Acre-Foot -- The amount of water that will cover one acre to a depth of
one foot. (Equals 43,560 cubic feet).
Backwater -- An unnaturally high stage in stream caused by obstruction
or confinement of flow, as by a dam, a bridge, or a levee. Its measure is the
excess of unnatural over natural stage, not the difference in stage upstream
and downstream from its cause.
Bank -- The lateral boundary of a stream confining water flow. The bank
on the left side of a channel looking downstream is called the left bank, etc.
Capacity -- The effective carrying ability of a drainage structure
facility. May also refer to storage capacity.
Channel -- Refers to a drainageway which has been created or extensively
modified for the purpose of conveying floodwaters, and is therefore, no longer
a watercourse in its natural condition.
Concentrated Flow -- Flowing water that has been accumulated into a
single fairly narrow s tream.
Confluence -- A junction of streams.
Control -- A section or reach of an open conduit or stream channel which
maintains a stable relationship between stage and discharge.
Conveyance -- A measure of the water carrying capacity of a stream or
Critical Depth -- Depth at which specific energy is a minimum.
Conditions the maximum flow will occur.
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 one.
Critical Slope -- That slope which the maximum flow will occur at the
minimum velocity. The slope or grade that is exactly equal to the loss of head
per foot resulting from flow at a depth that will give uniform flow at critical
depth; the slope of a conduit which will produce critical flow.
Critical Velocity -- Mean velocity of flow when flow is at critical
Culv ert -- A closed conduit, other than a bridge, which allows water to
pass under a road or highway.
Design Flood -- The peak discharge (when appropriate, the volume, state,
or wave crest elevation) of the flood associated with the probability of
exceedance selected for design.
Design Storm -- That particular storm which contributes runoff which the
drainage facilities were designed to handle. This storm is selected for design
on the basis of its probability of exceedance or average recurrence interval.
Detention Storage -- Surface water allowed to temporarily accumulate in
ponds, basins, reservoirs or other types of holding facilities and which is
ultimately returned to a watercourse or other drainage system as runoff is in
Discharge -- A volume of water flowing past a give point per unit of
time. Generally specified in cubic feet per second.
Diversion -- The change in character, location, di rection, or quantity
of flow of a natural drainage course. A deflection of flood water is not
Drainage System -- Usually a system of underground conduits and
collector structures which flow to a single point of discharge.
Drainageways -- Any path along which water flows when acted upon by
Ephemeral Stream -- A stream or reach of stream that flows briefly only
in direct response to precipitation.
Energy Dissipator -- A structure for the purpose of slowing the flow of
water and reducing the erosive forces present in any rapidly flowing body of
Energy Grade Line -- The line which represents the total energy gradient
along the channel. It is established by adding together the potential energy
expressed as the water surface elevation referenced to a datum and the kinetic
energy (usually expressed as velocity head) at points along the stream bed or
Energy Head -- The elevation of the hydraulic grade line at any section
plus the velocity head of the mean velocity of the water in that section.
Erosion -- The wearing away of a surface by some external force. In the
case of drainage terminology, this term generally refers to the wearing away of
the earth's surface by flowing water. It can also refer to the wear on a
structural surface by flowing water and the material carried therein.
Erosion and Scour -- The cutting or wearing away by the forces of water
of the banks and bed of a channel in horizontal and vertical directions,
Erosion and Accretion -- Loss and gain of land, respectively, by the
gradual action of a stream in shifting its channel by cutting one bank while it
builds on the opposite bank. Property is lost by erosion and gained by
accretion but not by avulsion when the shift from one channel to another is
sudden. Property is gained by reliction when a lake recedes.
Flap Gate -- This is a form of valve that is designed so that a minimum
force is required to push it open but when a greater water pressure is present
on the outside of the valve, it remains shut so as to prevent water from
flowing in the wrong direction. Construction is simple with a metal cover
hanging from an overhead rod or pinion at the end of a culvert or drain.
Flood -- In common usage, an event where a stream overflows its normal
banks. In frequency analysis it means an annual flood that may not overflow the
Flood Frequency -- Also referred to as an exceedance interval,
recurrence interval or return period; the average time interval between actual
occurrences of a hydrological event of a given or greater magnitude; the
percent chance of occurrence is the reciprocal of flood frequency, e.g., a 2
percent chance of occurrence is the reciprocal statement of a 50-year flood.
Probability of Exceedance.)
Floodplain -- Normally dry land areas subject to periodic temporary
inundation by stream flow or tidal overflow. Land formed by deposition of
sediment by water; alluvial land.
Floodproof -- To design and construct individual buildings, facilities,
and their sites to protect against structural failure, to keep water out or
reduce the effects of water entry.
Flood Routing -- Determining the changes in a floodwave as it moves
downstream through a valley or through a reservoir.
Flood Stage -- The elevation at which overflow of the natural banks of a
stream begins to cause damage in the reach in which the elevation is measured.
Flood Waters -- Former stream water which have escaped from a
watercourse (and its overflow channel) and flow or stand over adjoining lands.
They remain as such until they disappear from the surface by infiltration,
evaporatio n, or return to a natural watercourse. They do not become surface
waters by mingling with such waters, nor stream waters by eroding a temporary
Flood Wave -- The rise and fall in stream flow during and after a storm.
Floodway -- A zone of the floodplain, designated by FEMA under the
National Flood Insurance Program, which must be preserved for allowing passage
of the base flood discharge without increasing water elevations more than one
foot. Velocities and depths in the floodway are more hazardous than in rest of
Flow -- A term used to define the movement of water, silt, sand, etc.;
discharge; total quantity carried by a stream.
Flow Line -- A term used to describe the line connecting the low points
in a watercourse.
Flow Regime -- The system or order characteristic of streamflow with
respect to velocity, depth, and specific energy.
Freeboard -- (1) The vertical distance between the level of the water
surface usually corresponding to the design flow and a point of interest such
as a bridge beam, levee top or specific location on the roadway grade. (2) The
distance between the normal operating level and the top of the sides of an open
conduit; the crest of a dam, etc., designed to allow for wave action, floating
debris, or any other condition or emergency, without overtopping structure.
Gradient (Slope) -- The rate of ascent or descent expressed as a percent
or as a decimal as determined by the ratio of the change in elevation to the
Gradually Varied Flow -- A classification of flow where changes in depth
and velocity take place slowly over large distances, resistance to flow
dominates and acceleration forces are neglected.
Head -- Represents an available force equivalent to a certain depth of
water. This is the motivating force in affecting the movement of water. The
height of water above any point or plane or reference. Used also in various
compound expressions, such as energy head, entrance head, friction head, static
head, pressure head lost head, etc.
Headcutting -- Progressive scouring and degrading of a streambed at a
relatively rapid rate in the upstream direction, usually characterized by one
or a series of vertical falls.
Hydraulic Gradient -- A line which represents the relative force
available due to the potential energy available. This is a combination of
energy due to the height of the water and the internal pressure. In any open
channel, this line corresponds to the water surface. In a closed conduit, if
several openings were placed along the top of the pipe and open tubes inserted,
a line connecting the water surface in each of these tubes would represent the
hydraulic grade line.
Hydraulic Radius -- The cross sectional area of a stream of water
divided by the length of that part of its periphery in contact with its co
ntaining conduit; the ratio of area to wetter perimeter.
Hydrograph -- A graph showing stage, flow, velocity, or other property
of water with respect to time.
Hydrology -- The science dealing with the occurrence and movement of
water upon and beneath the land areas of the earth. Overlaps and includes
portions of other sciences such as meteorology and geology.
Hyetograph -- Graphical representation of rainfall intensity against
Incised Channel -- Those channels which 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.
Infiltration -- The passage of water through the soil surface into the
Inlet Time -- The time required for storm runoff to flow from the most
report point, in flow time. of a drainage area to the point where it enters a
drain o r culvert.
Intermittent Stream -- A watercourse that has significant flow for 30
days after the last significant storm or has a well-defined channel.
Invert -- The bottom of a drainage facility along which the lowest flows
Inverted Siphon -- A pipe for conducting water beneath a depressed
place. A true inverted siphon is a culvert which has the middle portion at a
lower elevation than either the inlet or the outlet and in which a vacuum is
created at some point in the pipe. A sag culvert is similar, but the vacuum is
not essential to its operation.
Isohyetal Line -- A line drawn on a map or chart joining points that
receive the same amount of precipitation.
Isohyetal Map -- A map containing isohyetal lines and showing rainfall
Lag -- Variously defined as time from beginning (or center of mass) of
rainfall to peak (or center of mass) of runoff.
Laminar Flow -- 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 velocity (as opposed to turbulent flow).
Lateral -- In a roadway drainage system, a drainage conduit transporting
water from inlet points to the main drain truck line.
Levee -- An embankment to prevent inundation.
Mean Annual Flood -- The flood discharge with a recurrence interval of
Meander -- In connection with streams, a winding channel usually in an
erodible, alluvial valley. A reverse or S-shaped curve or series of curves
formed by erosion of the concave bank, especially at the downstream end,
characterized by a curved flow and alternating shoals and bank erosions.
Meandering is a stage in the migratory movement of the channel, as a whole,
down the valley.
Meander Plug (Clay Plug) -- Deposits of cohesive materials in old
channel bendway s. These plugs are sufficiently resistant to erosion to serve
as essentially semi-permanent geological controls to advancing channel
Meander Scroll -- Evidence of historical meander patterns in the form of
lines visible on the inside of meander bends. ( particularly on aerial
photographs) which resemble a spiral or convoluted form in ornamental design.
These lines are concentric and regular forms in high sinuosity channels and are
largely absent in poorly developed braided channels.
Natural Stream -- Refers to a natural watercourse that has not been
significantly disturbed by development, and the native vegetation is therefore
Normal Depth -- The depth at which flow is steady and hydraulic
characteristics are uniform.
Normal Water Surface (Natural Water Surface) -- The surface associated
with flow in natural streams.
Open Channel -- Any conveyance in which water flows with a free surf
Outfall -- Discharge or point of discharge of a culvert or other closed
Peak Flow -- Maximum momentary stage or discharge of a stream in flood.
Permeability -- The property of soils which permits the passage of any
fluid. Permeability depends on grain size, void ratio, shape and arrangement of
Point of Concentration -- That point at which the water flowing from a
given drainage area concentrates.
Probability of Exceedance -- The statistical
probability, expressed as a percentage of a hydrologic event occurring or being
exceeded in any given year. the probability (p) of a storm or flood is the
reciprocal of the average recurrence interval (N)
Probable Maximum Flood - The flood discharge that may be expected from
the most seve re combination of critical meteorological and hydrological
conditions that are reasonable possible in the regional.
Rapidly Varied Flow -- A classification of flow where changes in depth
and velocity take place over short distances. acceleration forces dominate, and
energy loss due to friction is minor.
Reach -- The length of a channel uniform with respect to discharge,
depth, area, and slope. More generally, any length of a river or drainage
Regime -- The system of order characteristic of a stream; its behavior
with respect to velocity and volume, form of and changes in channel, capacity
to transport sediment, amount of material supplied for transportation, etc.,
Retention Storage -- Water which accumulates and ponds in natural or
excavated depressions in the soil surface with no possibility for escape as
Riparian -- Pertaining to the banks and other adjacent, terrestrial
environs of freshwater bodies, watercourses, and surface emergent aquifers,
whose imported waters provide soil moisture significantly in excess of that
available through local precipitation.
Risk -- The consequences associated with the probability of flooding
attributable to an encroachment. It shall include the potential for property
loss and hazard to life during the service life of the highway.
Runoff -- That part of the precipitation which appears in the surface
Scour - The result of erosive action of running water, primarily in
streams, excavating and carrying away material fro the bed and banks. Wearing
away by abrasive action.
Sediment -- Fragmentary material that originates from weathering of
rocks and is transported by, suspended in, or deposited by water.
Sheet Flow -- Any flow spread out and not confined; i.e., flow across a
flat open field.
Sinuosity -- The ratio of length of the river
to the length of the valley proper.
Slope -- (1) Gradient of a stream. (2) Inclination of the face of an
embankment, expressed as the ratio of horizontal to vertical projection; or (3)
The face of an inclined embankment or cut slope. In hydraulics it is expressed
as percent or in decimal form.
Slough -- (1) Pronounced SLU. A side or overflow channel in which water
is continually present. It is stagnant or slack; also a waterway in a tidal
marsh. (2) Pronounced SLUFF. Slide or slipping of a thin mantle of earth,
especially in a series of small movements.
Stage -- 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.
Steady Flow -- A flow in which the flow rate or quantity of fluid
passing a given point per unit of time remain s constant.
Storage -- Detention, or retention of water for future flow, naturally
in channel and marginal soils or artificially in reservoirs.
Storage Basin -- Space for detention or retention of water for future
flow, naturally in channel and marginal soils, or artificially in reservoirs.
Storm -- A disturbance of the ordinary, average conditions of the
atmosphere which, unless specifically qualified, may include any or all
meteorological disturbances, such as wind, rain, snow, hail, or thunder.
Storm Drain (or Storm-Drain System) -- is a combination of underground
conduits and surface-inlet structures constructed for the purpose of removing
runoff from the ground surface, usually from the street pavement, and conveying
it to some downstream discharge point.
Storm Water Management -- The recognition of adverse drainage resulting
from altered runoff and the solutions resulting from the cooperative efforts of
pu blic agencies and the private sector to mitigate, abate, or reverse those
Stream Power -- An expression used in predicting bed forms and hence bed
load transport in alluvial channels. It is the product of the mean velocity,
the specific weight of the water-sediment mixture, the normal depth of flow and
Stream Response -- Changes in the dynamic equilibrium of a stream by any
one, or combination of various causes.
Subcritical Flow -- In this state, gravity forces are dominant, so that
the flow has a low velocity and is often described as tranquil and streaming.
Also, that flow which has a Froude number less than one.
Street Conveyance -- is the process of utilizing an existing or newly
designed street to convey storm runoff to some downstream discharge point. Such
storm runoff is usually generated from adjacent lots and from the road surface
itself. This runoff is controlled, particularly for newly d esigned street
systems, in order to limit runoff quantity to a specified frequency flood
event, thus minimizing interference with vehicular traffic and containing the
runoff between the curbs and/or within the right-of-way/drainage easement.
Subdrain -- A conduit for collecting and disposing of underground water.
It generally consists of a pipe, with perforations in the bottom through which
water can enter.
Supercritical Flow -- In this state, inertia forces are dominant, so
that flow has a high velocity and is usually described as rapid, shooting and
torrential. Also, that flow which has a Froude number greater than one.
Surface Runoff -- (1) The movement of water on earth's surface, whether
flow is over surface of ground or in channels. (2) total precipitation minus
interception, evaporation, and infiltration.
Surface Storage -- Natural or man-made roughness of a land surface,
which stores some or all of the surface r unoff of a storm.
Surface Waters -- Surface waters are those which have been precipitated
on the land from the sky or forced to the surface in springs, and which 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 or water like lakes or seas.
Suspended Load -- Sediment that is supported by the upward components of
turbulent currents in a stream and that stay in suspension for appreciable
amount of time.
Swale -- A shallow, gentle depression in the earth's surface. this tends
to collect the waters to some extent and is considered in a sense as a drainage
course, although waters in a swale are not considered stream waters.
Thalweg -- The line following the
lowest part of a valley, whether under water or not. Usually the line following
the deepest part of the bed or channel of a river.
Time of Concentration -- The time required for storm runoff to flow from
the most remote point, in flow time, of a drainage area to the point under
Time to peak, Tp -- Time from beginning of rise to the peak
of the hydrograph.
Trash Rack -- A grid or screen across a stream designed to catch
Turbulent Flow -- That type of flow in which any particle may move in
any direction with respect to any other particle, and in which the head loss is
approximately proportional to the square of the velocity.
Undercut -- Erosion of the low part of a steep bank so as to compromise
stability of the upper part.
Unit Hydrograph -- A discharge hydrograph coming from one inch of direct
runoff distributed uniformly over the watershed, with the direct runoff
generated at a uniform rate during the given storm duration.
Unsteady Flow -- A flow in which the velocity changes with respect to
space and time.
Velocity Head -- A term used in hydraulics to represent the kinetic
energy of flowing water. This "head" is represented by a column of
standing water equivalent in potential energy to the kinetic energy of the
moving water calculated as (V2/2g) where the "V"
represents the velocity in feet per second and "g" represents the
potential acceleration due to gravity, in feet per second per second.
Watercourse -- A definite channel with bed and banks within which water
flows, wither continuously or in season. 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. The term does not include
artificial ch annels such as canals and drains, except natural channels,
trained or restrained by the works of man. Neither does it include depressions
or swales through which surface or errant water pass.
Watershed -- The area contributing direct runoff to a stream. Usually it
is assumed that base flow in the stream also comes from the same area.
Waterways -- That portion of a watercourse which is actually occupied by
Weir -- A low overflow dam or sill for measuring, diverting, or checking
Wetland -- A zone periodically or continuously submerged or having high
soil moisture, which has aquatic or riparian components or both, and is
maintained by imported water supplies in excess of those available through