson Laboratory, Report No.
133, December 1971.
(42)
Hedman, E. R. and W. K. Osterkamp. Streamflow Characteristics Related to Channel
Geometry of Streams in Western United States. U.S. Geological Water Supply Paper 2193.
U.S. Government Printing Office, Washington, DC, 1982.
(43)
Hiemstra, L. A. V. Joint Probabilities in the Rainfall—Runoff Relation. In Transportation
Research Record 261. TRB, National Research Council, Washington, DC, 1969.
(44)
Hiemstra, L. A. V. and B. M. Reich. Engineering Judgment and Small Area Flood Peak.
Colorado State University Hydrology Paper No. 19. Ft. Collins, CO, 1967.
(45)
Horner, W. E. Modern Procedure in District Sewer Design. Engineering News, Vol. 64,
p. 326, 1910.
(46)
Horner, W. W. and F. L. Flynt. Relation between Rainfall and Runoff from Small Urban Areas.
Trans. ASCE, Vol. 101. American Society of Civil Engineers, 1936.
(47)
Interagency Advisory Committee on Water Data. Guidelines for Determining Flood Flow
Frequency. Bulletin 17B. March 1982.
(48)
Jarrett, R. Mixed Population Flood Frequency Analysis in Colorado. In Transportation
Research Record 1073. TRB, National Research Council, Washington, DC, 1986.
(49)
Jens, S. W. Design of Urban Highway Drainage, the State-of-the-Art. Prepared for the Federal
Highway Administration, U.S. Department of Transportation, Washington, DC, 1979.
(50)
Jens, S. W. and M. B. McPherson. Hydrology of Urban Areas in Handbook of Applied
Hydrology, Section 20. McGraw-Hill Book Co., Inc., New York, NY, 1942.
2-56
Highway Drainage Guidelines
(51)
Leopold, L. B. Hydrology for Urban Land Planning—A Guidebook on the Hydrologic Effects
of Urban Land Use. U.S. Geological Survey Circular 554. U.S. Geological Survey,
Washington, DC, 1968.
(52)
Linsley, R. K., Jr., M. A. Kohler, and J. L. H. Paulhus. Hydrology for Engineers. 3rd ed.,
McGraw-Hill Book Company, New York, NY, 1982.
(53)
Lowham, H. W. Techniques for Estimating Flow Characteristics of Wyoming Streams. U.S.
Geological Survey Water Resources Investigations 76–112. 1976.
(54)
Lowham, H. W. Stream Flows in Wyoming. U.S. Geological Survey Water-Resources
Investigations Report 88-4045, 1988.
(55)
Matthai, H. F. Measurement of Peak Discharge at Width Contractions by Indirect Methods.
U.S. Geological Survey Techniques of Water-Resources Investigations, Book 3, Chapter A5.
U.S. Government Printing Office, Washington, DC, 1968.
(56)
McCain, J. F. and R. D. Jarrett. Manual for Estimating Flood Characteristics of Natural-Flow
Streams in Colorado. USGS and Colorado Water Conservation Board, 1976.
(57)
McCuen, R. H. and N. Miller. Conceptual and Empherical Comparison Methods for
Predicting Peak-Runoff Rates. In Transportation Research Record 922. TRB, National
Research Council, Washington, DC, 1983.
(58)
Meinzer, O. E., ed. Hydrology. McGraw-Hill Book Company, Inc., New York, NY, 1942.
(59)
Mitci, C. Determine Runoff by Simple Way. Water and Wastes Engineering, Vol. II, No. 1,
January 1974.
(60)
National Resources Conservation Service. National Engineering Handbook. Section 4,
Hydrology. U.S. Government Printing Office, Washington, DC, March 1985.
(61)
Newton, D. W. and J. C. Herrin. Assessment of Commonly Used Flood Frequency Methods. In
Transportation Research Record 896. TRB, National Research Council, Washington, DC,
1982.
(62)
Ragan, R. M. A Nomograph Based on Kinematic Wave Theory for Determining Time of
Concentration for Overland Flow. Civil Engineering Research Report No. 44. University of
Maryland, College Park, MD, December 1971.
(63)
Reich, B. and G. Aron. Magnitude and Frequency of Flood. CRC Critical Reviews in
Environmental Control, Vol. 6, Number 4, pp. 297-348. October 1976.
(64)
Reich, B. M. and D. R. Jackson. Flood Prediction Methods for Pennsylvania Highway
Crossings. Research report prepared by Pennsylvania State University for the Pennsylvania
Department of Transportation, 1971.
(65)
Rhodes, D. D. and G. William. Adjustments of the Fluvial System. Reference Section on SlackWater Deposits: A Geomorphic Technique for the Interpretation of Fluvial Paleohydrology by
Hydrology
2-57
Peter C. Patton, Victor R. Baker, and R. Craig Kochel. Kendall-Hunt Publishing Co., Iowa,
1979.
(66)
Ribeny, F. M. J. On the Chance of Culvert Washouts on a Long Railway Line. Institution of
Engineers, Hydrology Papers, Australia, 1971.
(67)
Riggs, H. C. Frequency Curves. U.S. Geological Survey Techniques of Water-Resources
Investigations, Book 4, Chapter A2. U.S. Government Printing Office, Washington, DC, 1968.
(68)
Sanders, T. G. Hydrology for Transportation Engineers. Prepared by Colorado State
University for the Federal Highway Administration, U.S. Department of Transportation,
Washington, DC, January 1980.
(69)
Sauer, V. B. Magnitude and Frequency of Urban Floods. In Transportation Research Record
1073. TRB, National Research Council, Washington, DC, 1986.
(70)
Sauer, V. B., W. O. Thomas, Jr., V. A. Stricker, and K. V. Wilson. Flood Characteristics of
Urban Watersheds in the United States. U.S. Geological Survey Water-Supply Paper 2207.
Prepared in cooperation with the Federal Highway Administration, U.S. Department of
Transportation, Washington, DC, 1983.
(71)
Schaake, J. C., Jr., J. C. Geyer, and J. W. Knapp. Experimental Examination of the Rational
Method. Proc., ASCE, Hydraulics Division Journal, HY6 Paper 5607. American Society of
Civil Engineers, New York, NY, 1967.
(72)
Sherman, L. K. Stream Flow from Rainfall by the Unit Graph Method. Engineering News
Record, Vol. 108. 1932.
(73)
Snyder, F. F. Synthetic Unit-graphs. Trans. American Geophysical Union, Vol. 19, 1938.
(74)
Stockton, C. W. and W. R. Boggess. Tree Ring Data: Valuable Tool for Reconstructing Annual
and Seasonal Streamflow and Determining Long Term Trends. In Transportation Research
Record 922, 1983.
(75)
Subcommittee on Hydrology, Inter-Agency Committee on Water Resources. Methods of Flow
Frequency Analysis, Notes on Hydrologic Activities, Bulletin No. 13. U.S. Government
Printing Office, Washington, DC, 1966.
(76)
Thomas, D. A. and M. A. Benson. Generalization of Stream Flow Characteristics from
Drainage Basin Characteristics. U.S. Geological Survey Water-Supply Paper No. 1975. 1970.
(77)
Tice, R. H. Delaware River Basin Flood Frequency. U.S. Geological Survey open-file report.
Trenton, NJ, 1958.
(78)
U.S. Bureau of Public Roads. Design of Roadside Drainage Channels. Hydraulic Design Series
No. 4. 1965.
(79)
U.S. National Resources Conservation Service. Urban Hydrology for Small Watersheds.
Technical Release No. 55, Second Edition. June 1986.
2-58
Highway Drainage Guidelines
(80)
USDA, National Resources Conservation Service. Computer Program for Project FormulationHydrology. Technical Release No. 20, May 1965.
(81)
Wacker, A. M. Highway Impact on a Mountain Stream. Colorado State University, M.S.
Thesis, June 1974.
(82)
Wahl, K. L. Determining Streamflow Characteristics Based on Channel Cross-Section
Properties. In Transportation Research Record 922. TRB, National Research Council,
Washington, DC, 1983.
(83)
Wilson, K. V. A Preliminary Study of the Effect of Urbanization on Floods in Jackson,
Mississippi. U.S. Geological Survey Professional Paper 575-D. U.S. Government Printing
Office, Washington, DC, 1967.
(84)
Wisler, C. O. and E. F. Brater. Hydrology. 2nd ed. John Wiley and Sons, Inc., New York,
1959.
CHAPTER 3
EROSION AND SEDIMENT CONTROL
IN HIGHWAY CONSTRUCTION
Chapter 3
Erosion and Sediment Control in
Highway Construction
3.1 INTRODUCTION
Soil erosion is a natural process whereby soil particles are dislodged by rainfall and carried away by
runoff. The removal rate of the soil particles is proportional to the intensity and duration of the
rainfall, the volume and characteristics of the water flow, and the terrain characteristics and soil
properties. This erosion process is accelerated where the land has been disturbed by removing the
vegetative or other natural protective cover of the soil.
Sedimentation is the natural process of deposition of the eroded soil. This eroded soil in the form of
sediment may contaminate lakes, streams and reservoirs, restrict drainage ways, plug culverts,
damage adjacent properties, and affect the ecosystems of streams.
Because modern highway construction may involve the disturbance of large land areas, control of
erosion and sedimentation is a major concern. A commitment to erosion and sedimentation prevention
during all phases of highway design construction and maintenance is stated in the AASHTO
publication A Policy on Geometric Design of Highways and Streets, 2004 (1).1
While much of the effort for control of erosion and sedimentation is expended during the construction
phase of a highway development, a successful program must address erosion and sedimentation
control during the planning, location, design, and future maintenance phases as well. This erosion and
sediment control program should be a plan of action and provision of contract documents to achieve
an acceptable level of control within established criteria and control limits. This plan of action is
analogous to an agency’s highway development process, which results in contract plans and
documents to provide and maintain transportation facilities based on certain criteria and controls.
This chapter will address the establishment of criteria and controls for erosion and sedimentation and
the consideration, process and measures that must be taken to achieve the desired result. The primary
thrust will be directed at water-related erosion and sedimentation although some of the practices are
also applicable in controlling wind erosion.
1
Numbers in parentheses refer to publications in “References” (Section 3.8).
3-2
Highway Drainage Guidelines
Figure 3-1. Highway Construction Can Disturb
Large Areas of Land
Figure 3-2. Bare Soil Exposed to Erosion
3.2 PURPOSE AND OBJECTIVES
The purpose of an erosion and sediment control program is to allow the development of a highway
facility while also accomplishing the three general erosion and sediment control objectives of
(1) limiting off-site effects to acceptable levels, (2) facilitating project construction and minimizing
overall costs, and (3) complying with Federal, State, and local regulations.
Erosion and Sediment Control in Highway Construction
3-3
The first objective is to limit off-site effects to acceptable levels. One problem with this approach is
that, not only are many of the effects uncertain, there is no universal agreement as to what constitutes
an undesirable effect. However, many off-site conditions are readily definable relative to the levels of
sediment that may cause damage. Examples include clear water streams, impoundments, and
developed areas. The designer of the erosion and sediment control measures should attempt to make
some determination of the type and magnitude of off-site effects to be expected, to determine whether
the effects will be detrimental, beneficial, or neutral, and temper the design accordingly. This
determination may require some prediction or estimates of the quantity of eroded material that would
be expected from the construction site. This information will allow an evaluation of what, if any,
control measures are required and their size and extent of application. Several acceptable procedures
for predicting soil loss quantities are presented in References (6), (19), and (27).
The second objective deals with integration of the erosion control measures into the construction
processes to facilitate construction and afford an overall cost effective program. Control measures
should be simple to construct, afford as little interruption to normal construction procedures as
practicable, and be effective in their operation. Much is lost when a shotgun approach is taken, where
the designer attempts to achieve total control of both erosion and sediment by calling for rigorous or
inflexible design plan measures of questionable effectiveness.
Central to the preparation of an erosion and sediment control plan is an evaluation of each site for
possible actions and their consequences. It is necessary to analyze the probable effects to be expected
from both the implementation of the control measures and their omission, the location of the effects,
whether or not the potential damage is acceptable, and the cost-effectiveness of the chosen action.
This analysis will establish if, and to what extent, a plan for erosion and sediment control is needed.
Figure 3-3. Roadway Slope Protected with Shoulder
Dike, Temporary Slope Drains, and Vegetation
The third objective is complying with Federal, State, and local regulations. As a result of the National
Environmental Policy Act of 1969, much attention has been directed to the control of erosion and
sedimentation. Promulgated by this concern are numerous State and Federal regulations and controls
governing land disturbing activities. At the Federal level, several Executive Orders (E.O.) and
regulations address erosion and sediment control requirements on Federally supported highway
activities. There are also Federal control requirements exerted by numerous agencies (USACE, U.S.
3-4
Highway Drainage Guidelines
EPA, U.S. FWS) through their administration of various permitting requirements (Section 404,
Section 402 of the Federal Water Pollution Control Act (FWPCA) and Sections 9 and 10 of the River
and Harbor Act). The National Pollutant Discharge Elimination System (NPDES) authorized under
40 CFR 122.26(b)(14)(x) addresses stormwater discharge from construction sites. In 1990, the Phase
I regulations were applied to construction activities which disturb greater than five acres (two
hectares). A subsequent signing of the Phase II Final Rule addresses disturbed areas between one acre
and five acres (0.4 hectare to two hectares).
Most States have enacted some form of an erosion and sediment control program through specific
legislated sediment control acts or as a part of their Section 208 (PL 92-500) planning. In most
instances, highway agencies are required to meet their State regulations.
Some basic principles hold true when developing an erosion and sediment control program for a
project. These include the following: erosion prevention is generally more effective than sediment
control; sediment control is generally more effective than the repair of damage caused by
uncontrolled sediment; and an erosion and sediment control plan carefully prepared for the specific
conditions to be expected for a particular project will be more effective than a generalized
nonspecific approach.
Figure 3-4. Silt Fence Protecting Adjacent Property
3.3 EROSION AND SEDIMENT-RELATED PLANNING AND LOCATION
CONSIDERATIONS
Effective erosion control begins in the planning and location of a highway route. All highway route
alternatives have a base erosion potential that varies from route to route. These alternative routes can
also present a range of potential sedimentation problems and controls. These sediment and erosion
sensitive areas should be identified and considered in selection of the final route location and the
establishment of criteria on which the control measures and procedures will be established.
Unless damage from erosion and sediment is considered in selecting a route location, the cost of
solving problems that may have been avoided sometimes becomes great. The total cost of erosion
Erosion and Sediment Control in Highway Construction
3-5
control measures on each of the alternatives under study must be considered as a part of the
economic analysis.
3.3.1 Identification of Erosion Sensitive Areas
All highway route alternatives have a base erosion potential that is dependent on soil types, terrain
features, and climate.
Some soil types are known to be more erosive than others and their identification is a valuable aid in
route selection. Information on soil erodibility can often be obtained from (1) soil and geological
maps and reports, (2) local agricultural offices, or (3) local highway personnel familiar with previous
work in particular soil types.
Areas with unstable or troublesome soils (e.g., landslide areas, loess soils, alluvial fans, some glacial
deposits) are potential problem areas when disturbed by highway construction. Soil reports and
investigations by knowledgeable engineers and engineering geologists can be made during the route
location stage to identify these areas.
The natural drainage pattern, including subsurface flow, should be examined for the alternative routes
considered. A dense pattern of steep gradient natural channels presents a greater erosive potential than
would a flatter gradient and more dispersed natural system. Subsurface flow can present problems
with slope stability in areas requiring extensive cut sections.
A knowledge of the geology of the area allows the highway engineer to detect problem areas and
anticipate subsidence, landslides, and erosion problems. Such areas and problems can sometimes be
avoided in route selection.
Terrain features are the result of past geologic and climatic processes. Erosion and deposition by
running water are major geologic processes in shaping the terrain. A study of the terrain and the
natural erosion can aid in judging the complexity of erosion and what control measures, if any,
are required.
Seasonal variations of climatic conditions (e.g., rainfall and snowmelt amounts, wind intensity and
direction, temperature extremes) can be identified for the expected construction phases and exposed
soil periods. This will allow evaluation of their effects on the potential erodibility of the route.
3.3.2 Identification of Sediment Sensitive Areas
During the planning and location stages of project development, areas of potential damage from
excessive sedimentation should be identified. These would include such things as water supply
sources, impoundments, irrigation systems, recreational waters, croplands, homes, wetlands,
developed areas, and streams with particularly sensitive ecological systems. This identification should
include threshold limits for the accelerated introduction of sediment into the system as a result of the
proposed project construction. This information will first assist in evaluating if the project can be
located in a particular area without potential damaging results and, secondly, it will provide the
criteria on which to base cost-effective erosion and sediment control measures. These threshold limits
are addressed in more detail in Reference (5).
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3.3.3 Coordination
The highway planning process requires contact and coordination with the private and public sectors
of society that may either have an interest in, or control of, the effects of proposed development. This
process provides a means to obtain input identifying erosion and sediment sensitive areas and
regulatory controls. Coordination within the highway agency is also imperative.
3.3.3.1 Coordination within the Transportation Agency
The development of an erosion and sediment control plan spans the entire planning, design, and
construction stages of a highway project development. To be successful, it is imperative that
communication be established and maintained throughout each stage of development to ensure a
coordinated effort.
The designer must be aware of the erosion and sediment sensitive areas identified during project
planning and any accepted criteria from others that would affect the control provisions included in the
plans. This information, along with a clear purpose for the control provisions, must be passed on to
those responsible for project construction and maintenance. Conversely, designers and planners must
be aware of what is practicable, reasonable, and necessary to achieve during construction and over the
life of the project when selecting design features and control criteria for use in developing the erosion
control plan.
3.3.3.2 Coordination with Other Agencies
Local Natural Resources Conservation Service (NRCS) offices of the U.S. Department of Agriculture
and various State agencies can provide valuable assistance in solving local erosion problems by
suggesting vegetation and other permanent and temporary control measures suitable for the locality.
Soil survey maps prepared by the NRCS can provide an indication of different erosive potentials.
USACE, U.S. EPA and U.S. FWS and other groups and agencies having interest in environmental
concerns can provide information and requirements regarding existing stream and impoundment
quality classification, their present and potential use, and the impact that differing levels of sediment
input may produce.
USGS and the State Resource Agency are primary sources for stream sediment and sedimentrelated data.
3.4 EROSION AND SEDIMENT-RELATED GEOMETRIC CONSIDERATIONS
Highway geometrics can be used to an advantage in minimizing soil erosion and potential
sedimentation problems and control measures. Project alignment and grade, the design cross section,
and the number and involvement of stream crossings and encroachments are geometric features that
may have a range of flexibility. Within this range of flexibility, adjustments can often reduce the
erosion and sediment damage potential or considerably lessen the requirements and cost of control.
The following is a discussion of these geometric features and their influence on erosion and sediment
considerations.
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3.4.1 Alignment and Grade
Alignment and grade of a highway are important to successful erosion control and their careful
selection may be an option available to the designer. The alignment may be shifted to eliminate or
minimize encroachment into a surface water environment. A change in grade may be used to avoid
intrusion into known erodible soil strata. Alignment and grade alternatives must be consistent with
highway safety criteria and should be blended and fitted to the natural landscape for minimization of
cut-and-fill sections to reduce erosion and costly maintenance. These geometric features should be
selected so that both ground and surface water can pass through the highway right-of-way or be
intercepted with minimum disturbance to streams and without causing serious erosion problems.
Whenever practical, stream crossings should be made at stable reaches of a stream, avoiding
meanders that are subject to rapid shifting and channel profiles that are degrading or aggrading. The
direction and amount of flood flow at various stages must be considered in the location of hydraulic
openings to avoid undue scour and erosion. To reduce the potential for problems, every effort should
be made to minimize the number of stream crossings and encroachments. Reference (4) is suggested
for further information on the proper location and design of stream crossings.
Figure 3-5. Adjusting Grades to Minimize Excavation
3.4.2 Cross Section
Slopes of the roadway cross section should be as flat as practicable and consistent with soil stability,
climatic exposure, geology, proposed landscape treatment, and maintenance procedures. The cross
section should be varied, if necessary, to minimize erosion and to be consistent with safety and
drainage requirements. Generally, good landscaping and drainage design are compatible with both
erosion control and vehicle safety.
Severe erosion of earth slopes is usually caused by a concentration of surface water flowing from the
area at the top of cut or fill slopes. Diversion dikes and ditches, either temporary or permanent, should
be included in the cross section to intercept and convey the runoff to a suitable outlet. These dikes
and ditches are discussed further in Section 3.5.1.3.
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Highway Drainage Guidelines
Serrated cut slopes aid in the establishment of vegetative cover on decomposed rock or shale slopes.
Serrations may be constructed in any material that is rippable or that will hold a vertical face until
vegetation becomes established.
Figure 3-6. Serrated Cut Slope Aids in Establishing
Vegetation in Shale Slope
3.5 PLAN DEVELOPMENT
Erosion control and sediment collection during construction are highly dependent on the temporary
and permanent measures contained in the plans and available to the construction force. This practice
is no longer satisfactory. The agency must provide sufficient measures and guidance through the
contract documents to ensure that a well conceived, economically justified, and timely implemented
erosion and sediment control plan is presented to the contract forces. Sufficient rights-of-way and
easements must be provided to allow proper construction and maintenance of temporary and
permanent control measures. The Contractor may want to modify the plans to meet its schedule and
work methods, subject to agency approval.
The erosion and sediment process suggest some basic principles for development of this control plan.
Some of these principles are:
design slopes consistent with soil properties;
limit the area of unprotected soil exposure;
minimize the duration of unprotected soil exposure;
protect soil with vegetative cover, mulch, or erosion-resistant material;
control concentrations of runoff;
retard runoff with planned engineering works; and
trap sediment with temporary or permanent barriers, basins, or other measures as close to the
source as possible.
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3.5.1 Temporary Erosion and Sediment Control Measures
Temporary erosion and sediment control measures can be defined as those devices or procedures
employed during construction to control erosion and sediment until such time that permanent
protection can be provided. These temporary measures can be categorized into three general areas of
effort: (1) measures that provide direct protection to the soil surface (ground cover, channel liners,
riprap); (2) measures that tend to control the runoff pattern to an area of acceptable flow conditions
(diversion dikes and ditches, shoulder berms, slope drains); (3) measures that serve to remove
sediment from waters by filtering or slowing the velocity of the sediment laden water to such an
extent that it can no longer keep the particles in suspension or moving along the channel bed (filter
berms, brush barriers, silt fences, check dams, sediment basins).
The following sections address the objective, application, construction, and maintenance of these
temporary erosion control measures.
3.5.1.1 Ground Cover
An effective ground cover is one of the best erosion control measures available. An effective ground
cover protects the soil surface from the erosive force of raindrops, promotes infiltration by reducing
the sealing tendency of the soil surfaces and provides a barrier and limitation to sheet runoff.
Temporary ground covers are generally vegetation, mulch, or a combination of the two. These covers
are used on disturbed areas that are not to final grade and will be exposed for a period of time or in
areas where seasonal limitations or a delay in final construction preclude permanent seeding.
A common type of temporary cover is a combination of a quick-growing native vegetation (e.g., rye,
with a straw or hay mulch to provide protection to the surface and seeds until the permanent
vegetative growth is established). In some instances, a heavy application of mulch (e.g., woodchips,
wood fibers, cellulose) is used in conjunction with the seeding as a temporary protective cover.
To be effective, these ground covers must be routinely inspected to ensure that they are functional
and in good repair.
Figure 3-7. Roadway Area Protected with Temporary
Vegetation
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Highway Drainage Guidelines
Figure 3-8. Completed Embankment Area Covered
with Temporary Vegetation
3.5.1.2 Channel Liners
Temporary channel liners are used to facilitate the establishment of a vegetative growth in a drainage
way or as protection prior to the placement of a permanent armoring. Such liners are placed where an
ordinary seeding and mulch application would not be expected to withstand the force in the
channel flow.
Some typical temporary channel liners are excelsior, jute, and paper mats and fiberglass roving.
Permanent soil-reinforcing mats and rock riprap serve as both a temporary and permanent
channel liner.
The jute and paper mats are placed in the channel section after the area has been seeded. They not
only provide protection from the erosive forces of the channel flow, they also retain moisture that is
beneficial to seed germination. The mat should be rolled to ensure a firm contact, then stapled to the
ground. This will help to prevent undermining. Check slots should also be used at maximum 15 m
(50 ft) intervals and at the ends of a roll. The slots are merely a penetration of the material into the
soil a minimum of 100 mm to 150 mm (4 in. to 6 in.).
Figure 3-9. Jute Mat Ditch Protection
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Fiberglass roving has become a widely used and effective channel liner. It not only provides good
protection during the development of vegetation, the fibers interlace within the developing root mat
and create a sod that is more resistant to erosion.
Figure 3-10. Fiberglass Roving
The ease of installation and minimal labor requirements are also popular factors in the use of
fiberglass roving. The roving is applied by dispersal of the fibers through an air spray nozzle. Care
must be taken to ensure that a uniform covering is provided over the entire area of anticipated water
flow. The roving is generally applied at the rate of 150 to 200 g/m2 (0.25 to 0.35 lb/yd2), and it is
tacked with asphalt at the rate of 1.1 to 1.6 L/m2 (0.25 to 0.35 gal/yd2)to hold it in place. The interval
between check slots should be no more than 15 m (50 ft).
Figure 3-11. Application of Asphalt Tack to Hold
Fiberglass Roving in Place
Permanent soil reinforcement mats are designed to act as a reinforcing matrix for vegetative roots.
They accomplish this by forming a composite system of a soil-filled matrix of polymeric fibers
entangled and penetrated by vegetative roots. Installation requirements are similar to other mat
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Highway Drainage Guidelines
materials—this is a requirement of a firm contact with the protected surface including some form of
stapling or staking to hold them securely.
Figure 3-12. Polyester Fiber Soil Reinforcement Mat
Rock riprap channel lining is generally thought of as a permanent liner and will be discussed in more
detail in Section 3.5.1.10. However, it is used, particularly in the smaller stone sizes, more in the
realm of a temporary liner to promote vegetative growth.
Figure 3-13. Small Stone Riprap Liner in Median Ditch
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Figure 3-14. Riprap Liner in Roadway Side Ditch
Reference (11) provides detailed information on the use and limitation of many channel liners.
3.5.1.3 Diversion Dikes and Ditches
Diversion dikes and ditches can be used to intercept surface runoff and direct it to a desirable
collection or discharge point. These dikes and ditches may be constructed to intercept and divert flow
before it reaches a graded area, or they may be provided within the graded area to control flow.
A berm ditch (intercept ditch) is commonly provided along the top of cut slopes for which the
direction of predominant slope of the adjacent natural terrain is toward the cut section. This
interception of flow before it reaches the steep cut slope will facilitate the establishment and
maintenance of a vegetative cover and prevent rill erosion. In many instances, these intercept ditches
are of a standard size and configuration. The hydraulics engineer should review each site to ensure
that the standard size is adequate to convey the expected discharge. The ditches should also be
checked for lining requirements.
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Highway Drainage Guidelines
Figure 3-15. Berm Ditch along Top of Roadway Cut
Several types of dikes and ditches can be used within the graded area to control surface runoff. One
type, sometimes referred to as a shoulder berm, is constructed along the top of newly constructed fill
slopes to intercept flow and divert it to a temporary slope drain or into a protected outlet at a grade
low point (see Figure 3-16). Generally, when the height of fill is less than 1.5 m (5 ft), it is preferable
to control any sediment runoff at the toe of slope. The shoulder berm is obliterated as the fill is raised
and should be reconstructed at the end of each day’s grading operation, especially if rain is expected.
To reduce the number of adjustments, a stage buildup of the fill as noted in Figure 3-19 may be
desirable.
Figure 3-16. Shoulder Berm
The shoulder berm should be routinely inspected for general conformity to the recommended section
and for proper and positive direction of any collected runoff to the protected outlet points.
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Figure 3-17. Shoulder Berm with Pipe Outlet
A diversion ditch is sometimes provided within the graded area across the slope to intercept runoff
before it reaches erosive velocities and volumes. The intercepted runoff is then conveyed at
nonerosive velocities onto stabilized areas. A small berm along the downhill edge of the ditch will
make it more effective and provide a disposal site for the excavated soil. A number of small ditches
along a grade may be more effective and more convenient in allowing passage of equipment than one
large ditch. These ditches should be routinely inspected and repaired as necessary, perhaps on a daily
basis if subject to frequent construction equipment traffic.
3.5.1.4 Filter Berms
A filter berm is a temporary ridge of porous material that can be stabilized in rows, banks, or mounds.
Crushed stone and gravel are common and effective materials for filter berm construction.
Figure 3-18. Filter around Catch Basins
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Highway Drainage Guidelines
Filter berms may be constructed across graded rights-of-way, around drainage inlets, and at other
locations where a relatively small volume of flow is expected. The berm retains sediment on site by
retarding and filtering the runoff. An added advantage is that the filter berm is traversable by
construction traffic when dry.
Figure 3-19. Filter Berm across Construction Road
The filter berms require frequent checks and maintenance as the stone becomes clogged. The general
maintenance procedure is to remove trapped sediment and replace fully clogged portions of the
barrier.
3.5.1.5 Temporary Slope Drains
A temporary slope drain is a device to carry storm runoff from one elevation to another (see Figure 320). It is used to convey storm runoff from the work area down unprotected slopes. A major area of
application is providing controlled outlets for shoulder berm ditches. Slope drains can be open chute
or closed conduit design.
Figure 3-20. Temporary Slope Drain
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Half pipe sections, wooden flumes, and trenches lined with erosive resistant materials (e.g., riprap,
plastic sheets, concrete) are commonly constructed open chute slope drains. Open chutes are
particularly susceptible to failure from overflow, a shift or slump of the fill slope face and overtaxing
of the lining material by high-velocity flow. For these reasons, the basic design and/or standards
including materials, sizing, and location should be reviewed by the hydraulics engineer.
Figure 3-21. Flexible Temporary Slope Drain
Metal, plastic, and flexible pipes are conduits often used as closed slope drains. While subject to
failures from disjoining, they generally are a more reliable down drain on long-steep slopes than open
chutes of comparable cost. The slope drain conduit should be properly anchored to prevent movement
and disjointing.
For either the open chute or closed conduit, the inlet is a critical point subject to failure. Special care
must be taken to funnel the runoff into the drain to prevent bypass, piping, or saturated soil failures.
Good compaction of material around the inlet including the berm is imperative. When long-term use
of a temporary slope drain inlet is dictated by construction staging, it is recommended that the inlet
area be further protected with asphalt paving or other appropriate sealants. Placing a cut-off wall at
the inlet should also be considered.
A metal end section with a short section of pipe through a berm provides a very satisfactory inlet. For
rigid pipe drains, elbows should be provided at the top and bottom. An elbow at the top allows
placement of the inlet a sufficient distance back from the fill face (see Figure 3-20). An elbow on the
outlet will allow redirection of the vertical flow component. A stage buildup of the fill as depicted in
Figure 3-20 could reduce the number of times a slope drain inlet would require modification. With
proper planning it may be possible to locate the temporary slope drain so that it can become part of
the permanent drainage system, thus avoiding unnecessary slope disturbance later in the project. The
slope drain outlet should be located on a well-stabilized area. Energy dissipators (e.g., dumped rock
or direct discharge into a sediment basin) may be required.
To ensure proper operation, temporary slope drains should be inspected after each storm for structural
integrity, blockage, and stability at the inlet.
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3.5.1.6 Brush Barriers
Brush barriers are temporary barriers constructed of boughs, limbs, root mat, and small logs (see
Figures 3-22 through 3-24). They are generally placed along the toe of slope of high-fill sections, and
are provided to retard sheet flow and retain sediment on-site by filtering sediment laden runoff.
Figure 3-22. Brush Barrier along Toe of Embankment
Figure 3-23. Brush Wrapped in Filter Cloth
Brush barriers are cost-effective and readily constructible on projects located through wooded areas
because materials from the clearing operation can be utilized in their construction. The barrier and
trapped sediment are generally left in place to decay and be covered by natural vegetation growth.
Therefore, consideration should be given to the aesthetic quality of such a structure if visible from the
finished roadway or adjoining developed properties.
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Figure 3-24. Brush Barrier
While no formal design is required, the barrier is generally 1 m to 1.5 m (3 ft to 5 ft) in height and 1.5
m to 3 m (5 ft to 10 ft) in width. Stakes or logs as shown in Figure 3-24 may be required to avoid
displacement of a barrier. Small brush should be intermingled throughout the barrier to limit voids
and assure a proper filtering action. In some instances, the performance of a barrier has been
enhanced by the addition of a wrapping of filter cloth. The barrier size can be variable, based on the
amount of material available and the judgment of the engineer as to what constitutes an adequate
structure for a particular site.
Periodic inspection should be made to check and repair breaching or undercutting of the barrier.
3.5.1.7 Silt Fences
A silt fence is a vertical barrier of filter fabric supported by a low fence. Sediment-laden water is
filtered as it passes through the fence retaining the sediment within the construction area and allowing
relatively sediment-free water to pass through.
Figure 3-25. Silt Fence Protecting Adjacent Stream
A geotextile fabric is commonly used for the filter material. The fence backing can be a variety of
materials, patterns, and sizes and employing a variety of support post sizes and spacings. However,
the backing system must be adequate to support the filter cloth and the anticipated sediment loading.
One standard silt fence detail that has proven to function satisfactorily through extensive use is shown
in Figure 3-26.
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Highway Drainage Guidelines
Figure 3-26. Silt Fence
Silt fences are placed around drop inlets, across minor swales and at the toe of fill slopes adjacent to
streams and developed property. The fence functions best when the flow is uniformly distributed
along its length as in sheet flow conditions. Therefore, the fence should be located and the grading
controlled to avoid concentrations of flow.
Figure 3-27. Silt Fence in Minor Median Swale
One area of particular susceptibility to failure is water flowing under the fence. To protect against this
failure, the bottom edge of the filter fabric should be placed in a trench or otherwise anchored
securely.
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3-21
Figure 3-28. Silt Fence with Stone Filter Outlet Protecting Stream
The fence should be inspected after every rain and when a sediment accumulation of approximately
50 percent of the filter height is observed, it should be removed and disposed of properly. The fabric
should be checked for rips, tears, and other types of deterioration and replaced as needed. When
vegetation is established on the construction area, the fence should be removed and the accumulated
sediment spread and seeded.
3.5.1.8 Check Dams
Check dams are temporary barriers constructed of rock, woven wire and brush, limbs, logs and/or
other durable material placed across a natural or artificial channel (see Figure 3-29). A check dam
serves to control both erosion and sediment. This is accomplished by the dam creating an area of
reduced velocity within the channel to promote the deposition of suspended sediment and provide a
trap for bed-load material. This area of reduced velocity and flattening of the energy gradient also
reduces the erosive forces on the channel sides.
Figure 3-29. Check Dam
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Highway Drainage Guidelines
Improperly placed or inadequately designed check dams are vulnerable to failure due to their location
in areas of concentrated flow. They must be designed and constructed with adequate spillways,
dissipator aprons, and tie-ins to the channel banks and/or bed to protect the channel and structure
during times of high runoff. The basic design should be reviewed by the hydraulics engineer.
Figure 3-30. Rock Check Dam with Filter Stone
Figure 3-31. Check Dam Weir
Figure 3-32. Riprap Check Dam
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A very common failure is a washout of the channel banks around the ends of the structure. This can
generally be attributed to inadequacies in the spillway design.
Check dams should be inspected on a frequent basis, at least after each major rainfall event, and
repairs made as necessary.
3.5.1.9 Straw Bales
Straw bales are used as filters along the toe of fill slopes around drainage inlets and across minor
swales. They function to retain sediment on-site by retarding and filtering runoff from the
disturbed area.
Straw bales have a low porosity and weight per unit volume. Therefore, their use must be limited to
situations where expected storm flow volumes are small and structural strength is not required. Straw
bales should never be used in live streams.
Figure 3-33. Straw Bales in a Roadside Channel
When constructing a barrier of straw bales, the bales must be securely bound and anchored with steel
pins or wooden stakes. One major cause of failure is water flowing under the bales. To prevent this,
the bale can be entrenched a few millimeters [inches] prior to staking.
Straw bale barriers in ditch lines should be extended a sufficient length so that the elevation of the
bottom of the end of the barrier is higher than the top of the lowest bale. This assures that when
inflow exceeds the percolation rate of the barrier, excess flow will be over the barrier rather than
around the end.
Because straw bales quickly deteriorate and become clogged with sediment, they should be examined
frequently and replaced as needed.
3.5.1.10 Riprap
Riprap is an assemblage of gravel, cobble, crushed stones, or broken concrete materials. It may be
used in layers of varying thickness, individual particle size and gradation in the following
applications: (1) to protect the banks of rivers and streams, (2) as a liner for ditches and channels,
(3) as a dissipator at the outlet of culverts and concrete ditches, and (4) as a general surface covering.
While riprap generally remains as a permanent feature, it can also be thought of as a temporary
erosion control measure because it promotes the establishment of a vegetative cover.
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Figure 3-34. Broken Concrete on Geotech
Fabric Providing Shore Protection
Figure 3-35. Riprap Ditch Liner
Figure 3-36. Riprap Used for Fish Habitat
Enhancement in Channel Change
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Erosion and Sediment Control in Highway Construction
3-25
Figure 3-37. Riprap Slope Stabilization
There are many instances where expected velocity and depth of flow in channel are such that a scour
problem would be anticipated. These conditions dictate some form of armoring of the area that would
be subjected to scour forces. Rock riprap is one widely used and effective armoring tool.
Rock riprap is aesthetically pleasing and provides a flexible lining that can adjust to foundation
changes. It is also porous, allowing infiltration and exfiltration of the protected soil. This eliminates
many hydrostatic pressure problems associated with rigid linings such as concrete.
While rock riprap can be an effective erosion resistant lining, it does have limitations of use and is
susceptible to damage. This damage susceptibility is focused in three areas: displacement of
individual stones by the forces of water flow or ice, a loss of foundation stability by leaching of the
underlying soil through the riprap layer, and undermining by scour.
The size of the individual riprap stones are important in combating displacement damage. Of as much
importance as the individual stone size is the provision of a well-graded, interlocking mass of stone.
This multi-stone contact and interlock within the layer provides greater resistance of the mass to
displacement than could be provided by the individual stones. Thus, it is important that riprap stone
be sized to resist displacement, and it must also be well-graded within the selected size range.
Wire-enclosed riprap can provide the desired protection where larger rock is not readily available.
Another method that has been found to enhance the strength of riprap is the lodging of loose riprap in
place through “plating.” This process, often referred to as “keyed riprap,” involves dropping a large
piece of steel plate on the rock to produce a tight uniform blanket with a smooth face. Greater
stability is afforded by the keyed stone due to the reduced drag on the individual stones and the
increase in the angle of repose produced by the compact mass (21).
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Highway Drainage Guidelines
Figure 3-38. Wire-Enclosed Riprap
A well-graded, interlocking mass of riprap stone will also present fewer voids through which the flow
can attack the foundation soil. In some situations (e.g., reservoir shore protection, steep graded
ditches, highly erosive foundation soils), it is desirable to provide additional protection from this
leaching action. The most common methods of providing this additional protection are one or a
combination of the following: (1) increasing the thickness of the riprap layer, (2) providing a stone
filter blanket between the riprap and underlying soil, or (3) placing a geotextile fabric under the riprap
stone to serve as a filter.
Potential undermining damage to riprap can be minimized by properly designed beginning and
ending points and setting of foundation depths based on scour predictions.
Timely and properly installed, and correctly sized and graded rock riprap can provide an effective
erosion resistant lining and is an important tool in controlling temporary and long-term erosion
problems. Good sources for riprap design are References (11) and (12).
3.5.1.11 Sediment Basins
Sediment basins are storage areas provided by either excavation and/or the provision of a dam or
barrier. They are constructed for the primary purpose of trapping and storing sediment and are usually
constructed in channels and drainageways on or downslope from construction sites. They range in
size from small excavated traps with a volume of one cubic meter [one cubic yard] or less to large
impoundments with volumes measured in hectare-meters [acre-feet].
The location and design of sediment basins is determined by the expected sediment/water runoff and
the degree of downstream protection required. These factors will be discussed in the following
sections.
3.5.1.11.1 Planning and Location
While most small sediment basins (traps) can be included in the project erosion and sediment control
plans, many are located by the engineer and contractor to meet specific needs that develop during
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3-27
grading operations. The contract documents should ensure that the engineer and contractor have this
flexibility.
These small basins are excavated pits, and they are used effectively in many locations. Common sites
are (1) around drop inlets, (2) in swales and small ditches, (3) at the outlet of temporary slope drains,
and (4) in conjunction with check dams and silt fences. In many applications, there are no outlet
drains for the basin, and trapped water is removed by evaporation or percolation into the adjoining
soil. This percolation must be anticipated and considered in areas where soil saturation could present
stability problems. Therefore, to protect the structural integrity of the roadway, the use of this type of
device is discouraged in close proximity to fill slopes or in areas designated for future pavement.
Figure 3-39. Sediment Trap around Drop Inlet
Figure 3-40. Sediment Trap in Channel at Toe of Fill
Figure 3-41. Sediment Trap in Combination with
Silt Fence in Roadway Ditch
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Highway Drainage Guidelines
The location of large sediment basins requiring a dam and spillway structure are generally included in
the project plans, because they are designed for a specific site and usually require additional right-ofway. These structures can be quite costly, and their need and cost effectiveness must be evaluated.
This determination begins in the planning stage with the identification of sediment sensitive
downstream conditions. It also involves the evaluation of the use of other measures within the
construction area that may be more cost effective.
Figure 3-42. Large Sediment Basin with Pipe Riser
and Riprap Spillways
Figure 3-43. Existing Pond Can Be Used for Sediment
Basin and Restored Subsequent to Construction Period
If a large basin is justified, the site must be reviewed for the most effective placement. This would
include consideration of access for necessary cleanout and maintenance of the dam and spillway,
disposal of the removed sediment, and a reasonable adaptability of a dam and impoundment to the
site.
Large impoundments should be designed with public health, safety, and nuisance abatement in mind.
This criteria assumes greater importance when locating a basin in or close to a developed area.
CHAPTER 4
HYDRAULIC DESIGN OF CULVERTS
Chapter 4
Hydraulic Design of Culverts
4.1 INTRODUCTION
The function of a culvert is to convey surface water across or from the highway right-of-way. In
addition to this hydraulic function, it must also carry construction and highway traffic and earth
loads; therefore, culvert design involves both hydraulic and structural design. The hydraulic and
structural designs must be such that risks to traffic, of property damage, and of failure from floods are
consistent with good engineering practice and economics. This chapter is concerned with the
hydraulic aspects of culvert design and makes reference to structural aspects only as they are related
to the hydraulic design.
Structures measuring more than 20 ft (6.1 m) along the roadway centerline are conventionally
classified as bridges. Many longer structures, however, are designed hydraulically and structurally as
culverts. Culverts, as distinguished from bridges, are usually covered with embankment and are
composed of structural material around the entire perimeter, although some are supported on spread
footings with the streambed serving as the bottom of the culvert. Bridges are not designed to take
advantage of submergence to increase hydraulic capacity even though some are designed to be
inundated under flood conditions. For economy and hydraulic efficiency, culverts should be designed
to operate with the inlet submerged during flood flows, if conditions permit. At many locations, either
a bridge or a culvert will fulfill both the structural and hydraulic requirements for the stream crossing.
Structure choice at these locations should be based on construction and maintenance costs, risk of
failure, risk of property damage, traffic safety, and environmental and aesthetic considerations. Some
of the advantages of culverts are better traffic safety and lower maintenance costs than bridges.
Culverts do not have bridge railing, which can be a hazard, or a bridge deck, which is subject
to deterioration.
Culverts are usually considered minor structures, but they are of great importance to adequate
drainage and the integrity of the highway facility. Although the cost of individual culverts is usually
relatively small, the total cost of culvert construction constitutes a substantial share of the total cost of
highway construction. Similarly, the total cost of maintaining highway hydraulic features is
substantial, and culvert maintenance may account for a large share of these costs. Improved traffic
service and a reduction in the total cost of highway construction and maintenance can be achieved by
judicious choice of design criteria and careful attention to the hydraulic design of each culvert.
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4.2 DATA COLLECTION
For purposes of this section, site information from whatever source is broadly classified as survey
data. Sources of data include aerial or field survey; interviews; water resource, fish and wildlife, and
planning agencies; newspapers; and floodplain zoning studies. Complete and accurate survey
information is necessary to design a culvert to best serve the requirements of a site. The individual in
charge of the drainage survey should have a general knowledge of drainage design and coordinate the
data collection with the hydraulics engineer. The amount of survey data gathered should be
commensurate with the importance and cost of the proposed structure.
4.2.1 Topographic Features
The survey should provide the designer with sufficient data for locating the culvert and may aid in
determining the hydraulic design controls. All significant physical features and culture in the vicinity
of the culvert site should be located by the survey, and especially those features that could be affected
by the installation or operation of the culvert. Such features as residences, commercial buildings,
croplands, roadways, and utilities can influence a culvert design; therefore, their elevation and
location should be obtained.
The extent of survey coverage required for culvert design is related to topography and stream slope.
In streams with relatively flat slopes, the effects of structures may be reflected a considerable distance
upstream and require extensive surveys to locate features that may be affected by the culvert
installation.
4.2.2 Drainage Area
Drainage area is an important factor in estimating the flood potential; therefore, the area of the
watershed should be carefully defined by means of survey, photogrammetric maps, U. S. Geological
survey (USGS) topographic maps or a combination of these.1
In locations where accurate definition of drainage areas from maps is difficult, the map information
should be supplemented by survey. Noncontributing areas, such as contributing to sinkholes and
playa lakes, may need to be defined. The survey should note land usage, type and density of
vegetation, and any constructed changes or developments (e.g., dams) which could significantly alter
runoff characteristics.
4.2.3 Channel Characteristics
The physical characteristics of the existing stream channel should be described by the survey. For
purposes of documentation and design analysis, sufficient channel cross sections, a streambed profile
and the horizontal alignment should be obtained to provide an accurate representation of the channel,
including the floodplain area. The channel profile should extend beyond the proposed culvert location
far enough to define the slope and locate any large streambed irregularities (e.g., headcutting).
1
Maps for all areas of the United States can be ordered from the U.S. Geological Survey, Map Distribution,
Federal Center, Box 25286, Denver, CO 80225.
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4-3
General characteristics helpful in making design decisions should be noted. These include the type of
soil or rock in the streambed, the bank conditions, type and extent of vegetal cover, amount of drift
and debris, ice conditions, and any other factors that could affect the sizing of the culvert and the
durability of culvert materials. Photographs of the channel and the adjoining area can be a valuable
aid to the designer and serve as excellent documentation of existing conditions.
4.2.4 Fish Life
Survey data should include information regarding the value of the stream to fish life and the type of
fish found in the stream. The necessity to protect fish life and to provide for fish passage can affect
many decisions regarding culvert, channel change and riprap designs and construction requirements
for protection of the stream environment. Data required, and criteria for design and construction, are
generally available from State and Federal fish and wildlife agencies. A culvert designed for fish
passage is discussed in more detail in Section 4.7.4.
4.2.5 Highwater Information
Reliable, documented highwater data, when available, can be a valuable design aid. Often, the
designer must rely upon highwater marks as the only basis on which to document past floods.
Highwater marks can also be used to check results of flood-estimating procedures, establish highway
grade lines and locate hydraulic controls, but considerable experience is necessary to properly
evaluate highwater information.
Data related to highwater should be taken in the vicinity of the proposed structure, but it is sometimes
necessary to use highwater marks from upstream or downstream points. The location of the highwater
mark with respect to the proposed structure should be recorded. Highwater elevations should be
referenced to the project data.
If highwater information is obtained from residents, the individuals should be identified and the
length of residency indicated. Other sources for data include commercial and school bus drivers, mail
carriers, law enforcement officers, highway and railroad maintenance personnel, or other persons who
have frequently traveled through the area over a long period of time.
Unusual highwater elevations should be examined to ascertain whether irregularities existed during
the flood, such as blockage of the channel from drift, ice, or backwater from stream confluences.
4.2.6 Existing Structures
Considerable importance should be placed on the hydraulic performance of existing structures, and all
information available should be gathered in the survey. The performance of structures some distance
either upstream or downstream from the culvert site can be helpful in the design. Local residents,
highway maintenance personnel, or others can furnish important highwater data and dates of flood
occurrences at such structures.
Data at existing structures should include the following, if available:
date of construction;
major flood events since construction and dates of occurrence;
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Highway Drainage Guidelines
performance during past floods;
scour indicated near the structure;
type of material in streambeds and banks;
alignment and general description of structure, including condition of structure, especially noting
abrasion, corrosion, or deterioration;
alignment and general description of structure, including dimensions, shape, and material and
flowline invert elevations;
highwater elevations with data and dates of occurrence;
location and description of overflow areas;
photographs;
silt and drift accumulation;
evidence of headcutting in stream;
appurtenant structures (e.g., energy dissipators, debris control structures, stream grade control
devices); and
as-built plan of structure.
4.2.7 Field Review
The engineer designing drainage structures should be thoroughly familiar with the watershed site
under consideration. Much can be learned from the survey notes, but the most complete survey
cannot adequately depict all watershed site considerations or substitute for a personal inspection by
the designer. Often, a plans-in-hand inspection by the designer and the construction engineer will
prove mutually beneficial by improving the drainage design and reducing construction problems.
4.3 CULVERT LOCATION
Culvert location deals with the horizontal and vertical alignment of the culvert with respect to both
the stream and the highway. It is important to the hydraulic performance of the culvert, to stream
stability, to construction and maintenance costs, and to the safety and integrity of the highway.
The horizontal and vertical alignment are important in maintaining a sediment-free culvert.
Deposition occurs in culverts because the sediment transport capacity of flow within the culvert is
often less than in the stream. The following factors contribute to deposition in culverts:
at moderate flow rates, the culvert cross section is larger than that of the stream, thus the flow
depth and sediment transport capacity is reduced;
point bars form on the inside of stream bends, and culvert inlets placed at bends in the stream will
be subjected to deposition in the same manner. This effect is most pronounced in multiple-barrel
culverts with the barrel on the inside of the curve often becoming almost totally plugged with
sediment deposits; and
abrupt changes to a flatter grade in the culvert or in the channel adjacent to the culvert will induce
deposition. Gravel and cobble deposits are common downstream from the break in grade because
of the reduced transport capacity in the flatter section.
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Deposition usually occurs at flow rates smaller than the design flow rate. The deposits may be
removed during larger floods, dependent upon the relative transport capacity of flow in the stream
and in the culvert, compaction and composition of the deposits, flow duration, ponding depth above
the culvert, and other factors.
4.3.1 Plan
Plan location deals basically with the route the flow will take in crossing the right-of-way. Regardless
of the degree of sinuosity of the natural channel within the right-of-way, a crossing is generally
accomplished by using a straight culvert either normal to or skewed with the roadway centerline.
Ideally, a culvert should be placed in the natural channel (see Figure 4-1). This location usually
provides good alignment of the natural flow with the culvert entrance and outlet, and little structural
excavation and channel work are required.
Figure 4-1. Culvert Located in Natural Channel
Where location in the natural channel would require an inordinately long culvert, some stream
modification may be in order (see Figure 4-2). Such modifications to reduce skew and shorten
culverts should be carefully designed to avoid erosion and siltation problems.
Culvert locations normal to the roadway centerline are not recommended where severe or abrupt
changes in channel alignment are required upstream or downstream of the culvert. Short radius bends
are subject to erosion on the concave bank and deposition on the inside of the bend. Such changes
upstream of the culvert result in poor alignment of the approach flow to the culvert, subject the
highway fill to erosion and increase the probability of deposition in the culvert barrel. Abrupt changes
in channel alignment downstream of culverts may cause erosion on adjacent properties.
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Highway Drainage Guidelines
Figure 4-2. Methods of Culvert Location Where Location in the Natural Channel
Would Involve an Inordinately Long Culvert
In flat terrain, drainage is often provided by excavated channels. Highway planning should be
coordinated with the drainage authority where drainage improvements are planned. Where planned
channels are not at the location of natural drainage swales, concurrent channel and highway
construction is desirable. If concurrent construction is not possible, it will be necessary to provide
highway culverts for the existing drainage pattern. The drainage authority may contribute toward
modifications to accommodate future channel construction, revise drainage plans to conform with
highway culvert locations, or make the necessary changes in highway drainage at the time of
channel construction.
Culvert construction in live stream environments frequently necessitates the installation of temporary
diversion channels to carry the stream around the work site. The temporary diversion channels need
protective linings to prevent erosion. At times, it may also be necessary to develop a staged
construction sequence that will permit a portion of the work to be done; stream flow is then diverted
through the completed portion of the culvert while the remainder of the culvert installation is
constructed. Additional information on temporary erosion and sediment control measures that can be
used at a construction site may be found in Chapter 3, “Erosion and Sediment Control in Highway
Construction,” of the Highway Drainage Guidelines.
4.3.2 Profile
Most culvert locations approximate the natural streambed, though other locations may be chosen for
economy in the total cost to construct and maintain. Modified culvert slopes, or slopes other than that
of the natural stream, can be used to arrest stream degradation, induce sedimentation, improve the
hydraulic performance of the culvert (Section 4.5.6.4), shorten the culvert or reduce structural
requirements. Modified slopes can also cause stream erosion and deposition; therefore, slope
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4-7
alterations should be given special attention to ensure that detrimental effects do not result from
the change.
Channel changes often are shorter and steeper than the natural channel. A modified culvert slope can
be used to achieve a flatter gradient in the channel so that degradation will not occur.
Figure 4-3 illustrates some possible culvert profiles.
Figure 4-3. Possible Culvert Profiles
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Highway Drainage Guidelines
Where channel excavation is planned, culvert invert elevations can be established to accommodate
drainage requirements if concurrent channel and highway construction is possible. If concurrent
construction is not feasible, a joint or cooperative project should be investigated so that highway
culverts can be designed and constructed to serve current highway drainage requirements and future
needs for land drainage.
4.4 CULVERT TYPE
Selection of culvert type includes the choice of material, shape, and cross section, and the number of
culvert barrels. Total culvert cost can vary considerably depending upon the culvert type selected. Fill
height, terrain, foundation condition, fish passage, shape of the existing channel, roadway profile,
allowable headwater, stream stage-discharge and frequency-discharge relationships, cost and service
life are some of the factors that influence culvert-type selection.
4.4.1 Shape and Cross Section
The shape of a culvert is not the most important consideration at most sites, so far as hydraulic
performance is concerned. Rectangular, arch, or circular shapes of equal hydraulic capacity are
generally satisfactory. It is often necessary, however, for the culvert to have a low profile because of
the terrain or because of limited fill height. Construction cost, the potential for clogging by debris,
limitations on headwater elevation, fill height, and the hydraulic performance of the design
alternatives enter into the selection of the culvert shape. Design and construction specifications and
methods of determining maximum cover for some shapes and materials are included in publications
of AASHTO, FHWA, the American Society of Testing Materials, various State highway agencies and
others (1, 6, 7, 24).2 Several commonly used culvert shapes are discussed in the following sections.
4.4.1.1 Circular
The most commonly used culvert shape is circular. This shape is preferred due to the available
structural options for various fill heights. Various standard lengths of circular pipe in standard
strength classes are usually available from local suppliers at reasonable cost. The need for cast-inplace construction is generally limited to culvert end treatments and appurtenances.
4.4.1.2 Pipe Arch and Elliptical
Pipe arch and elliptical shapes are generally used in lieu of circular pipe where there is limited cover
or overfill. Structural strength characteristics usually limit the height of fill over these shapes except
when the major axis of the elliptical shape is laid in the vertical plane. When compared to circular
sections, these shapes are more expensive for equal hydraulic capacity because of the additional
structural material required.
4.4.1.3 Box or Rectangular
A culvert of rectangular cross section can be designed to pass large floods and to fit nearly any site
condition. A rectangular culvert lends itself more readily than other shapes to low allowable
2
Numbers in parentheses refer to publications in “References” (Section 4.15).
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4-9
headwater situations, because the height may be decreased and the total span increased to satisfy the
location requirement. The required total span can consist of one or multiple cells. Modified box
shapes in the form of hexagons or octagons have been used and proved economical under certain
construction situations. The longer construction time required for cast-in-place boxes can be an
important consideration in the selection of this type of culvert. Precast concrete and metal box
sections have been used to overcome this disadvantage.
4.4.1.4 Arches
Arch culverts have application in locations where less obstruction to a waterway is a desirable feature
and where foundations are adequate for structural support. Such structures can be installed to
maintain the natural stream bottom for fish passage, but the potential for failure from scour must be
carefully evaluated. Structural plate metal arches are limited to use in low-cover situations, but have
the advantage of rapid construction and low transportation and handling costs. This is especially
advantageous in remote areas and in rugged terrain.
4.4.1.5 Multiple Barrels
Culverts consisting of more than one barrel are useful in wide channels where the constriction or
concentration of flow is to be kept to a minimum. Low roadway embankments offering limited cover
may require the use of a series of small openings. The barrels may be separated by a considerable
distance to maintain flood flow distribution. The practice of altering channel geometry to
accommodate a wide culvert will generally result in deposition in the widened channel and in the
culvert. Where overbank flood flow occurs, relief culverts with inverts at the floodplain elevation
should be used to avoid the need for channel alteration.
In the case of box culverts, it is usually more economical to use a multiple structure than a wide
single span. In some locations, multiple barrels have a tendency to catch debris, which clogs the
waterway. They are also susceptible to ice jams and the deposition of silt in one or more barrels.
Alignment of the culvert face normal to the approach flow and installation of debris control structures
can help to alleviate these problems. To avoid widening of the natural channel, provide overflow
(flood) relief, support environmental preservation, and reduce sedimentation and debris problems, it is
good practice to install one barrel of the multiple-barrel culverts at the flow line of the stream, while
the other barrels are set at a slightly higher invert elevation. For more detail, see Reference (39).
4.4.2 Materials
The selection of the material for a culvert is dependent upon several variables (e.g., durability,
structural strength, roughness, bedding conditions, abrasion and corrosion resistance, water tightness).
The culvert materials used are:
concrete (reinforced and non-reinforced),
steel (smooth and corrugated),
corrugated aluminum,
vitrified clay,
plastic,
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bituminous fiber,
cast iron,
wood, and
stainless steel.
Water and soil environment, construction practices, availability of materials and costs vary
considerably depending on location; therefore, listing criteria for selecting culvert material appears to
be impracticable as a general guideline. Discussions on the use of certain materials from the durability
and hydraulic standpoint are given in Sections 4.5, 4.6, and 4.10.
The most economical culvert is one which has the lowest total annual cost over the design life of the
structure. The initial cost should not be the only basis for culvert material selection. Replacement
costs and traffic delay are usually the primary factors in selecting a material that has a long service
life. If two or more culvert materials are equally acceptable for use at a site, including hydraulic
performance and annual costs for a given life expectancy, consideration should be given to material
selection by the contractor.
4.4.3 End Treatments
Culvert end structures, prebuilt or constructed-in-place, are attached to the ends of a culvert barrel to
reduce erosion, inhibit seepage, retain the fill, improve the aesthetics and hydraulic characteristics,
and make the ends structurally stable. Several common types of culvert ends are listed in the
following subsections.
4.4.3.1 Projecting
A culvert is considered to have a projecting inlet or outlet when the culvert barrel extends beyond the
face of the roadway embankment. This common type of culvert end has no end treatment and is
vulnerable to various types of failures. It is the least desirable from the hydraulic standpoint when
used as an inlet to corrugated metal, thin-edged barrels. Rigid sectional pipe is vulnerable to
displacement at culvert outlets, if not adequately supported. The projecting end is economical, but its
appearance is not pleasing and its use should be limited to smaller culverts placed at minor locations,
such as at driveways and in ditches where there would be little safety hazard to traffic.
4.4.3.2 Mitered
A mitered culvert end is formed when the culvert barrel is cut to conform with the plane of the
embankment slope. This type of treatment is used primarily with large metal culverts to improve the
aesthetics of the culvert ends. It is structurally inadequate to withstand hydraulic, earth, and impact
loads unless it is well anchored and protected. The hydraulic performance of this type of inlet is
approximately the same as a thin-edged projecting inlet.
4.4.3.3 Pipe End Sections
Pipe end sections, sometimes called flared or terminal end sections, are prefabricated metal or precast
concrete sections placed onto the ends of culverts (Figure 4-4). These sections are used to retain the
embankment and improve the aesthetics, but usually do not improve the structural stability of the
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culvert end. Commonly used pipe end sections do not improve the hydraulic performance of culverts
appreciably over the performance of a headwall (for inlet improvements, see Section 4.5.6).
Figure 4-4. Flared-End Section
4.4.3.4 Headwalls and Wingwalls
Headwalls and wingwalls are generally cast-in-place concrete structures commonly constructed on
the ends of culvert barrels for the following reasons:
to retain the fill material and reduce erosion of embankment slopes;
to improve hydraulic efficiency;
to provide structural stability to the culvert ends and serve as a counter weight to offset buoyant
or uplift forces; and
to inhibit piping (see Section 4.6.2).
Although headwalls are sometimes skewed to the culvert barrel to fit the embankment slope, an
alignment normal to the direction of flow provides a more hydraulically efficient opening. Minor
warping of the fill can accommodate this more favorable orientation at most locations (see
Figure 4-5).
Wingwalls aid in maintaining the approach velocity, align and guide drift, and funnel the flow into
the culvert entrance. Wingwalls should be flush with box culvert barrels to avoid snagging drift.
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Figure 4-5. Fill Warped to Fit Culvert Headwall Normal to Culvert
4.5 HYDRAULIC DESIGN
The hydraulic design of a culvert consists of an analysis of the performance of the culvert in
conveying flow from one side of the roadway to the other. To meet this conveyance function
adequately, the design must include consideration of the variables discussed in the following sections.
4.5.1 Design Flood Discharge
The flood discharge used in culvert design is usually estimated on the basis of a preselected
recurrence interval, and the culvert is designed to operate in a manner that is within acceptable limits
of risk at that flow rate. Refer to Chapter 2, “Hydrology,” of the Highway Drainage Guidelines for a
discussion of the selection of the design flood frequency and the estimation of flood magnitudes.
Recognizing that floods cannot be estimated precisely and that it is seldom economically feasible to
design for the very rare flood, all designs should be reviewed using a larger review flood for the
extent of probable damage should the design flood be exceeded. The performance curve of Section
4.5.5.2 should include this larger review flood.
4.5.2 Headwater Elevation
Any culvert that constricts the natural stream flow will cause a rise in the upstream water surface to
some extent. The total flow depth in the stream measured from the culvert inlet invert is termed
headwater. Design headwater elevations and selection of design floods should be based on these risk
considerations:
damage to adjacent property,
damage to the culvert and the roadway,
Hydraulic Design of Culverts
traffic interruption,
hazard to human life, and
damage to stream and floodplain environment.
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Potential damage to adjacent property or inconvenience to owners should be of primary concern in
the design of all culverts. In urban areas, the potential for damage to adjacent property is greater
because of the number and value of properties that can be affected. If roadway embankments are low,
flooding of the roadway and delay to traffic are usually of primary concern, especially on highly
traveled routes.
Culvert installations under high fills may present the designer an opportunity for use of a high
headwater or ponding to attenuate flood peaks. If deep ponding is considered, the possibility of
catastrophic failure should be investigated because a breach in the highway fill could be quite similar
to a dam failure. When headwater depths will exceed, say 6 to 8 m (20 to 25 ft) for the estimated 100year flood, the roadway embankment will function as a dam, and an appropriate investigation should
be made to evaluate the risk in case of the occurrence of a larger flood or blockage of the culvert by
debris. In some instances, design of the highway fill as a dam and use of emergency facilities (e.g.,
spillways, relief culverts) should be considered as alternative designs to the construction of larger
structures or changes in the roadway profile.
The study of culvert headwater should include verification that watershed divides are higher than
design headwater elevations. If the divides are not sufficiently high to contain the headwater, culverts
of lesser depths or earthen training dikes may be used, in some instances, to avoid diversion across
drainage divides. In flat terrain, drainage divides are often undefined or non-existent and culverts
should be located and designed for least disruption of the existing flow distribution. In these
locations, culverts can be considered to have a common headwater elevation, though this will not be
precisely so. Figure 4-6 illustrates a design technique that can be used to select culvert sizes in this
type of terrain.
Figure 4-6. A Design Technique for Selecting Culvert Sizes in Flat Terrain
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4.5.3 Tailwater
Tailwater is the flow depth in the downstream channel measured from the invert at the culvert outlet.
It can be an important factor in culvert hydraulic design because a submerged outlet may cause the
culvert to flow full rather than partially full.
A field inspection of the downstream channel should be made to determine whether there are
obstructions that will influence the flow depth. Tailwater depth may be controlled by the stage in
another stream, headwater from structures downstream of the culvert, reservoir water surface
elevations, tide stages, or other downstream features.
4.5.4 Outlet Velocity
The outlet velocity of culverts is the velocity measured at the downstream end of the culvert, and it is
usually higher than the maximum natural stream velocity. This higher velocity can cause streambed
scour and bank erosion for a limited distance downstream from the culvert outlet. Local scour at or
near the culvert outlet should not be confused with degradation and headcutting in the stream.
Variation in shape and size of a culvert seldom has a significant effect on the outlet velocity except at
full flow. The slope and roughness of the culvert barrel are the principal factors affecting outlet
velocity. If the outlet velocity of a culvert is believed to be detrimental and it cannot be reduced
satisfactorily by changing the barrel roughness or adjusting the barrel slope, it may be necessary to
use some type of outlet protection or energy dissipation device. Inspection of existing culverts in the
area will be helpful in making this judgment. Various types of outlet treatment are included in
Section 4.5.8.
4.5.5 Culvert Hydraulics
The culvert size and type can be selected after the determination of the design discharge, culvert
location, tailwater, and controlling design headwater. The hydraulic performance of culverts is
complex, and the flow characteristics for each site should be analyzed carefully to select an
economical installation, which will perform satisfactorily over a range of flow rates. Headwater and
capacity computations can be made by using mathematical equations, electronic computer programs
or nomographs. References (27), (30), (32), (33), and (39) are widely used for the hydraulic design
of culverts.
Flood routing through a culvert is an alternative culvert-sizing practice that evaluates the effect of
temporary upstream ponding caused by the culvert’s backwater. In some instances, a culvert should
be sized on the basis of the flood routing concept, depending on the amount of temporary storage
involved and the degree of environmental concern and flood hazard. The flood-routing procedure
requires three basic data inputs:
an inflow hydrograph,
an elevation versus storage relationship, and
an elevation versus discharge relationship.
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4-15
A complete inflow hydrograph, not just the peak discharge, must be generated. Elevation, often
denoted as stage, is the parameter that relates storage to discharge providing the key to the floodrouting solution.
4.5.5.1 Conditions of Flow
The two major conditions of culvert flow are inlet control and outlet control. For each type of control,
a different combination of factors is used to determine the hydraulic capacity of a culvert. Prediction
of the condition of culvert flow is difficult; therefore, most designers assume that the culvert will flow
with the most adverse condition. This assumption is both conservative and expeditious. With the aid
of a computer analysis program (e.g., HY8 (27)), it is possible to analyze both inlet and outlet flow
conditions easily to determine which condition should prevail.
4.5.5.1.1 Inlet Control
A culvert operates with inlet control when the flow capacity is controlled at the entrance by the depth
of headwater and the entrance geometry, including the barrel shape, cross sectional area and the inlet
edge. Sketches to illustrate inlet control flow for unsubmerged and submerged projecting entrances
are shown in Figure 4-7.
For a culvert operating with inlet control, the roughness and length of the culvert barrel and outlet
conditions (including tailwater) are not factors in determining culvert hydraulic performance. The
entrance edge and the overall entrance geometry have much to do with culvert performance in this
type of flow; therefore, special entrance designs can improve hydraulic performance and result in a
more efficient and economical culvert. Types of entrances are discussed in Section 4.5.6.
Figure 4-7. Inlet Control
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4.5.5.1.2 Outlet Control
In outlet control, the culvert hydraulic performance is determined by the factors governing inlet
control plus the controlling water surface elevation at the outlet and the slope, length, and roughness
of the culvert barrel. Culverts operating in outlet control may flow full or partly full, depending on
various combinations of the above factors. In outlet control, factors that may affect performance
appreciably for a given culvert size and headwater are barrel length, roughness and tailwater depth.
Although entrance geometry is a factor, only minor improvement in performance can be achieved by
modifications to the culvert inlet.
Typical types of outlet control flow are shown in Figure 4-8.
Figure 4-8. Outlet Control
4.5.5.2 Performance Curves
Performance curves are plots of discharge versus culvert headwater depth or elevation. A culvert may
operate with outlet or inlet control over the entire range of flow rates, or control may shift from the
inlet to the outlet. For this reason, it is necessary to plot both inlet and outlet control curves to develop
the culvert performance curve.
In culvert design, the designer usually selects a design flood frequency, estimates the design
discharge for that frequency, and sets an allowable headwater elevation based on the selected design
flood and considerations cited in Section 4.5.2. There are, however, uncertainties in estimating flood
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peaks for any desired recurrence interval and a probability that the design frequency flood will be
exceeded during the life of the project. (See Chapter 2, “Hydrology,” of the Highway Drainage
Guidelines) Because of the uncertainties, it is necessary for the designer to develop information from
which he can evaluate the culvert performance or headwater capacity relationship over a range of
flow rates. With this information on culvert performance, the risks involved in the event of large
floods can be evaluated. This evaluation should include the probability of occurrence, the possibility
of traffic interruption by flow over the highway, and damages that would occur to the highway and
other property.
Performance curves aid in the selection of the culvert type, including size, shape, material and inlet
geometry, which fulfills site requirements at the least annual cost. The curves also may reveal
opportunities for increasing the factor of safety and improving the hydraulic capacity at little or no
increase in cost. A typical culvert performance curve is shown in Figure 4-9. Flood frequency has
been added to the abscissa to aid in evaluating the risk of exceeding the design headwater with the
selected culvert design.
Figure 4-9. Performance Curves for Single Box Culvert
90° Wingwall
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Highway Drainage Guidelines
4.5.6 Entrance Configurations
Entrance configuration is defined as the cross sectional area and shape of the culvert face and the type
of inlet edge. When a culvert operates in inlet control, headwater depth and the entrance configuration
determine the culvert capacity, and the culvert barrel usually flows only partially full. Entrance
geometry refinements can be used to reduce the flow contraction at the inlet and increase the capacity
of the culvert without increasing the headwater depth. The degree of refinement warranted is
dependent upon the slope and roughness of the culvert barrel, headwater elevation controls, tailwater,
design flood discharge, and the probability of exceedance, risk of damage, construction costs, and
other factors. Performance curves are an indispensable aid in evaluating the degree of inlet refinement
that is warranted (30).
In connection with inlet improvements, two points should be emphasized. First, culverts operating in
outlet control usually flow full at the design flow rate. Therefore, inlet improvements on these
culverts only reduce the entrance loss coefficient, ke, which results in only a small decrease in the
required headwater elevation. Second, inlet improvements are made for the purpose of causing a
culvert flowing with inlet control to flow full or nearly full at the design discharge. It should be
recognized that outlet control may govern for discharges that are higher than the design flood peak,
and the rate of increase in headwater with increasing discharge is greater for outlet control than inlet
control. Because of uncertainties in estimating flood peaks and the chance that the design frequency
flood will be exceeded, the risk of damage from larger floods may warrant incorporating an increased
factor of safety in culvert capacity at some sites.
Table 4-1 gives entrance loss coefficients, ke, for computing entrance losses for outlet control flow. In
inlet control, the effect of the entrance configuration is inherent in empirical charts and nomographs
for the headwater discharge relationships developed from research (30, 32, 33, 39).
Various types of culvert entrances are shown in Figures 4-10 through 4-18 and are discussed in the
following sections. References (30) and (39) contain a full discussion of inlet improvements, design
charts, and procedures.
4.5.6.1 Conventional
Commonly used inlets consist of projecting culvert barrels or projecting inlets, cast-in-place concrete
headwalls, precast or prefabricated end sections, and culvert ends mitered to conform to the fill slope
or step-mitered to approximate the fill slope. For a given headwater elevation, the conventional bell or
groove end of a concrete pipe has a greater capacity than a square-edged inlet, whether projecting or
in a headwall, and a square-edged inlet has greater capacity than a thin-edged, mitered or projecting
inlet. Although the entrance loss coefficient cannot be used in computing the headwater elevation for
culverts operating with inlet control, the efficiency of the various inlets for both inlet and outlet
control is, in general, indicated by the key values shown in Table 4-1. Conventional inlets are shown
in Figures 4-10 through 4-14.
Hydraulic Design of Culverts
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Table 4-1. Entrance Loss Coefficients (Outlet Control, Full, or Partly Full)
ª y2 º
H e = ke « »
¬ 2g ¼
Type of Structure and Design of Entrance
Pipe, Concrete
Mitered to conform to fill slope
End section conforming to fill slope
Projecting from fill, sq. cut end
Headwall or headwall and wingwalls:
Square-edge
Rounded (radius = 1/12D)
Socket end of pipe (groove-end)
Projecting from fill, socket end (groove-end)
Beveled edges, 33.7° or 45° bevels
Side- or slope-tapered inlet
Pipe, or Pipe-Arch, Corrugated Metal
Projecting from fill (no headwall)
Mitered to conform to fill slope, paved, or unpaved slope
Headwall or headwall and wingwalls square-edge
End section conforming to fill slope
Beveled edges, 33.7° or 45° bevels
Side- or slope-tapered inlet
Box, Reinforced Concrete
Wingwalls parallel (extension of sides)
Square-edged at crown
Wingwalls at 10° to 25° or 30° to 75° to barrel
Square-edged at crown
Headwall parallel to embankment (no wingwalls)
Square-edged on 3 edges
Rounded on 3 edges to radius of 1/12 barrel
dimension, or beveled edges on 3 sides
Wingwalls at 30° to 75° to barrel
Crown edge rounded to radius of 1/12 barrel
dimension, or beveled top edge
Side- or slope-tapered inlet
Coefficient, ke
0.7
0.5*
0.5
0.5
0.2
0.2
0.2
0.2
0.2
0.9
0.7
0.5
0.5*
0.2
0.2
0.7
0.5
0.5
0.2
0.2
0.2
* “End section conforming to fill slope,” made of either metal or concrete, are the sections commonly
available from manufacturers. From limited hydraulic tests, they are equivalent in operation to a
headwall in both inlet and outlet control. Some end sections, incorporating a closed taper in their
design, have a superior hydraulic performance. These latter sections can be designed using the
information given for the beveled inlet.
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Figure 4-10. Thin-Edge Projecting Inlet
Figure 4-11. Groove End Projecting Inlet
Figure 4-12. Square-Edge Inlet in Headwall with Wingwalls
Highway Drainage Guidelines
Hydraulic Design of Culverts
4-21
Figure 4-13. Mitered Inlet with Slope Paving
Figure 4-14. Step-Mitered Inlet
4.5.6.2 Beveled
Bevels similar to, but larger than, chamfers on the inlet edges of a culvert are the simplest type of
inlet improvement. The bevels may be plane surfaces or rounded and are proportioned according to
the culvert barrel or face dimensions. The top and sides of box culverts and the perimeter of other
shapes should be beveled, except that bevels may be omitted from that portion of the perimeter of
round and arch shapes that is tangential to an inlet apron. The bell or groove end of a concrete pipe is
equal in performance to a beveled entrance and is superior to the performance of a square-edged inlet
in a headwall. The entrance of a thin-walled culvert can be improved by incorporating the thin edge in
a headwall or in a headwall with bevels.
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Highway Drainage Guidelines
Bevels also improve the performance of culverts operating with outlet control, but not as much as
with inlet control. The entrance loss coefficient, ke, is reduced by the use of beveled edges, and they
should be considered because little additional cost is involved.
A beveled inlet is shown in Figure 4-15.
Figure 4-15. Beveled Inlet with Headwall
4.5.6.3 Side-Tapered Inlets
Further increase in culvert capacity by reducing the flow contraction at the entrance is possible by use
of an enlarged face area and a transition from the enlarged face to the culvert barrel. On a box culvert,
this is called a side-tapered inlet because the inlet face is the same height as the culvert barrel and the
transition from face size to barrel size is accomplished by tapering the sidewalls. Side-tapered or
flared inlets for pipe culverts may have a face in the shape of an oval, a circle, or a rectangle. Flared
or warped wingwalls or a simple headwall may be used with this type of inlet.
The intersection of the transition section and the barrel is termed the throat section. For side-tapered
inlets, the hydraulic control may be at the face or at the throat. Because flow contraction at the throat
is less than at the face and the throat is at a lower elevation, it is advantageous to design side-tapered
inlets so that control will be at the throat. This is accomplished by making the face sufficiently large
that control will be at the throat at most flow rates.
The advantages of a side-tapered inlet for culverts flowing in inlet control are increased flow capacity
or lower headwater elevation for a given flow rate and a possible reduction in the size of culvert
barrel. Some increase in forming costs may be experienced for the transition or inlet section, but any
such increased cost has been difficult to detect in those built to date.
Side-tapered inlets are shown in Figures 4-16 and 4-17.
Hydraulic Design of Culverts
Figure 4-16. Side-Tapered Inlet on Box Culvert
Figure 4-17a. Side-Tapered Inlet
for Corrugated Pipe Culvert
Figure 4-17b. Side-Tapered Inlet
for Concrete Pipe Culvert
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Highway Drainage Guidelines
4.5.6.4 Slope-Tapered Inlets
Slope-tapered inlets are similar to side-tapered inlets except that the slope in the transition section is
steeper than the slope of the culvert barrel. With control at the throat, more head is available at the
control section and, at given headwater elevations, culvert capacity is greater than with other inlet
configurations. The total annual cost of various alternative designs should be considered in culvert
selection. If a slope-tapered inlet is hydraulically feasible, the increased costs for structural excavation
should be offset by advantages of increased culvert flow capacity and/or reduced culvert barrel size
and cost. Slope-tapered inlets should not be used in streams that require fish passage.
Slope-tapered inlets can be used on either rectangular or circular culverts, but circular culverts require
a special transition to the barrel section.
Figure 4-18 shows a slope-tapered inlet under construction.
A full discussion of inlet improvements and design aids are contained in References (30) and (39).
Figure 4-18. Slope-Tapered Inlet under Construction
4.5.7 Barrel Characteristics
In inlet control flow, culvert barrel characteristics of roughness, length, and slope do not affect culvert
capacity. It should be under-stood, however, that these characteristics often determine whether or not
the culvert will flow with inlet or outlet control. With a given culvert slope, a rough pipe will flow
with outlet control at a lower discharge than a smooth pipe. Therefore, there may be advantages at
some sites in the use of smooth barrel materials on steep slopes where the safety factor in capacity
can be increased by improving the headwater elevation-discharge relationship for relatively large
flow rates.
Hydraulic Design of Culverts
4-25
Barrel characteristics of roughness, length, slope, shape, and size enter into the determination of
culvert capacity when flow is in outlet control. In outlet control, the head to overcome friction losses
in the barrel is a part of the total headwater depth required to pass the flow through the culvert. It is
common practice for highway engineers to use the Manning equation to calculate these losses.
Manning “n” values can be found by use of Reference (39), Appendix B. Precise “n” values are not
warranted in most culvert design.
For full flow, it has been found that the roughness coefficient of small-diameter corrugated metal pipe
with helical corrugations is less than for pipe with annular corrugations. However, the helix angle
decreases with increasing pipe diameter and the advantage disappears. For this reason, and because
culverts rarely flow full for the entire length, the same Manning “n” values are recommended for
annular and helical corrugated pipe larger than 1500 mm (60 in.).
4.5.8 Outlet Design
It is customary to use similar end treatments at the inlet and outlet of a culvert. Often, such designs
are satisfactory but, in many instances, they should be different because they serve different purposes.
In general, culvert outlet end treatment does not affect culvert capacity. The exception to this would
be an energy dissipation device, which raised the pressure line or effective tailwater at the outlet and
caused the culvert to flow with outlet control rather than inlet control. Outlet structures are used for
three purposes:
to retain the embankment;
to provide structural support for the end of the culvert (see Section 4.6.1); and
to inhibit scour damage to the roadway embankment, downstream channel, and adjacent property.
Scour at culvert outlets is caused by high-velocity flow, flow confined to a lesser width and greater
depth than in the natural channel, and eddies resulting from flow expansion. Scour prediction is
somewhat subjective because the velocity at which erosion will occur is dependent upon the
characteristics of the channel bed and bank material, velocity, and depth of flow in the channel and at
the culvert outlet, velocity distribution, and the amount of sediment and other debris in the flow.
Scour developed at the outlet of similar existing culverts in the vicinity is always a good guide in
estimating potential scour at the outlet of proposed culverts.
Scour does not develop at all suspected locations because the susceptibility of the stream to scour is
difficult to assess and the flow conditions that will cause scour do not occur at all flow rates. At
locations where scour is expected to develop only during relatively rare flood events, the most
economical solution may be to repair damage after it occurs.
At many locations, use of a simple outlet treatment (e.g., headwalls, cutoff walls, aprons of concrete
or riprap) will provide adequate protection against scour. At other locations, use of a rougher culvert
material may be sufficient to prevent damage from scour.
When the outlet velocity will greatly exceed the maximum velocity in the downstream channel,
consideration should be given to energy dissipation devices (e.g., stilling basins, riprap basins). It
should be recognized, however, that such structures are costly, many do not provide protection over a
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Highway Drainage Guidelines
wide range of flow rates, some require a high tailwater to perform their intended function, and the
outlet velocity of most culverts is not high enough to form a hydraulic jump that is efficient in
dissipating energy. Therefore, selection and design of an energy dissipation device to meet needs at a
site requires a thorough study of expected outlet flow conditions and the performance of various
devices. The cost of dissipation devices may dictate the design that provides outlet protection from
low-frequency flood discharges and accepts the damage caused by larger floods.
Design information for some of the more commonly used energy dissipators is contained in
References (8), (13), (17), (20), (28), (29), and (44). The design of energy dissipators should take into
consideration the difficulties they may cause for fish passage and other environmental concerns. For
more details about fish passage, see Section 10.7.4 in the Highway Drainage Guidelines.
4.6 SPECIAL HYDRAULIC CONSIDERATIONS
In addition to the hydraulic considerations discussed in the preceding sections, other factors must be
considered to assure the integrity of culvert installations and the highway.
4.6.1 Anchorage
The forces acting on a culvert inlet during high flows are variable and highly indeterminate. Vortices
and eddy currents cause scour which can undermine the culvert inlet, erode the embankment slope,
and make the inlet vulnerable to failure. Flow is usually constricted at the inlet, and inlet damage (see
Figure 4-19) or lodged drift can accentuate this constriction. The large unequal pressures resulting
from this constriction are, in effect, buoyant forces that can cause entrance failures, particularly on a
corrugated metal pipe with mitered, skewed, or projecting ends (22).
Figure 4-19a. Damage to Culvert Inlets from Hydraulic Forces and Drift
Hydraulic Design of Culverts
4-27
Figure 4-19b. Damage to Culvert Inlets from Hydraulic Forces and Drift
Anchorage at the culvert entrance helps to protect against these failures by increasing the dead load
on the end of the culvert, thus protecting against bending damage, and by protecting the fill slope
from the scouring action of the flow. End anchorage can be in the form of slope paving, concrete
headwalls or grouted stone, but the culvert end must be anchored to the end treatment to be effective.
In some locations, prefabricated metal end sections should also be anchored to increase their
resistance to failure.
Culvert ends need anchorage at many locations. Sectional rigid pipe is susceptible to separation at the
joints when scour undermines the ends. Tiebars are commercially available to prevent separation of
concrete pipe joints. Metal culvert ends projected into ponds, tidal waters, or through levees are
susceptible to failure from buoyant forces if tide gates are used or if the ends are damaged by debris.
Figures 4-20 and 4-21 show culverts that failed from buoyant forces at the inlet end.
Figure 4-20. Culvert and Roadway Fill Failure from
Buoyant Forces—Culvert Carried Downstream
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Highway Drainage Guidelines
Figure 4-21. Bending at Culvert Inlet from Buoyant Forces—Both Ends of
Culvert Are Seen in This View
4.6.2 Piping
Piping is a phenomenon caused by seepage along a culvert barrel, which removes fill material,
forming a hollow similar to a pipe, hence the term “piping” (see Figure 4-22). Fine soil particles are
washed out freely along the hollow, and the erosion inside the fill may ultimately cause failure of the
culvert or the embankment. Piping may also occur through open joints into the culvert barrel.
The possibility of piping can be reduced by decreasing the velocity of the seepage or by decreasing
the quantity of seepage flow. Methods of achieving these objectives are discussed in the following
sections.
Figure 4-22. Void from Piping along Culvert Barrel—Inadequate
Space between Pipes for Good Compaction
Hydraulic Design of Culverts
4-29
4.6.2.1 Joints
To decrease the velocity of the seepage flow, it is necessary to increase the length of the flow path
and thus decrease the hydraulic gradient. The most direct flow path for seepage and thus the highest
hydraulic gradient is through open pipe joints. Therefore, it is important that culvert joints be as
watertight as practical. If piping through joints could become a problem, flexible, long-lasting joints
should be specified as opposed to mortar joints.
4.6.2.2 Anti-Seep Collars
Piping should be anticipated along the entire length of the culvert when ponding above the culvert is
planned. Anti-seep or cutoff collars increase the length of the flow path, decrease the hydraulic
gradient and the velocity of flow, and thus the probability of pipe formation. Anti-seep collars usually
consist of bulkhead type plates or blocks around the entire perimeter of the culvert. They may be of
metal or of reinforced concrete and, if practical, dimensions should be sufficient to key into
impervious material. Reference (16) is recommended for longitudinal spacing and dimension
requirements.
Figure 4-23 shows anti-seep collars installed on a culvert under construction.
Figure 4-23. Anti-Seep Collars
4.6.2.3 Weep Holes
Weep holes are sometimes used to relieve uplift pressure. Filter materials should be used in
conjunction with the weep holes to intercept the flow and to prevent the formation of piping channels.
The filter materials should be designed as underdrain filter so they will not become clogged and so
piping cannot occur through the pervious material and the weep hole. Geotextile filter material (28)
should be placed over the weep hole to keep the pervious material from being carried into the culvert.
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Highway Drainage Guidelines
Weep holes may not be required in culverts, and their use is becoming less prevalent. If drainage of
the fill behind the culvert wall is believed necessary, a separate underdrain system should be installed.
4.6.3 Junctions and Bifurcations
It is sometimes necessary to combine the flow of two culverts into a single barrel. The junction
should be designed so that a minimum amount of turbulence and adverse effect on each branch will
result. This is accomplished by considering the flow momentum in each branch and numerous other
variables such as the timing of peak flows (e.g., low flow in one branch and high flow in the other).
Supercritical flow velocities add to the complexity of the problem. References (11) and (14) and other
technical publications treat the subject of junctions for supercritical flow. In critical locations,
laboratory verification of junction design is advisable.
If a bifurcation in flow is necessary or desirable, it is recommended that the flow division be
accomplished outside the culvert barrel. Problems with clogging by debris and the desired
proportioning of flow between branches can be handled much more easily outside of the culvert.
4.6.4 Training Walls
Where supercritical flow conditions prevail in a curved approach to a culvert, training walls are
needed to align flow with the culvert inlet and to equalize flow rates in the barrels of multiple barrel
culverts. In locations where overtopping of the channel or culvert or inefficient operation could result
in catastrophic failure, laboratory verification of the training wall design is advisable.
Training walls may also be required at culvert outlets to align flow with the downstream channel if
this alignment cannot be accomplished in the culvert barrel.
Design of the training wall shown in Figure 4-24 was verified by laboratory testing, and the wall has
been proven by operation during floods.
Figure 4-24. Training Wall
Hydraulic Design of Culverts
4-31
4.6.5 Sag Culverts
A sag culvert, often called an inverted siphon, is not a siphon because the pressure in the barrel is not
below atmospheric. Sag culverts of pipe or box section are used extensively to carry irrigation water
under highways. They are used infrequently for highway drainage and should be avoided on
intermittent or alluvial streams because of problems with siltation and stagnation.
Hydraulically, a sag culvert operates with outlet control, and losses through the culvert can be
computed by the procedures used for conventional culverts. Bend losses can be added to the usual
losses, but these losses are usually negligible because of low velocities. Bend loss coefficients can be
found in References (5), (16), and (39).
4.6.6 Irregular Alignment
At some locations, it may be desirable to incorporate bends, either in plan or profile, in the culvert
alignment. When irregular alignment is advisable or desirable, bends should be as gradual and as
uniform as is practical to fit site conditions. Changes in alignment may be accomplished either by
curves or angular bends. When large changes are necessary, mild bends, such as 15 degrees at
intervals of 15 m (50 ft), should be used. Passage of debris should be considered in selecting the
angle, interval, and number of bends used to accomplish the change in alignment.
If the culvert operates with inlet control, bend losses do not enter into the headwater computation. If it
operates with outlet control, typically, bend losses will be small. In critical locations, they should be
calculated and added to the usual losses. Bend loss coefficients can be found in References (5), (16),
and (39).
4.6.7 Cavitation
The phenomenon known as cavitation occurs as a result of local velocity changes at surface
irregularities that reduce the pressure to the vapor limit of the liquid. Tiny vapor bubbles form at the
point of lowest pressure and are carried downstream into a zone of higher pressure where they
collapse. As the countless bubbles collapse, extremely great local pressure is transmitted radially
outward at the speed of sound, followed by a negative pressure wave that may lead to a repetition of
the cycle. Boundary materials in the vicinity are subjected to rapidly repeated stress reversals and
may fail through fatigue (43). Surface pitting is the first sign of such a failure.
Cavitation is seldom a problem in highway culverts because of relatively low velocities and because
flow rates are not sustained for a long period. Abrasion damage is sometimes mistaken for cavitation
damage.
4.6.8 Tidal Effects and Flood Protection
Where areas draining through culverts are adversely affected by tide or flood stages, flap gates may
be desirable to prevent backflow. Sand, silt, debris, or ice will cause these gates to require
considerable maintenance to keep them operative. Head losses due to the operation of flap gates may
be computed using loss coefficients furnished by the manufacturer.
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Highway Drainage Guidelines
4.7 MULTIPLE-USE CULVERTS
Culverts often serve purposes in addition to drainage. There are cost advantages of multiple-use, but
one purpose or the other is often inadequately served. The cost advantages of multiple-use should be
weighed against the possible advantages of separate facilities for each use.
4.7.1 Utilities
It is sometimes convenient to locate utilities in culverts, particularly if jacking, boring, or an open cut
through an existing highway can be avoided by such a location. The space occupied in the culvert is
usually relatively small, and the obvious effects on culvert hydraulic performance can be
insignificant. Consideration of this multiple-use, however, should include recognition of the flood
flow and debris hazard to the utility and the probability of reduced culvert capacity from debris
caught on the utility line. Also, increased stream scour often occurs at pipelines at the upstream and
downstream ends of culverts. This multiple use is not generally recommended if separate facilities are
practicable.
4.7.2 Stock and Wildlife Passage
Culverts can serve both for drainage and for stock and wildlife passes. Culvert size may be
determined either by hydraulic requirements or by criteria established for the accommodation of the
stock or game that will use the structure. Criteria for the accommodation of stock and wildlife are not
included in these guidelines. Scour protection at the outlet may be necessary to ensure acceptable
access conditions for livestock. As with other multiple-use culverts, satisfactory performance for both
intended uses should be assured or separate facilities provided.
4.7.3 Land Access
Culverts often serve both as a means of land access and drainage, particularly on highways with
controlled access. This use is common in areas where land use on both sides of the highway is under
common control. The culvert size will generally be determined by the physical dimensions of the
equipment or vehicles that will make use of the facility. Scour protection not considered necessary for
hydraulic reasons may be required at the outlet to facilitate access to the culvert. Where a low-flow
culvert is placed at a lower elevation than the multiple-use culvert, precautions against headcutting
from the stream to the outlet of the multiple-use culvert may be necessary. Good drainage at the
culvert ends is necessary to the successful use of culverts for land access.
4.7.4 Fish Passage
In some locations, the need to accommodate migrating fish is an important consideration in the design
of a stream crossing. New roadway locations should be coordinated with State fish and wildlife
agencies at an early date so stream crossings that require fish passage can be identified. These
agencies normally request provision for fish passage for all streams with fish migrations and streams
that have suitable habitat to support fish runs. Questions regarding fish passage criteria should be
reviewed in the field during project development and discussed with the agency making the request.
At some locations, the agency may request that the culvert design include a fish barrier to prevent
migration of rough fish into an upstream lake.
Hydraulic Design of Culverts
4-33
When fish passage is requested, the priority order of alternatives is:
highway relocation to avoid the crossing,
construction of a bridge, and
construction of a suitable culvert.
Many fish and wildlife agencies have established design criteria for fish passage through culverts.
These include maximum allowable velocity, minimum water depth, maximum culvert length and
gradient, type of structure, and construction scheduling.
Several types of culvert installations have been used satisfactorily for fish passage (35, 37). These
include:
Open-Bottom Culverts. Culverts supported on spread footings to permit retention of the natural
stream bed. The culvert size must be adequate to maintain natural stream velocities at moderate
flows, and the foundation must be in rock or scour-resistant material (see Figure 4-25).
Oversized or Depressed Culverts. Oversized culverts with the bottom of the culvert placed
below the stream bed so that gravel will deposit and develop a nearly natural streambed within
the culvert (see Figure 4-26). Sometimes, baffles are necessary to hold gravel and rock in place.
Culverts with Baffles. Many baffle configurations have proved to be satisfactory. A number of
baffle configurations are shown in Chapter 10, Figures 10-13, 10-14, and 10-15.
Weirs. Use of a weir in the channel downstream of the culvert (see Figure 4-27), constructed so
as to maintain the desired depth through the culvert, is probably the most practical way to meet a
minimum water depth requirement for a given species of fish. The weir must be of substantial
design to withstand flood flows, and provisions must be made for fish to bypass the weir. The
bypass provided is dependent on the species of fish. References (19) and (46) will aid in the
design of weirs and bypasses for fish passage.
Special Treatment. In wide, shallow streams, one barrel of a multiple-barrel culvert can be
depressed to carry low flow or weirs can be installed at the upstream end of some barrels to
provide for fish passage through other barrels at low flow.
Timing. When fish passage is required, consideration must be given to the time of the year that
the culvert will be installed. Fisheries agencies will usually provide dates when spawning will
occur to limit stream disturbance during this period.
Figure 4-25. Culvert on Footings to Retain Streambed for Fish Passage
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Highway Drainage Guidelines
Figure 4-26. Culvert Invert Placed below Streambed—Baffles Used to
Hold Gravel in Place and Provide Natural Streambed for Fish Passage
Figure 4-27. Weirs Downstream of Culvert to Facilitate Fish Passage
The addition of baffles in culverts to aid fish passage may cause the culvert to flow with outlet control
at relatively low flow rates. Neglecting the culvert area occupied by the baffles does not adequately
account for energy losses from turbulence generated by the baffles. Reference (40) is recommended
for the determination of hydraulic performance of culverts with baffles.
Hydraulic Design of Culverts
4-35
4.8 IRRIGATION
Conventional culverts and sag culverts are often used to convey irrigation water under a highway.
Freeboard in irrigation canals is usually small, and the hydraulic design of the culvert should be such
that service to irrigable lands will not be impaired by loss of head in the culvert.
Culvert construction in irrigation canals should be scheduled to avoid conflict with the irrigation
season and supervised carefully to minimize the possibility of sediment disrupting the water supply.
4.9 DEBRIS CONTROL
Accumulation of debris at a culvert inlet can result in the culvert not performing as designed. The
consequences may be damages from inundation of the road and upstream property.
The designer has three options for coping with the debris problem: retain the debris upstream of the
culvert, attempt to pass debris through the culvert or use a bridge (42, 45).
If the debris is to be retained by an upstream structure or at the culvert inlet, frequent maintenance
may be required. If debris is to be passed through the structure or retained at the inlet, a relief opening
should be considered, either in the form of a vertical riser or a relief culvert placed higher in the
embankment (see Figure 4-28).
It is often more economical to construct debris control structures after problems develop because
debris problems do not occur at all suspected locations.
Figure 4-28. Vertical Riser for Relief
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Highway Drainage Guidelines
4.9.1 Debris Control Structure Design
The design of a debris control structure must be preceded by a thorough study of the debris problem.
Among the factors to be considered are:
type of debris;
quantity of debris;
expected changes in type and quantity of debris due to future land use;
streamflow velocity in vicinity of culvert entrance;
maintenance access requirements;
availability of storage area;
standard of planned maintenance for debris removal; and
assessment of damage due to debris clogging, if protection is not provided.
Reference (42) will aid in the design of a debris control structure.
4.9.2 Maintenance
Provisions for maintenance access are necessary for debris control structures. For high embankments,
this may be difficult. If access to the debris control structure is not practical, a parking area for
mechanical equipment such as a crane may be necessary to remove debris without disrupting traffic.
Many debris barriers require cleaning after every storm. The standard or frequency of maintenance
should be considered in selecting the debris control structure. If a low standard of maintenance is
anticipated, the designer should choose to pass the debris through the structure.
4.10 SERVICE LIFE
Commonly used culvert materials are durable at most locations, but some soil and water
environments are hostile and service life must be a consideration in material selection and culvert
design. Conditions that affect the service life of culvert materials are corrosion, abrasion, and freezing
and thawing action. Measures to increase service life are sometimes costly, and the total annual cost
should be considered when designs are prepared. Periodic culvert replacement may be the most
feasible alternative. Driveway culverts, for instance, are generally easy to replace and traffic service
would not be a problem when replacement becomes necessary. Culverts under high-traffic volume
highways or high fills, on the other hand, are more difficult and costly to repair or replace, and more
precaution against failure from a hostile environment is warranted.
Many of the conditions that affect service life can be evaluated and service life estimated prior to the
selection of culvert material. The type and degree of protection needed can then be determined
(References (10), (12), (14), (26), (31), (36), (38), and (41)). One of the most reliable methods
available to the designer is to examine existing culverts in the same stream channel or in similar
streams in the same area. For a more detailed discussion on service life and durability, see the
Highway Drainage Guidelines, Chapter 14 (5).
Hydraulic Design of Culverts
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4.10.1 Abrasion
Abrasion loss is the erosion of culvert materials by the bed load carried by streams (see Figure 4-29).
The principal factors to be considered are the frequency and duration of runoff events, which
transport significant amounts of abrasive materials, the character and volume of the bed load and the
resistance of the culvert material to abrasion.
Figure 4-29. Loss of Culvert Material from Abrasion
In some locations, culverts can be protected from abrasion by use of debris control structures to
remove the abrasive sediment load from the flow (see Section 4.9.1).
Provision for abrasive wear can be made by the use of sacrificial thickness of structural material in
the invert. In metal culverts, the sacrificial material may be either additional metal thickness or
portland cement concrete invert paving. Provision for abrasion in concrete culverts generally consists
of requiring additional cover over reinforcing steel and more durable concrete mixes.
Invert treatment of planking, with metal plate or railroad rails, channels or other steel shapes placed
longitudinally in the bottom of the culvert, can be used where severe abrasion is anticipated or
experienced (see Figure 4-30).
Figure 4-30. Downstream End of Culvert Treatment
for Protection against Abrasion
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Highway Drainage Guidelines
4.10.2 Corrosion
Environmental conditions that are generally considered to contribute to the corrosion of metal culvert
pipe are acidic and alkaline conditions in the soil and water and the electrical conductivity of the soil.
Another contributing factor in corrosion is the frequency and duration of flows transporting bed
loads, which abrade or otherwise damage protective coatings.
Saltwater causes corrosion of steel and, depending on the salt concentration, will corrode aluminum.
Experience with aluminum in saltwater environments to date indicates that aluminum culverts are
fairly resistant to corrosion at such locations. Coated aluminum may be considered in alkaline
environments or where other metals (e.g., iron, copper) or their salts are present. Experience has not
been good with metals in organic muck in estuarine environments. Concrete deteriorates slowly in
contact with chlorides, sulphates, and certain magnesium salts. Alternate wetting and drying with
seawater is also detrimental to concrete. In general, most culvert materials exposed to seawater
require some type of protection to assure adequate service life.
Coal mines and certain other mining operations can produce free acid or acid-forming elements that
are corrosive to many types of culvert materials. Vitrified clay and bituminous and fiber-bonded
coatings have been successfully used in severe acid environments as culvert lining materials. Plastic
pipe has been used in this type of environment and appears to successfully resist deterioration. Care
must be taken to prevent plastic pipe from direct exposure to the sunlight (ultraviolet rays) and from
fire hazard.
Alkaline water and soils containing sulphates and carbonates cause rapid deterioration of concrete
culverts. This deterioration can be retarded by the use of Type V and other limited calcium aluminates
cement or higher cement content concretes.
Protection of metal culverts from corrosion usually consists of bituminous fiber-bonded coating or
mill-applied thermoplastic coating. Conclusions regarding the use of protective coatings are not
consistent. Some States have found significant increases in service life while others have concluded
that such coatings are not cost effective. Fiber-bonded metal appears to give better resistance to
deterioration. Bituminous coatings are not successful in highly hostile environments because of
insufficient bond to the metal and damage to the coatings in handling and placing. Bituminous
coatings are vulnerable to petroleum wastes and spills and to destruction by fire. All coatings are
vulnerable to abrasion.
Mill-applied thermoplastic coatings on corrugated metal culverts are of more uniform thickness, less
subject to damage in handling and installation, and have fewer manufacturing flaws than bituminous
coats. They are superior to bituminous coatings in abrasion resistance and, although experience is
relatively short, it appears that culverts with these coatings will survive for a reasonable period in
corrosive environments.
A National Cooperative Highway Research Program (NCHRP) Synthesis Report 50 (1978) and
Reference (26) provide guidelines for the selection of durable materials and protective measures for
various corrosive environments.
Hydraulic Design of Culverts
4-39
4.11 SAFETY
The primary responsibility for traffic safety in the hydraulic design of culverts is met by providing
structures adequate to avoid hazardous flooding and failure of highways. It is also important that
culverts be located so that the structure will present a minimum hazard to traffic.
Culvert ends should be located outside the safe recovery area, where possible, and continued across
medians, except where safe recovery areas can be provided otherwise. Some culvert ends can be
made traffic safe by the use of traversable grates, but only if the grates will not become a hazard by
causing the highway to flood. Grate hydraulic capacity and the potential for clogging by debris must
be considered before selecting this method for making culvert ends traffic safe (3, 23, 34). See Design
of Small Canal Structures (16) for a hazard classification system (Classes A to F), which is based on
adjacent land use, and for the design of appropriate safety devices.
At locations where culvert ends cannot be located outside the safe recovery area and where grates
would be impractical or unsafe, guardrail protection should be provided.
Culverts can also be an attractive nuisance and a hazard to children. At locations where long culverts
could be a hazard, fencing or grates could be provided to prevent entry.
4.12 DESIGN DOCUMENTATION
Design data should be assembled in an orderly fashion and retained for future reference. The amount
and detail of documentation for each culvert site should be commensurate with the risk and the
importance of the structure. Post-construction review of data and documentation may be necessary for
the following reasons:
the performance of structures over a period of time is very helpful in evaluating design policies
and procedures and the validity of design assumptions;
in the event of failure, contributing factors can be identified and considered in the design of
replacement structures;
source of information when structure is replaced, extended or improved;
source of information for the design of other structures in the vicinity; and
source of information in the event of litigation.
4.12.1 Compilation of Data
Data can be compiled in a variety of ways and should include these items as appropriate:
copies of all pertinent correspondence,
topography of site,
drainage area map,
stream profile and cross sections,
historical highwater documentation,
information on existing structures in the vicinity,
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Highway Drainage Guidelines
hydrologic design computations,
hydraulic design calculations and culvert performance curves,
foundation investigations,
economic analysis of structure selection,
as-built plan, and
material service life analysis.
4.12.2 Retention of Records
Provisions should be made to retain records of culvert designs until the highway is reconstructed or
the culverts replaced. Records may be retained in design files or on microfilm and should be readily
available when needed for reference or review.
4.13 HYDRAULIC-RELATED CONSTRUCTION CONSIDERATIONS
Assembly or construction, bedding, and backfill are as important to satisfactory culvert service as the
hydraulic and structural design. In addition, there are hydraulic-related factors that should be
considered by construction engineers.
4.13.1 Verification of Plans
Plans should be checked to verify that site conditions have not changed from the time of location
surveys to construction. Changes in culvert design required because of differences between location
and construction surveys should be made in consultation with the design engineers. Some changes
could significantly affect either the hydrology at the site or the hydraulic performance of the culvert
designed for the site.
Changes in land use in the watershed (e.g., clear cutting of forests, urbanization) can change the
hydrology at the site and debris considerations used in the design. Development near the site could
change damage risk considerations for the design.
Changes in stream alignment and profile can result in different flow conditions than those for which
the design was prepared. Changes in headwater elevation-capacity relations and outlet velocity may
require consideration of changes in culvert type, size, or shape, and of the need for protection against
scour at the outlet.
4.13.2 Temporary Erosion Control
During construction, care should be taken to minimize the erosion at culvert inlets and outlets and
siltation within the culvert. The ideal condition is to design a temporary channel outside the area
needed for the culvert installation that will allow for a dry installation. Temporary erosion methods
are discussed in Chapter 3, “Erosion and Sediment Control in Highway Construction,” of the
Highway Drainage Guidelines and Reference (21).
Hydraulic Design of Culverts
4-41
4.13.3 Construction and Documentation
Records should be kept of the construction of each culvert installation. The final location and slope of
the culvert should be recorded on the “as-built” plans. This information is useful for evaluating
overall performance of the installation.
Construction personnel are encouraged to inform the designer of any difficulties that are encountered
and to make suggestions to improve future designs.
4.14 HYDRAULIC-RELATED MAINTENANCE CONSIDERATIONS
Culvert designs should be prepared recognizing that all structures require periodic maintenance
inspection and repair. Where possible, some means should be provided for personnel and equipment
access to the structures to facilitate this activity. Culverts must be kept in good repair and reasonably
clean at all times if they are to function as intended (1). However, many culverts are installed so as to
accumulate sediment along the invert to provide an environmentally acceptable streambed through
the structure. The maintenance personnel should be aware of this fact so that they do not
inadvertently remove the desired sediment.
Maintenance personnel should advise design engineers of culvert locations that require considerable
annual maintenance. It may be that the maintenance is not necessary to the integrity of the structure or
a problem may exist that should be corrected by a design modification.
4.14.1 Maintenance Inspections
Culvert failures can be both disastrous and expensive. A comprehensive program for maintaining
culverts in good repair and operating condition will reduce the probability of failures and prove to be
cost effective. The program should include periodic inspections with supplemental inspections
following flood events. Conditions that appear to require remedial construction should be referred to
the hydraulics engineer for the design of corrective measures. For guidelines on culvert inspection,
see Reference (25).
4.14.2 Flood Records
An inspection of culverts should be made during and after major floods to observe the culvert
operation and record highwater marks. Conditions that require corrective maintenance should be
noted including debris accumulations, silting, erosion, piping, scour, and structural damage.
Performance information that reflects a need for design or construction changes or unusually large
flood peaks should be submitted to the hydraulic design section for review.
4.14.3 Reconstruction and Repair
Maintenance inspections will often reveal the need for major repairs, culvert appurtenant structures
(e.g., energy dissipators), extensive scour protection and sometimes reconstruction. The repair of
various types of culvert distress and failures is discussed in References (1) and (9) and the Highway
Drainage Guidelines, Chapter 8.
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Extensive and costly repair, construction and reconstruction should be coordinated with the hydraulic
design section. This is advisable particularly when conditions have changed from those that prevailed
at the time the existing culvert was designed. Urbanization or other changes in the watershed,
channelization of the stream, flood control storage, or any of numerous other changes that affect
hydrology may require reconsideration of the culvert type and size, allowable headwater elevations
and acceptable risk at the culvert site. Physical changes at the site and in the stream (e.g., aggradation,
degradation) may make it advisable to reconstruct rather than undertake major repairs or
modifications.
Most culvert replacements by maintenance forces should be coordinated with design for possible
revisions in structure geometry and size. Culvert failures may occur because of unusual floods,
inadequate size or for reasons not related to hydraulic adequacy (e.g., piping, scour, corrosion,
abrasion, inadequate foundation, buoyancy). For this reason, overflow over the roadway or culvert
failure may require replacement with a larger culvert, a change in inlet geometry of the existing
culvert, replacement with an equivalent culvert, and precautions against failure from other causes or
an identical replacement culvert may be indicated.
4.15 REFERENCES
Note: Some FHWA references are available online at www.fhwa.dot.gov/bridge/hydpub.htm.
(1)
AASHTO. Maintenance Manual. American Association of State Highways and Transportation
Officials, Washington, DC, 1999.
(2)
AASHTO. Roadside Design Guide. American Association of State Highway and Transportation
Officials, Washington, DC, 2002.
(3)
AASHTO. AASHTO LRFD Bridge Design Specifications, SI Units or Customary Units, 3rd Ed.
American Association of State Highways and Transportation Officials, Washington, DC, 2004.
(4)
AASHTO. Model Drainage Manual. American Association of State Highways and
Transportation Officials, Washington, DC, 2005.
(5)
AASHTO. “Culvert Inspection and Rehabilitation.” Chapter 14 in Highway Drainage
Guidelines. American Association of State Highways and Transportation Officials, Washington,
DC, 2007.
(6)
ACPA. Concrete Pipe Handbook. American Concrete Pipe Association, Irving, TX, 1980.
(7)
AISI. Handbook of Steel Drainage and Highway Construction Products, 3rd ed. American Iron
and Steel Institute, Washington, DC, 1983.
(8)
ASCE. Symposium of Stilling Basins and Energy Dissipators. Journal of the Hydraulics
Division, Proceedings Symposium Series No. 5, 8 papers with discussions. American Society of
Civil Engineers, Reston, Virginia, 1961.
Hydraulic Design of Culverts
(9)
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Ballinger, C. A. and P. G. Drake. Culvert Repair Practices Manual. FHWA-RD-94-096
(Volume 1) and FHWA-RD-95-089 (Volume 2). Federal Highway Administration, U.S.
Department of Transportation, Washington, DC, 1995.
(10) Beaton, J. L. and R. F. Stratfull. Field Test for Estimating Service Life of Corrugated Metal
Pipe. Highway Research Board Proceedings, Volume 41. Highway Research Board,
Washington, DC, 1962, pp. 255–272.
(11) Behlke, C. E. and H. D. Pritchett. The Design of Supercritical Flow Channel Junctions.
Highway Research Record Number 123. Highway Research Board, Washington, DC, 1966,
pp. 17–35.
(12) Berg, V. E. Culvert Performance Evaluation. Washington State Highway Commission,
Department of Highways, 1965.
(13) Bohan, J. P. Erosion and Riprap Requirements at Culvert and Storm-Drain Outlets.
Miscellaneous Paper H-70-2. U.S. Army Waterways Experiment Station, Vicksburg, MS, 1970.
(14) Braley, S. A. Acid Drainage from Coal Mines. Trans. AIME (Mining Branch), Volume 190.
American Institute of Mining, Metallurgical, and Petroleum Engineers, Littleton, CO, 1951,
pp. 703–707.
(15) Bureau of Reclamation, U.S. Department of Interior. “Safety.” Chapter 9 in Design of Small
Canal Structures. U.S. Government Printing Office, Washington, DC, 1978.
(16) Bureau of Reclamation, U.S. Department of Interior. Design of Small Dams. U.S. Government
Printing Office, Washington, DC, 1987.
(17) Chang, F.M. and M. Karim. Erosion Protection for the Outlet of Small and Medium Culverts.
South Dakota State University, Brookings, South Dakota and South Dakota Department of
Highways, Pierre, SD, 1970.
(18) Chow, V. T. Open Channel Hydraulics. McGraw-Hill, New York, NY, 1970, pp. 512–516.
(19) Clay, C. H. Design of Fishways and Other Fish Facilities. Queen’s Printer, Ottawa, Canada,
1961.
(20) Corry, M. L., P. L. Thompson, F. J. Watts, J. S. Jones, and D. L. Richards. The Hydraulic
Design of Energy Dissipators for Culverts and Channels. FHWA-EDP-86-110, Hydraulic
Engineering Circular No. 14. Federal Highway Administration, U. S. Department of
Transportation, Washington, DC, 1983.
(21) Dunkley, C. L. Suggestions for Temporary Erosion and Siltation Control Measures. Federal
Highway Administration, U.S. Department of Transportation, Washington, DC, 1973.
(22) Edgerton, R. C. Culvert Inlet Failures—A Case History. In Highway Research Board Bulletin
286. Highway Research Board, Washington, DC, 1961, pp. 13–21.
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Highway Drainage Guidelines
(23) FHWA. Handbook of Highway Safety Design and Operating Practices. Federal Highway
Administration, U.S. Department of Transportation, Washington, DC, 1973.
(24) FHWA. Structural Design Manual for Improved Inlets and Culverts. FHWA-IP-83-6. Federal
Highway Administration, U.S. Department of Transportation, Washington, DC, June 1983.
(25) FHWA. Culvert Inspection Manual. Supplement to Bridge Inspector’s Training Manual,
FHWA-IP-86-2. Federal Highway Administration, U.S. Department of Transportation,
Washington, DC, July 1986.
(26) FHWA. Durability of Special Coatings for Corrugated Steel Pipe. FHWA-FLP-91-006. Federal
Highway Administration, U.S. Department of Transportation, Washington, DC, 1991.
(27) FHWA. Hydraulic Computer Program HY8—Hydraulic Analysis of Highway Culverts. Federal
Highway Administration, U.S. Department of Transportation, Washington, DC, 1997
(http://www.fhwa.dot.gov/engineering/hydraulics/software.cfm).
(28) FHWA. Geosynthetic Design and Construction Guidelines. FHWA-HI-95-038. Federal
Highway Administration, U.S. Department of Transportation, Washington, DC, 1995, revised
1998.
(29) Fletcher, B. P. and J. L. Grace, Jr. Practical Guidance for Estimating and Controlling Erosion
at Culvert Outlets. Miscellaneous Paper H-12-5. U.S. Army Waterways Experiment Station,
Vicksburg, MS, 1972.
(30) Harrison, L. J., J. L. Morris, J. M. Normann [sp?], and F. L. Johnson. Hydraulic Design of
Improved Inlets for Culverts. Hydraulic Engineering Circular No. 13. Federal Highway
Administration, U.S. Department of Transportation, Washington, DC, 1972.
(31) Haviland, J. E., P. J. Bellair, and V. D. Morrell. Highway Research Report Number 242:
Durability of Corrugated Metal Culverts. Highway Research Board, Washington, DC, 1968,
pp. 41–66.
(32) Herr, L. A. Hydraulic Charts for the Selection of Highway Culverts. Hydraulic Engineering
Circular No. 5. Federal Highway Administration, U.S. Department of Transportation,
Washington, DC, 1965.
(33) Herr, L. A. and H. G. Bossy. Capacity Charts for the Hydraulic Design of Highway Culverts.
Hydraulic Engineering Circular No. 10. Federal Highway Administration, U.S. Department of
Transportation, Washington, DC, 1965.
(34) Highway Research Board. Traffic-Safe and Hydraulically Efficient Drainage Structures.
NCHRP Synthesis Report No. 9. Highway Research Board, Washington, DC, 1969.
(35) Kay, A. R. and R. B. Lewis. Passage of Anadromous Fish Thru Highway Drainage Structures.
Research Report 629110. State of California, Department of Public Works, Division of
Highways, 1970.
Hydraulic Design of Culverts
4-45
(36) Lowe, T. A. and A. H. Koeph. Corrosion Performance of Aluminum Culvert. In Highway
Research Record No. 56. Highway Research Board, Washington, DC, 1964. pp. 98–115.
(37) McClellan, T. J. Fish Passage Through Highway Culverts. Federal Highway Administration,
Region 10, U.S. Department of Transportation, Portland, OR, 1970.
(38) Nordin, E. F. and R. F. Stratfull. A Preliminary Study of Aluminum as a Culvert Material. In
Highway Research Record No. 95. Highway Research Board, Washington, DC, 1965, pp. 1–70.
(39) Normann, J. M. and Associates. Hydraulic Design of Highway Culverts. HDS No. 5. FHWA-
IP-85-15 and dual unit FHWA-NHI-01-020. Federal Highway Administration, U.S. Department
of Transportation, Washington, DC, 1985 and 2001, respectively.
(40) Normann, J. M. Hydraulics Aspects of Fish-Ladder Baffles in Box Culverts. Federal Highway
Administration, U.S. Department of Transportation, Washington, DC, 1974.
(41) Peterson, D. E. Evaluation of Aluminum Alloy for Use in Utah’s Highways. Utah State
Department of Highways, 1973.
(42) Reihsen, G. and L. J. Harrison. Debris Control Structures. Hydraulic Engineering Circular
No. 9. Federal Highway Administration, U.S. Department of Transportation, Washington, DC,
1971.
(43) Rouse, H. Engineering Hydraulics. John Wiley & Sons, Inc., New York, NY, 1949.
(44) Simons, D. B., M. A. Stevens, and F. J. Watts. Flood Protection at Culvert Outlets. Report
No. CER-69-70DBS-MAS-FJW4. Colorado State University, Ft. Collins, Colorado and
Wyoming State Highway Department, Cheyenne, WY, 1970.
(45) State of California. California Culvert Practice, 2nd ed. State of California, Department of
Public Works, Division of Highways, 2000.
(46) Watts, F. J. Design of Culvert Fishways. Water Resources Research Institute, University of
Idaho, Moscow, ID, 1974.
CHAPTER 5
THE LEGAL ASPECTS OF HIGHWAY DRAINAGE
Chapter 5
The Legal Aspects of
Highway Drainage
5.1 INTRODUCTION
Attorneys who have worked with highway engineers on drainage problems understand engineers’
frustrations that specific legal rules are not available for use as guides in their work. The work of
engineers generally involves the application of principles founded in mathematics and the physical
laws of nature. In contrast to this, drainage law seeks to strike a balance between often conflicting
interests of adjoining property owners. Generally, the law recognizes that owners may make certain
reasonable uses of their land without liability, even though there may be some effect on the
neighboring land. Certain other uses, however, may be held to be an unreasonable interference,
entitling the injured party to damages and an abatement of the interference. Drainage problems are
increasing with increasing land development, including highway construction and promise to become
even more numerous and vexatious as property owners are becoming increasingly aware that legal
recourse is available.
The objective of this chapter is to emphasize the importance of the legal aspects of highway drainage.
Although drainage laws vary from state to state and a proper conclusion regarding liability in one
state may not be true in another, the following generalizations can be made:
A goal in highway drainage design should be to perpetuate natural drainage, insofar as
practicable.
The courts look with disfavor upon infliction of damage that could reasonably have been avoided,
even where some alteration in flow is legally permissible.
The basic laws relating to the liability of governmental entities are undergoing change, with a
trend toward increased governmental liability.
Drainage laws are also undergoing change, with the result that older and more specific standards
are being replaced by more flexible standards that tend to depend on the circumstances of the
particular case.
Heretofore, an understanding of applicable drainage law has not been adequately stressed as a
qualification for engineers who are responsible for drainage facility planning, design, construction,
operation, and maintenance. This chapter was written by engineers for engineers to provide
information and guidance on the hydraulics engineer’s role in the legal aspects of highway drainage.
Although written from the viewpoint of design engineers, the chapter should be equally useful to
maintenance engineers who must take action to alleviate existing problems. It should not in any way
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be treated as a manual upon which to base legal advice or make legal decisions. The types of drainage
laws and rules applicable to highway facilities and the types of drainage claims commonly associated
with highways are discussed, and the involvement of the hydraulics engineer in the legal area is
described in general terms. It is not a summary of all existing drainage laws, and case citation is not
used. Most emphatically, this chapter is not intended as a substitute for legal counsel.
One of the principal objectives of this chapter is to generate sufficient interest in drainage law,
terminology, rules, and applications that engineers will be motivated to study available literature and
become better qualified to deal with this aspect of highway drainage.
It should be stressed that, unless circumstances dictate otherwise, an engineer should never attempt to
address a question of law without the aid of legal counsel. The water law of the United States is in
such a confused posture that it is extremely difficult for attorneys well-versed in law to arrive at a
solution to some of the problems. In most areas of water law, the law is neither black nor white but is,
in fact, gray, and legal counsel is necessary to determine in what shade of gray the given
circumstances fall. Another objective of this chapter is to impress on engineers the importance of
gaining sufficient interest in the legal aspects of highway drainage and sufficient knowledge of the
subject that they will recognize situations that warrant advice from legal counsel.
In dealing with water law, engineers should recognize that the State is generally held to a higher
standard than a private citizen. This is true even though the State should enjoy the same rights and
liabilities, and there is no law that says that the State should be treated differently.
There are numerous publications on the legal aspects of drainage and water laws, including some
dealing with drainage laws and the highway agency in a particular State. These publications are
especially useful in the States for which they were written; however, such information can be useful
and applicable in other States as well. Several references are listed in Section 5.9.
5.2 LAWS IN GENERAL
The descending order of law supremacy is Federal, State, and local and, except as provided for in the
statutes of the higher level of government, the superior level is not bound by laws, rules or regulations
of a lower level. Many laws of one level of government are passed for the purpose of enabling that
level to comply with or implement provisions of laws of the next higher level. In some instances,
however, a lower level of government may promulgate a law, rule or regulation that would require an
unreasonable or even illegal action by a higher level. An example is a local ordinance that would
require an expenditure of State funds for a purpose not intended in the appropriation. State permit
requirements are an example of law supremacy. Federal agencies do not secure permits issued by
State agencies, except as required by Federal law.
Many of the questions relative to conflicts in laws of different levels of government involve
constitutional interpretation and must be determined case by case. Such conflicts should be referred to
the highway agency’s legal counsel before any action is taken.
The Legal Aspects of Highway Drainage
5-3
5.3 FEDERAL LAWS
Federal law consists of the Constitution of the United States, Acts of Congress, regulations that
government agencies issue to implement these acts, Executive Orders issued by the President and
case law. Acts of Congress are published immediately upon issuance in slip law form and are
cumulated for each session of Congress and published in the United States Statutes at Large.
The Federal Register, which is published daily, provides a uniform system for making regulations
and legal notices available to the public. The following items are published in the Federal Register:
Presidential Proclamations and Executive Orders,
Federal agency regulations and documents having general applicability and legal effect,
documents required to be published by Act of Congress, and
other Federal agency documents of public interest.
Executive Orders have a wide scope, ranging from personnel appointments to prescribing rules and
regulations under the Trading-with-the-Enemy Act. Most relate to the conduct of government
business or to organization of the executive departments, but many have wider significance. An
Executive Order has never been defined by law or regulation. In a general sense, every act of the
President authorizing or directing that an act be performed is an executive order, but there are
legitimate differences of opinion regarding the papers that should be included in such a classification.
Beginning in June 1938, Executive Orders have been published by the Office of the Federal Register
in the supplements to Title 3 of CFR. Executive Order No. 10006 of October 9, 1948, required
current publication of all Presidential Proclamations and Executive Orders in the Federal Register.
All regulations in force are published in codified form in the CFR at least annually. The CFR is not as
inclusive as the Federal Register for it contains only regulations of general application presently in
force. Unlike the Federal Register, it does not include temporary rules, statements or policy or
interpretive rules.
Federal law does not deal with drainage per se, but many laws have implications which affect
drainage design. These include laws concerning flood insurance and construction in flood hazard
areas, navigation and construction in navigable waters, water pollution control, environmental
protection, protection of fish and wildlife and coastal zone management. Federal agencies formulate
and promulgate rules and regulations to implement these laws, and highway hydraulics engineers
should attempt to keep informed regarding proposed and final regulations.
Some of the more significant Federal laws affecting highway drainage are listed below with a brief
description of their subject area.
Department of Transportation Act (80 Stat. 941, 49 U.S.C. 1651 et seq.). This Act established
the Department of Transportation and sets forth its powers, duties and responsibilities to
establish, coordinate, and maintain an effective administration of the transportation programs of
the Federal Government.
Federal-Aid Highway Acts (23 U.S.C. 101 et seq.). The Federal-Aid Highway Act provides for
the administration of the Federal-Aid Highway Program. Proposed Federal-aid projects must be
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Highway Drainage Guidelines
adequate to meet the existing and probable future traffic needs and conditions in a manner
conducive to safety, durability, and economy of maintenance and must be designed and
constructed according to standards best suited to accomplish these objectives and to conform to
the needs of each locality.
The Federal-Aid Highway Act of 1970 (84 Stat. 1713, 23 U.S.C. 109(h)) provides for the
establishment of general guidelines to assure that possible adverse economic, social, and
environmental effects relating to any proposed Federal-aid project have been fully considered in
developing the project. In compliance with the Act, the Federal Highway Administration issued
process guidelines for the development of environmental action plans.
The Federal-Aid Highway Act of 1966 (80 Stat. 766), amended by the Act of 1970 (84 Stat. 1713),
required the issuance of guidelines for minimizing possible soil erosion from highway construction.
In compliance with these requirements, the Federal Highway Administration issued guidelines that
are applicable to all Federal-aid highway projects. These guidelines are included in 23 CFR 650,
Subpart B.
The following Sections contain brief discussions of other Federal laws and regulations, current on the
date of publication of these guidelines, which significantly affect highway drainage design.
5.3.1 National Environmental Policy Act (NEPA)
The National Environmental Policy Act of 1969 (NEPA) (42 U.S.C. 4321–4347) declares the national
policy to encourage a productive and enjoyable harmony between man and his environment; to
promote efforts that will prevent or eliminate damage to the environment and biosphere and stimulate
the health and welfare of man; and to enrich the understanding of the ecological systems and natural
resources important to the nation.
Section 102 of NEPA requires that, to the extent possible, policies, regulations, and laws of the
United States shall be interpreted and administered in accordance with NEPA and that all Federal
agencies shall ensure that presently unquantified environmental amenities and values may be given
appropriate consideration in decision making, along with economic and technical considerations.
Section 102(2)(c) requires that all Federal agencies, with respect to major Federal actions
significantly affecting the environment, submit to the Council on Environmental Quality (CEQ) a
detailed statement on (1) the environmental impact of the proposed action, (2) any adverse
environmental effects that cannot be avoided should the proposal be implemented, (3) alternatives to
the proposed action, (4) the relationship between local short-term uses of man’s environment and the
maintenance and enhancement of long-term productivity, and (5) any irreversible or irretrievable
commitments of resources that would be involved in the proposed action should it be implemented.
CEQ guidelines for preparing and reviewing environmental statements were issued with the objective
of building into Federal agency decision-making processes, an appropriate and careful consideration
of the environmental aspects of proposed actions. The guidelines specify that environmental impact
statements will cover the following:
A description of the proposed action including information and technical data adequate to permit
a careful assessment of environmental impact by commenting agencies.
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The probable impact of the proposed action on the environment, including impact on ecological
systems (e.g., wildlife, fish and marine life).
Any probable adverse effects that cannot be avoided, such as water or air pollution, undesirable
land use patterns, damage to life systems, urban congestion, threats to health, or other
consequences adverse to the environmental goals.
Alternatives to the proposed action that might avoid some or all of the adverse environmental
effects.
The relationship between local short-term uses of man’s environment and the maintenance and
enhancement of long-term productivity.
Any irreversible and irretrievable commitments of resources that would be involved in the
proposed action should it be implemented.
Where appropriate, a discussion of problems and objections raised by other Federal, State, and
local agencies and by private organizations and individuals in the review process and the
disposition of the issues involved.
Federal-aid highway policy, published in 23 CFR 771 states:
It is the policy of the Federal Highway Administration that in the development of a project
a systematic interdisciplinary approach be used to assess engineering considerations and
beneficial and adverse social, economic, environmental, and other effects; that efforts be
made in developing projects to improve the relationship between man and his environment,
and to preserve the natural beauty of the countryside and natural and cultural resources; that
project development involve consultation with local, State and Federal agencies, and the
public; that decisions be made in the best overall public interest based upon a balanced
consideration of the need for fast, safe and efficient transportation, public services, and
social, economic, and environmental effects, and national environmental goals.
NEPA and the implementing guidelines from Council on Environmental Quality (CEQ) and FHWA
clearly have an effect on highway drainage design insofar as the impacts on water quality and
ecological systems are concerned.
5.3.2 Flood Insurance
The Flood Disaster Protection Act of 1973 (PL 93-234, 87 Stat. 975) and the more current act, 42
U.S.C. 4001–4127, denies Federal financial assistance to flood prone communities that fail to qualify
for flood insurance. Formula grants to States are excluded from the definition of financial assistance,
and the definition of construction in the Act does not include highway construction; therefore, Federal
aid for highways is not affected by the Act. The Act does require communities to adopt certain land
use controls to qualify for flood insurance. These land use requirements could impose restrictions on
the construction of highways in floodplains and floodways in communities that have qualified for
flood insurance. A floodway, as used here and as used in connection with the National Flood
Insurance Program, is that portion of the floodplain required to pass a flood that has a one percent
chance of occurring in any one-year period with no significant increase in profile due to marginal
confinement.
It is possible to comply with the Federal requirements regarding the encroachment of a highway on a
floodplain and still be faced with future legal liabilities because of the impact of the highway on the
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floodplain and the stream. Hydraulics engineers should review these potential liabilities and ensure
that their evaluation is considered when the final highway location is made.
Regulations pertaining to Federal flood insurance are contained in 44 CFR 59-77, National Flood
Insurance Policy. This subject is discussed further under Section 5.2 of the Flood Disaster Protection
Act of 1973.
5.3.3 Navigable Waters
Navigable waters of the United States are waters that have been used in the past, are now used or are
susceptible to use as a means to transport interstate commerce. A more complete definition is
included in the Glossary of these guidelines. Authorization of structures or work in navigable waters
of the United States is required by Section 9, 10, and 11 of the River and Harbor Act of 1899 (30 Stat.
1151, 33 U.S.C. 401, 403, and 404), and Section 404 of the Federal Water Pollution Control Act
Amendments (FWPCA) of 1972 (PL 92-500, 86 Stat. 816, 33 U.S.C. 1344).
Section 9 of the River and Harbor Act (33 U.S.C. 401) prohibits the construction of any dam or dike
across any navigable water of the United States without Congressional consent and approval of the
plans by the Chief of Engineers, USACE and the Secretary of the Army. The instrument of
authorization is designated a permit. Section 9 authority with regard to bridges and causeways was
transferred to the Secretary of Transportation by the Department of Transportation Act of 1966 (80
Stat. 941, 49 U.S.C. 1165g(6)(A)), and the authority to approve plans and issue permits was delegated
to the Coast Guard.
Section 10 of the River and Harbor Act of 1899 (33 U.S.C. 403) prohibits the unauthorized
obstruction or alteration of any navigable water of the United States. A USACE permit is required for
the construction of structures other than a bridge or causeway or excavation or deposition of material
in such waters.
Section 11 of the River and Harbor Act of 1899 (33 U.S.C. 404) authorizes the Secretary of the Army
to establish harbor lines. Work channelward of those lines requires approval of the Secretary of the
Army, and work shoreward requires Section 10 permits.
Section 404 of the FWPCA (33 U.S.C. 1344) prohibits the unauthorized discharge of dredged or fill
material in navigable waters. The instrument of authorization is termed a permit, and the Secretary of
the Army, acting through the Chief of Engineers, USACE, has responsibility for the administration of
the regulatory program. For purposes of Section 404 of the FWPCA, the definition of navigable
waters includes all coastal waters, navigable waters of the United States to their headwaters, streams
tributary to navigable waters of the United States to their headquarters, inland lakes used for
recreation or other purposes that may be interstate in nature, and wetlands contiguous or adjacent to
the above waters.
The issuance of any of the above permits is contingent on receipt of a water quality certification or
waiver of certification from the State in which the discharge originates stating the State waives
certification or that the proposed work will meet effluent limitations and standards established
pursuant to the FWPCA (Section 401, PL 92-500). The Administrator of the U.S. EPA is authorized
to prohibit the use of any area as a disposal site when it is determined that the discharge of materials
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at the site will have an unacceptable adverse effect on municipal water supplies, shellfish beds, and
fishery areas, wildlife, or recreational areas (Section 404(c), PL 92-500).
Section 402(p) of the FWPCA requires the U.S. EPA to establish final regulations governing
stormwater discharge permit application requirements under the National Pollutant Discharge
Elimination System (NPDES) program. The permit application requirements pertain to stormwater
discharges associated with industrial activity; discharges from large municipal separate stormwater
systems (systems serving a population of 250,000 or more) and discharge from medium municipal
separate stormwater systems (systems serving a population of 100,000 or more but less than 250,000).
In response to this requirement, the U.S. EPA published in the November 16, 1990 Federal Register
the regulations for NPDES permit application requirements for the above-mentioned stormwater
discharges.
Individual or group permits are required for all storm sewer systems under the municipal separate
stormwater discharge designation.
Highway construction activities are classified as industrial activities. The requirements for stormwater
discharges associated with industrial activities involving any disturbance of one acre, approximately
0.4 hectare of surface area or greater, which is not part of a large common plan of development or
sale, call for application of an individual permit, group application or general permit.
It is not necessary to apply separately for permits that are most commonly obtained from the USACE
(Section 10, River and Harbor Act of 1899 and Section 404, PL 92-500). Permits for navigation
clearances obtained from the Coast Guard (Section 9, River and Harbor Act of 1899) formerly
included authorization for associated work that required a USACE permit under Section 10 of the
River and Harbor Act of 1899. It is now necessary to obtain both a Coast Guard permit for navigation
clearances and USACE permits (Section 10, River and Harbor Act and Section 404, FWPCA) for
associated work in navigable waters of the United States.
The regulations governing issuances of permits for work in navigable waters of the United States are
contained in Title 33 of the Code of Federal Regulations (33 CFR) that covers “Navigation and
Navigable Waters.” Regulations for U.S. EPA discharge permits are published in 40 CFR.
Section 208 of the FWPCA requires the Governor of each State to identify each area within the State
that has a significant water quality control problem. The boundaries of such areas are to be
established and an organization capable of developing effective area-wide waste treatment
management plans designated. The designated organization must have a continuing area-wide waste
treatment management planning process in operations within a year of designation. The plan must be
certified by the Governor and submitted to the Administrator of U.S. EPA within two years after the
planning process is in operation. The plan must include identification of treatment works necessary to
meet the needs of the area over a 20-year period including any requirement for urban stormwater
runoff systems and a program to provide the necessary financial arrangements for the development of
such works. The plan must also include a process to identify construction activity-related sources of
pollution and set forth procedures and methods (including land use requirements) to control such
sources, to the extent feasible.
Hydraulics engineers should be aware of the designated planning agencies in the State and the
implications of the plans for highway construction, operation, and maintenance. Because of long
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experience in erosion and sediment control, personnel in highway agencies are uniquely qualified to
contribute to the planning process in the identification of construction activity-related sources of
pollution and procedures and methods to control such sources of pollution.
U.S. EPA regulations for the implementation of Section 208 of the FWPCA are contained in
40 CFR 126.
5.3.4 Fish and Wildlife
The Fish and Wildlife Act of 1956 (16 U.S.C. 742a et seq.), the Migratory Game-Fish Act (16 U.S.C.
760c-760g), and the Fish and Wildlife Coordination Act (16 U.S.C. 661-666c) express the concern of
Congress with the quality of the aquatic environment as it affects the conservation, improvement, and
enjoyment of fish and wildlife resources. The Fish and Wildlife Coordination Act requires that
“whenever the waters of any stream or body of water are proposed or authorized to be impounded,
diverted, the channel deepened, or the stream or other body of water otherwise controlled or modified
for any purpose whatever, including navigation and drainage, by any department or agency of the
United States, or by any public or private agency under Federal permit or license, such department or
agency shall first consult with the United States Fish and Wildlife Service (U.S. FWS), Department of
the Interior, and with the head of the agency exercising administration over the wildlife resources of
the particular State with a view to the conservation of wildlife resources by preventing loss of and
damage to such resources and providing for the development and improvement thereof.”
U.S. FWS’s role in the permit review process is to review and comment on the effects of a proposal
on fish and wildlife resources. It is the function of the regulatory agency (e.g., USACE, USCG) to
consider and balance all factors, including anticipated benefits and costs in accordance with NEPA in
deciding whether to issue the permit (40 FR 55810, December 1, 1975).
Even though the Fish and Wildlife Coordination Act provides for a consulting role for the U.S. FWS
and the head of the State agency, a “Memorandum of Understanding” between the Secretary of the
Army and the Secretary of the Interior has considerably broadened the role of the Fish and Wildlife
Service in the USACE permit programs (42 FR 37158, July 19, 1977). By this memorandum of
understanding, it was agreed that the two Departments (Interior and Army) would coordinate and
cooperate fully in the discharge of mutual responsibilities to control and prevent water pollution and
to conserve natural resources. The memorandum further stipulates that the Secretary of the Army will
seek the advice and counsel of the Secretary of the Interior on difficult cases and will carefully
evaluate the advantages and benefits in relation to the resultant loss or damage. After evaluation, the
permit will either be denied or will stipulate conditions that are determined to be in the public interest,
including provisions that will assure compliance with water quality standards. The memorandum also
includes an appeals procedures for instances of disagreement between the District Engineer of the
USACE and U.S. FWS that ultimately leads to the respective Secretaries of the Departments. Failure
to agree at this level could lead to termination of the understanding.
A similar memorandum of understanding between the Departments of Interior and Transportation
does not exist, and the USCG has retained the authority granted to the Department of Transportation
by enabling legislation.
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Regulations of the USFWS are published in 50 CFR Chapter 1. Guidelines for the review of fish and
wildlife aspects of proposals in or affecting navigable waters are contained in 40 FR 55810,
December 1, 1915.
5.3.5 Tennessee Valley Authority (TVA)
The Tennessee Valley Authority Act of 1933, as amended (16 U.S.C. 831-831dd), confers broad
powers on the Tennessee Valley Authority (TVA) related to the unified conservation and
development of the Tennessee River Valley and surrounding area. In particular, Section 26A of the
Act requires that TVA’s approval be obtained prior to the construction, operation, or maintenance of
any dam, appurtenant works, or other obstruction affecting navigation, flood control, or public lands
or reservations along or in the Tennessee River or any of its tributaries.
Approval or disapproval of applications for construction, operation, or maintenance of structures has
been assigned to the Director of the Division of Reservoir Properties of TVA. Legislation, including
NEPA and FWPCA, have declared congressional policy that agencies should administer their
statutory responsibilities so as to restore, preserve, and enhance the quality of the environment and
should cooperate in the control of pollution. Under this policy, a water quality certification from the
State having jurisdiction (Section 401, PL 92-500) is required, and TVA may require an
environmental assessment prior to issuance of the permit.
The regulations governing issuances of TVA permits are contained in Title 18 of the CFR
(18 CFR 304).
5.3.6 Coastal Zone Management
The Coastal Zone Management Act of 1972 (PL 92-583, amended by PL 94-310; 86 Stat. 1280, 16
U.S.C. 145, et seq.) declares that it is national policy (1) to preserve, protect, develop and, where
possible, to restore or enhance the resources of the Nation’s coastal zone; (2) to encourage and assist
the States to exercise effectively their responsibilities through the development and implementation of
management programs to achieve wise use of land and water resources, giving full consideration to
ecological, cultural, historic, and aesthetic values and to the needs for economic development; (3) for
all Federal agencies engaged in programs affecting the coastal zone to cooperate and participate in
effectuating the purposes of the Act; and (4) to encourage the development of coastal zone
management programs. With respect to the implementation of such programs, it is the national policy
to encourage cooperation among various State and regional agencies, particularly regarding
environmental problems.
The coastal zone management programs are to (1) identify the boundaries, (2) define permissible land
and water uses, (3) inventory and designate areas of particular concern, (4) identify means by which
the State proposes to control land and water use, as by constitutional amendment, legislative action,
regulations and judicial decisions, (5) develop broad guidelines on priority of uses, and (6) describe
the organizational structure proposed to implement the program. Approval of a coastal zone
management program is vested with the Secretary of Commerce.
Each Federal agency conducting or supporting activities directly affecting the coastal zone shall
conduct or support those activities in a manner that is, to the maximum extent practicable, consistent
with approved State management programs. After final approval of a State’s management program,
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any applicant for a required Federal license or permit to conduct an activity affecting land or water
use in the coastal zone must provide a certification that the proposed activity complies with the
State’s approved program and will be conducted in a manner consistent with the program. The
certification must also be furnished to the State’s designated agency and that agency must notify the
permitting Federal agency of its concurrence or objection to the applicant’s certification. No license
or permit will be granted until the designated agency has concurred with the applicant’s certification.
The designated State agency is allowed six months to furnish notification of concurrence or, in the
absence of comment, the concurrence will be conclusively presumed.
State and local agencies applying for Federal assistance must indicate the views of the designated
State agency as to the relationship of the activity to the approved management program for the coastal
zone. The Federal agency cannot approve proposed projects that are inconsistent with a coastal
State’s management program, unless the Secretary of Commerce finds that the project is consistent
with the purpose of the Act or necessary to national security.
The Office of Coastal Zone Management, NOAA, U.S. Department of Commerce, has responsibility
for administering provisions of the Coastal Zone Management Act of 1972. Rules of administering
Federal requirements for consistency with approved coastal zone management programs will be
published as 15 CFR 930.
5.3.7 Executive Orders
Presidential Executive Orders have the effect of law in the administration of programs by Federal
agencies. Executive Order (E.O.) 11296 issued in August 1966, because of ever-increasing flood
losses, directed Federal agencies to avoid uneconomic, hazardous and unnecessary use of floodplains.
In May 1972, the Water Resources Council (WRC) published Guidelines for Federal Executive
Agencies for Flood Hazard Evaluations containing guidance for the implementation of provisions of
the Executive Order. Federal-aid highway drainage designs, to qualify for Federal-aid participation,
must meet minimum requirements established to comply with the provisions of the Executive Order
and the Water Resources Council Guidelines. These requirements were published in the Federal
Register, April 26, 1979 (44 FR 24678), and in 23 CFR 650 Subpart A.
E.O. 11988, May 24, 1977, requires each Federal agency, in carrying out its activities, to take action
(1) to reduce the risk of flood loss, to minimize the impact of floods on human safety, health and
welfare, and to restore and preserve the natural and beneficial values served by floodplains; (2) to
evaluate the potential effects of any actions it may take in a floodplain, to ensure its planning
programs reflect consideration of flood hazards and floodplain management; and (3) submit a report
to the CEQ and the WRC on the status of procedures and the impact of the Order on the agency’s
operations. E.O. 11988 revoked E.O. 11296.
E.O. 11990, May 24, 1977, orders each Federal agency (1) to take action to minimize the destruction,
loss or degradation of wetlands and to preserve and enhance the natural and beneficial values of
wetlands; (2) to avoid undertaking or providing assistance for new construction in wetlands unless the
head of the agency finds that there is no practicable alternative and all practicable measures are taken
to minimize harm that may result from the action; (3) to consider factors relevant to the proposal’s
effects on the survival and quality of the wetlands; and (4) to amend existing or issue new procedures
to comply with the Order.
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A recently executed Memorandum of Agreement (MOA) between the Department of the Army
(DOA) and the U.S. EPA provides guidance on mitigation for wetland impact by highway
construction.
5.4 STATE LAWS
State drainage law is derived mainly from two sources: (1) common law and (2) statutory law.
Common law is that body of principles that developed from immemorial usage and custom and that
receives judicial recognition and sanction through repeated application. These principles developed
without legislative action and are embodied in the decisions of the courts. Common law is a large and
important segment of drainage law because it generally applies to adjoining properties having
sufficient differences in elevation to cause natural drainage.
Statutory laws of drainage are enacted by legislatures to enlarge, modify, clarify, or change the
common law applicable to particular drainage conditions. This type of law is derived from
constitutions, statues, ordinances, and codes.
In general, the common law rules of drainage predominate unless they have been enlarged or
superseded by statutory law. In most instances, where statutory provisions have been enacted, it is
possible to determine the intent of the law. If, however, there is a lack of clarity in the statute, the
point in question may have been litigated for clarification. In the absence of either clarity of the
statute or litigation, a definitive statement of the law is not possible, although the factors that are
likely to be controlling may be indicated.
5.4.1 Common Law
State drainage laws originating from common law, or court-made law, first classified the water that
was being dealt with, after which the rule that was pertinent to the particular classification was
applied to obtain a decision. These common law concepts are briefly summarized in the following
sections. The classifications applicable to the various conditions are first indicated, and then the
prevailing ruling concepts are described.
5.4.1.1 Classification of Waters
Any discussion, arbitration, or litigation of water laws and problems by laymen and professionals
alike is usually handicapped by varying use of terms that classify, define and quantify natural waters.
The law governing watercourses is substantially different from the law governing surface waters.
Therefore, some amount of definition is needed, but it is not the intent in this chapter to attempt to
establish universal water terminology for highway agencies. The reader of material on the subject of
water law must interpret the material on the basis of experience and by consulting State statutes or
legal opinions for the proper definition of some of the terms used.
The first step in the evaluation of a drainage problem is to classify the water as surface water, stream
water, floodwater, or groundwater. These terms are defined in the following sections. Once the
classification has been established, the rule that applies to the particular class of water determines
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responsibilities with respect to disposition of the water. It must be recognized that water law differs
from state to state, and many issues can be resolved only through proper utilization of legal counsel.
5.4.1.1.1 Surface Waters
Surface waters are those waters that have been precipitated on the land from the sky or forced to the
surface in springs and that have then spread over the surface of the ground without being collected
into a definite body or channel. They appear as puddles, sheet or overland flow and rills, and continue
to be surface waters until they disappear from the surface by infiltration or evaporation or until, by
overland or vagrant flow, they reach well-defined watercourses or standing bodies of water.
5.4.1.1.2 Stream Waters
Stream waters are former surface or groundwaters that have entered and now flow in a well-defined
natural watercourse, together with other waters reaching the stream by direct precipitation or rising
from springs in the bed or banks of the watercourse. They continue as stream waters as long as they
flow in the watercourse, including overflow and multiple channels and the ordinary or low water
channel.
A watercourse, in the legal sense, refers to a definite channel with bed and banks within which water
flows either continuously or intermittently. A watercourse is continuous in the direction of flow and
may extend laterally beyond the definite banks to include overflow channels contiguous to the
ordinary channel. In semi-arid areas, a channel may be considered a watercourse even though it only
carries water during periods of storm, provided it does regularly carry flows at such times. The term,
however, is defined differently in different jurisdictions. In some States, natural depressions in the
earth’s surface that do not have a defined bed and banks are commonly classified as swales or draws
and do not constitute a watercourse in the legal sense.
Artificial channels (e.g., canals, drains) are not watercourses except as natural channels are trained or
restrained by the works of man.
5.4.1.1.3 Floodwaters
Floodwaters are former stream waters that have escaped from a watercourse (and its overflow
channels) and flow or stand over adjoining lands. They remain floodwaters until they disappear from
the surface by infiltration or evaporation or return to a natural watercourse. They do not become
surface waters by mingling with such waters nor stream waters by eroding a temporary channel.
Surface waters do not become floodwaters, no matter how fast or deep or where they flow, unless en
route they have entered a natural watercourse and escaped. They have not escaped if they run in an
overflow channel or in an outer channel of a braided stream. They are floodwaters only if they have
been stream waters and have completely escaped from the natural watercourse, including its collateral
channels.
Floodwaters are distinguished from surface waters by the fact that floodwaters have broken away
from a stream and surface waters have not yet become a part of the stream.
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5.4.1.1.4 Groundwaters
In legal consideration, groundwaters are divided into two classes: percolating waters and underground
streams.
The term “percolating waters” generally includes all waters that pass through the ground beneath the
surface of the earth without a definite channel. Percolating waters are those that seep, ooze, filter, and
otherwise circulate through the subsurface strata without definite or defined channels or in a course
that is unknown and not discoverable from surface indications without excavation for that purpose.
Percolating waters may be either rain waters that are slowly infiltrating through the soil or they may
be waters seeping through the banks or bed of a stream that have so far left the bed and other waters
as to have lost their character as part of its flow.
The general rule is that all underground waters are presumed to be percolating and, to take them out
of the percolating class, the existence and course of a permanent channel must be clearly shown. The
fact that underground waters may come together at places so as to form veins or rivulets or even
underground channels does not destroy their character as percolating waters as long as they are
unknown and undiscoverable except by excavation. A well will be presumed to be from percolating
water in the absence of proof to the contrary.
Underground streams are waters passing through the ground beneath the surface in permanent,
distinct, well-defined channels. Waters in an underground stratum are not percolating, in the common
law sense of the term, where they are in such immediate connection with the surface stream as to
make them part of the stream, although it may be difficult to distinguish between percolating waters
and subterranean stream waters in a particular case. Where a stream sinks into the ground, pursues a
subterranean course for some distance and then emerges again, the part beneath the surface is not
percolating water. Water flowing underground in an unbroken and well-defined channel constitutes a
watercourse and is generally governed by law applicable to surface streams, rather than by law
applicable to percolating waters.
5.4.1.2 Surface Water Rules and Applications
Two major rules have been developed by the courts regarding the disposition of surface waters. One
is known as the civil law rule of natural drainage. The other is referred to as the common enemy
doctrine, which treats surface waters as a common enemy. Modification of both rules has tended to
bring them somewhat closer together and, in some states, the original rule has been replaced by a
compromise rule known as the reasonable use rule.
5.4.1.2.1 Civil Law Rule (Natural Drainage Rule)
The civil law rule is based upon the perpetuation of natural drainage. One court, in applying the rule,
gave the following reasons for its use:
“As water must flow, and some rule in regard to it must be established where land is held
under the artificial titles created by human law, there can clearly be no other rule at once so
equitable and so easy of application as that which enforces natural laws. There is no
surprise or hardship in this, for each successive owner takes whatever advantages or
inconvenience nature has stamped upon his land.” (Gormley v. Sanford 52 Ill.158 (1869)).
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The following is a frequently quoted statement of the civil law rule:
“. . . every landowner must bear the burden of receiving upon his land the surface water
naturally falling upon land above it and naturally flowing to it therefrom, and he has the
corresponding right to have the surface water naturally falling upon his land or naturally
coming upon it, flow freely therefrom upon the lower land adjoining, as it would flow
under natural conditions. From these rights and burdens, the principle follows that he has a
lawful right to complain of others, who, by interfering with natural conditions, cause such
surface water to be discharged in greater quantity or in a different manner upon his land,
than would occur under natural conditions. This is the settled law of this (civil law rule)
state . . .” (Heier v. Krull. 160 Cal 441 (1911)).
The civil law rule obviously is a strict one. Further, it is a rule that tends to interfere with the
development of land to its highest and best use. Under the rule, any diversion of water onto the land
of another constitutes a technical trespass. It was only natural that such a rule would be modified.
Many civil law rule jurisdictions recognize an exception in urban areas. This, in and of itself, creates
a problem as to what constitutes an urban area. Another general exception permits upper owners to
gradually increase the drainage of surface waters from their land by means of cultivation of the soil
for agricultural purposes. Generally, where strict application of the rule has caused hardship, the
courts have tended to make an exception.
One important facet should be noted here. It has been generally held that there is no diversion if
surface waters are, for a reasonable purpose, gathered together and discharged into the stream that is
their natural means of drainage. Increased flow from land development must be disposed of in some
manner, and the streams of nature generally constitute the legally recognized channels for such
purposes. In some States, this rule holds true even though the stream channel is inadequate to
accommodate the increased flow. The burden is upon successive lower owners to pass the increased
stream flow through their property.
5.4.1.2.2 Application of the Civil Law Rule
(1)
Damming Back Water. The civil law rule, at least before modification, appears to forbid the
lower owner from damming back the natural flow of surface water. This seems to follow, of
course, the theory that the lower owners must accept the surface water naturally flowing on
them. However, it appears that lower owners have the right to dam back water or artificial
drainage that has been unlawfully thrown upon them.
(2)
Augmenting Natural Drainage. It appears generally that natural drainage may be augmented
as the civil law rule is now modified. Surface waters may be accelerated and increased in
volume so long as no additional areas are tapped from which surface water otherwise would
not have flowed. The tapping of additional watershed areas is usually referred to as a diversion
and is generally prohibited in civil law jurisdictions.
(3)
Collecting and Discharging Water. The civil law rule appears to be that a property owner
may not artificially collect surface waters and discharge them en masse on the lower owner to
the latter’s damage. In other words, in the proper improvement of land, an upper landowner
may, to some extent, augment or concentrate the natural drainage but may not gather the
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surface waters artificially and dump them on the property below to its injury. It has been held
that not only the amount of water caused to flow on the lower land, but also the manner of
collection and release, and the intermittent increase in volume or destructive force or its
direction to a more vulnerable point of invasion are important.
5.4.1.2.3 Common Enemy Doctrine
The common enemy doctrine permits landowners to fend off surface waters as they see fit, which is
the complete opposite of the civil law rule. Under the strict form of this theory, surface waters are
regarded as a common enemy which landowners may fight as they deem best, regardless of the harm
they may cause to others. The common enemy doctrine, in its stated form, is clearly a harsh one and,
therefore, was bound to be modified. In most jurisdictions, it has been made subject to a limitation
that owners must use their land so as not to unreasonably or unnecessarily damage the property of
others.
5.4.1.2.4 Application of the Common Enemy Doctrine
(1)
Damming Back Water. Under the common enemy doctrine in unmodified form, there is no
liability for casting surface waters on the land of an upper owner by the construction of a fill so
as to form a dam. This situation is generally avoided in highway construction by the
installation of adequate drainage facilities. The right to dam against surface waters has been
substantially limited by various modifications of the doctrine. It has been held that the casting
back or damming of waters must be reasonable and with due regard for the rights of others.
(2)
Augmenting Natural Drainage. Under the common enemy doctrine, even as modified, there
seems to be little doubt that owners of upper land, acting in the reasonable use of their property
and without negligence, may augment the flow of surface water to the land below, either by
increasing the volume or by changing the mode of flow.
(3)
Collecting and Discharging Water. The common enemy and civil law rules appear to be most
alike in this area. A number of jurisdictions with the common enemy doctrine or modifications
thereof have held that it is unlawful to collect, concentrate and discharge surface waters on
lower owners to their damage or injury. (It should be noted that courts in States throughout the
Union have had difficulty in determining the parameters or the definition of the words
collection, concentration and discharge).
5.4.1.2.5 Reasonable Use Rule
The problems created by the early attempts at specific rules have led to the application, in some
states, of the reasonable use rule. Under this rule, the possessors of land incur liability only when their
harmful interference with the flow of surface waters is unreasonable. One court, in applying this rule,
stated it as follows:
“In effecting a reasonable use of land for a legitimate purpose a landowner, acting in good
faith, may drain his land of surface waters and cast them as a burden upon the land of
another, although such drainage carries with it some waters which otherwise would never
have gone that way but would have remained on the land until they were absorbed by the
soil or evaporated in the air, if (a) there is a reasonable necessity for such a drainage;
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(b) reasonable care be taken to avoid unnecessary injury to the land receiving the burden;
and (c) if the utility or benefit accruing to the land drained reasonably outweighs the
gravity of the harm resulting to the land receiving the burden; and (d) if where practicable it
is accomplished by reasonably improving and aiding the normal and natural system of
drainage according to its reasonable carrying capacity, or if, in the absence of a practicable
natural drain, a reasonable and feasible artificial drainage system is adopted.” (Enderson v.
Kelehan, 226 Minn. 163, 32 N.W. 2d 286 (1948)).
5.4.1.2.6 Application of the Reasonable Use Rule
Under the reasonable use rule, possessors of land are legally privileged to make a reasonable use of
their land even though the flow of surface waters is altered thereby and causes some harm to others.
Possessors of land incur liability, however, when their harmful interference with the flow of surface
waters is unreasonable. The issue of reasonableness or unreasonableness is a question of fact to be
determined in each case upon consideration of all relevant circumstances. In determining the question
of reasonableness under the reasonable use rule, it is proper to take into consideration such factors as
the amount of harm caused, the foreseeability of the harm that results, the purpose or motive with
which the possessor acted, and other relevant matters such as whether the ability of the possessor’s
use of the land outweighs the gravity of the harm that results to a neighbor from alteration of the flow
of the surface waters.
5.4.1.3 Stream Water Rules
Much of the law regarding stream waters is founded on a common law maxim that states “water runs
and ought to run as it is by natural law accustomed to run.” Thus, as a general rule, any interference
with the flow of a natural watercourse to the damage of another will result in liability. This may
involve augmentation, obstruction and detention, or diversion of a stream. However, there are
qualifications.
Where natural watercourses are unquestioned in fact and in permanence and stability, there is little
difficulty in application of the rule. Highways cross channels on bridges or culverts, usually with
some constriction of the width of the channel and obstruction by substructure within the channel, both
causing backwater upstream and acceleration of flow downstream. The changes in regime must be so
small as to be tolerable by adjoining owners, or there may be liability for any damages suffered.
Surface waters from highways are often discharged into the most convenient watercourse. The right is
unquestioned if those waters were naturally tributary to the watercourse and unchallenged if the
watercourse has adequate capacity. However, if all or part of the surface waters have been diverted
from another watershed to a small watercourse, any lower owner may complain and recover for
ensuing damage.
Applications of law become more complicated when the regime of a channel is changed. If, for
example, upper owners change the character of the watershed so that stream waters are increased in
volume, lower owners are obligated by the common law to accept the increase, there being no
diversion. They are not obligated to improve the channel through their lands, although they may
choose to do so for self-protection. The question is unsettled as to whether upper owners can compel
lower owners to improve their formerly adequate channel if the increased flow is detained and backs
up on lands of the upper owners. Where the lower owner is the highway that had provided an
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adequate bridge or culvert, some contend that the facility should be enlarged promptly on demand of
the upper owner; others insist the highway has the alternative of maintaining the overtaxed facility, at
least until some substantial change is required by reconstruction of the highway itself or retirement of
the facility at the end of its useful life.
Law does not define any measure of adequacy for improved channels. The highway does not escape
liability for obstruction of a channel by providing a bridge or culvert to convey a 30-year flood, a
100-year flood, or even a 1,000-year flood. In common law, an extremely rare flood event might be
called an “act of God,” defined as “a direct, sudden, and irresistible action of natural forces, such as
could not humanly have been foreseen or prevented.” However, such a plea is a weak defense in
claims for damages, for engineers foresee and can provide for stream waters of great magnitude. The
law does not prescribe with specificity an all-inclusive and acceptable standard. Accordingly,
engineering decisions are generally based on considerations of risk and economics.
5.4.1.4 Floodwater Rule
In common law, floodwaters are treated as a “common enemy” of all people, lands, and property
attacked or threatened by them. Anyone, including owners of highways, can act in any reasonable
way to protect themselves and their property from the common enemy. They may obstruct its flow
from entering their land, backing or diverting water onto lands of another without liability. Generally,
they may discharge such water from their land onto land of a neighbor without penalty, by gravity or
pumping, by diverting dikes or ditches, or by any other reasonable means.
Again, the test of “reasonableness” has frequently been applied, and liability can result where
unnecessary damage is caused. Ordinarily, the highway designer should make provision for overflow
in areas where it is foreseeable that it will occur. There is a definite risk of liability if such waters are
impounded on an upper owner or, worse yet, are diverted into an area where they would not
otherwise have gone. Merely to label waters as “floodwaters” does not mean that they can be
disregarded. In a California decision, State engineers were issued the following warning: “. . .merely
to label waters as ‘floodwaters’ does not, as we see it, necessarily give to the State carte blanche to
dispose of said waters regardless of the reasonableness of methods employed and the quantity of
damage that individual landowners may suffer as a result.”
5.4.1.5 Groundwater Rules
Underground waters have been held to be part of the real property in which they are situated; the
owners of the land own the underground water by the same title by which they own the land itself and
the clay, gravel, coal, or oil within it even though those items of property differ in component parts.
An overlying owner has been held to have rights analogous to those of a riparian owner. So, rights to
the use of underground waters, whether flowing, stored or percolating, by the overlying owner or
appropriator are analogous and equal to riparian rights against subsequent claimants, are part and
parcel of the land and, as such, are real property. Generally speaking, an overlying right is the right of
the owner of the land to take water from the ground underneath for use on the land and within the
basin or watershed; the right is based on ownership of the land and is appurtenant thereto. As between
adjoining overlying owners, the rights are correlative and are referred to as belonging to them in
common.
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The general rule for flowing waters is that where a subterranean stream flows in a distinct, permanent,
well-known, and defined channel, it is governed by the same rules as apply to a natural watercourse
on the surface. The owners of land beneath which it flows have the same rights with respect to it as
riparian proprietors have with respect to a stream on the surface, conditioned on the water coming to
the land in a natural flow and regardless of whether it is under pressure.
Percolating water is generally regarded as part of the soil in which the owner of the land has a
property right. The right is limited to waters that percolate through the soil from natural causes; the
law does not vest in a landowner a right to a continuance of percolation from another’s land due to
irrigation or to other artificial causes.
If percolating waters escape naturally to other lands, the title of the former owner is gone.
Landowners may prevent the escape of such waters from their land, if they can do so. (They have no
right to follow them into the lands of another and there capture, control or reduce them to possession).
5.4.2 Statutory Law
The inadequacies of the common law or court-made laws of drainage led to a gradual enlargement
and modification of the common law rules by legislative mandate.
In the absence of statute, the common law rules adopted by State courts determine surface water
drainage rights. If the common law rules have been enlarged or superseded by statutory law, the
statute prevails.
In general, statutes have been enacted that affect drainage in one way or another, as described in the
following subject areas.
5.4.2.1 Eminent Domain
In the absence of an existing right, public agencies may acquire the right to discharge highway
drainage across adjoining lands through the use of the right of eminent domain. Eminent domain is
the power of public agencies to take private property for public use. Whenever the right of eminent
domain is exercised, the constitutional requirement of just compensation for property taken or
damaged for public use must be met.
An important corollary of the right of eminent domain is the suit in inverse condemnation in which
the property owner alleges that private property has been taken or damaged without just
compensation. The public agency could have taken the property by eminent domain, but properties
are sometimes overlooked or considered speculative or intangible so that the owners initiate the
eminent domain action inversely. Inverse condemnation is further described in Section 5.7.1, “Inverse
Condemnation.”
5.4.2.2 Water Rights
The water right that attaches to a watercourse is a right to the use of the flow, not ownership of the
water itself. This is true under both the riparian doctrine and the appropriation doctrine. This right-ofuse is a property right, entitled to protection to the same extent as other forms of property, and is
regarded as real property. After the water has been diverted from the stream flow and reduced to
possession, the water itself becomes the personal property of the riparian owner or the appropriator.
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The Western law of water rights embraces the common law doctrine of riparian rights and the
statutory doctrine of prior appropriation. The principles underlying these two doctrines are
diametrically opposed, the former being based on the ownership of land contiguous to a stream,
without regard to the time of use or to any actual use at all, and the latter on the time of use and on
actual use without regard to the ownership of land contiguous to the watercourse.
Generally, the important thing for highway hydraulics engineers to keep in mind in the matter of
water rights is that proposed work in the vicinity of a stream should not impair either the quality or
quantity of flow of any water rights to the stream.
5.4.2.2.1 Riparian Doctrine
Under the riparian doctrine, lands contiguous to watercourses have prior claim to waters of the stream
solely by reason of location and regardless of the relative productive capacities of riparian and
nonriparian lands.
By law, the right to the use of water under the riparian doctrine is incidental to the ownership of
riparian land. The general rule is that the acquisition of the land automatically results in acquisition of
the right. Only land contiguous to or abutting upon a natural stream or lake is riparian land. Further
limitations on land for which riparian rights may be claimed are that it must lie within the watershed
of the stream or body of water to which it is contiguous, and it must be within the bounds of the
original grant from the sovereign of land contiguous to the stream.
Under the strict riparian doctrine, the owner of riparian land is entitled to have the stream flow by or
through the land undiminished in quantity and unpolluted in quality, except that any riparian
proprietor may make whatever use of the water required for domestic and household purposes and for
the watering of farm animals. The doctrine has been generally modified to allow each proprietor to
make such use of the water for the irrigation of his riparian land as is reasonable in relation to the
same stream. The right does not depend upon the use of the water and, therefore, nonuse does not
result in its loss.
5.4.2.2.2 The Doctrine of Prior Appropriation
The riparian doctrine was developed under climactic conditions vastly different from those in our
western States. In the arid areas of the West, water is essential to agriculture, and the quantity of
water available is far short of the quantity required for the farming of all agricultural lands. As water
is much less abundant than arable land, the problem is to distribute the water supplies where they can
be most beneficially and economically used. A doctrine was needed which laid greater emphasis upon
beneficial use and afforded protection to enterprises based upon the feasibility of directing waters and
applying them to lands, whether or not contiguous to watercourses. The doctrine of prior
appropriation meets this need to a greater extent than the doctrine of riparian rights.
The essence of the doctrine of prior appropriation is the exclusive right to divert water from a source
when the water supply naturally available is not sufficient for the needs of all those holding rights to
its use. Such exclusive right depends upon the effective date of the appropriation, the first in time
being the first in right. As the volume of flow in the stream drops, the diversion gates of the
appropriators are closed in the reverse order of their priorities. As the volume increases, the diversion
gates are opened in the order of priority. The priority does not depend upon the location of one’s
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point of diversion. The first appropriation may be at the headwaters of the stream or at its mouth, and
later appropriations are junior in all respects, regardless of whether their points of diversion are
upstream or downstream from the diversion of the senior appropriator. Consequently, before any
appropriator may legally divert any water under their own right, they must allow sufficient water to
pass their headgate to supply fully the requirements of all downstream appropriators whose priorities
are senior to their own, regardless of what their own water needs may be. With the doctrine of prior
appropriation, the water right becomes part of a specific parcel of land and cannot be arbitrarily
removed therefrom without approval and consent of the State.
5.4.2.3 Flood Control, Drainage and Irrigation Districts
Flood control, drainage, and irrigation districts are created or are authorized by statute in some States.
These districts are organizations of landowners of an area with a common interest. The districts are
established as governmental or quasi-governmental agencies in status and may lie within the
geographic boundaries of one or more local or county governmental entities. The boundaries of the
districts are often based on watersheds though irrigation districts may transcend boundaries of major
watersheds where transbasin diversion is involved. These agencies are often a source of information
(e.g., ma