Geoscience Reference
In-Depth Information
Extended detention basin-enhanced —An extended detention basin modified to increase pol-
lutant removal by providing a shallow marsh in the lower stage of the basin.
Filter bed —The section of a constructed filtration device that houses the filtering media.
Filter strip —An area of vegetation, usually adjacent to a developed area, constructed to
remove sediment, organic matter, and other pollutants from runoff in the form of sheet
flow.
First lush —The first portion of runoff, usually defined as a depth in inches, considered to
contain the highest pollutant concentration resulting from a rainfall event.
Floodplain —For a given flood event, that area of land adjoining a continuous water course
which has been covered temporarily by water.
Flow splitter —An engineered hydraulic structure designed to divert a portion of storm flow
to a BMP located out of the primary channel, or to direct stormwater to a parallel pipe
system, or to bypass a portion of baseflow around a BMP.
Forebay —Storage space, commonly referred to as a sediment forebay, located near a storm-
water BMP inlet that serves to trap incoming coarse sediments before they accumulate in
the main treatment area.
Freeboard —Vertical distance between the surface elevation of the design high water and the
top of a dam, levee, or diversion ridge.
Frequency (design storm frequency) —The recurrence interval of storm events having the
same duration and volume. The frequency of a specified design storm can be expressed
either in terms of exceedance probability or return period.
Exceedance probability —The probability that an event having a specified volume and
duration will be exceeded in one time period, usually assumed to be one year. If a
storm has a one- percent chance of occurring in any given year, then it has an exceed-
ance probability of 0.01.
Return period —The average length of time between events having the same volume
and duration. If a storm has a one- percent chance of occurring in any given year, then
it has a return period of 100 years.
Gabion —A flexible woven wire basket composed of rectangular cells filled with large cob-
bles or riprap. Gabions may be assembled into many types of structures such as revetments,
retaining walls, channel liners, drop structures, diversions, check dams, and groins.
Geographic Information System (GIS) —A method of overlaying spatial land and land use
data of different kinds. The data are referenced to a set of geographical coordinates and
encoded in a computer software system. GIS is used by many localities to map utilities and
sewer lines and to delineate zoning areas.
Grassed swale —An earthen conveyance system that is broad and shallow with check dams
and vegetated with erosion resistant and flood tolerant grasses, engineered to remove pol-
lutants from stormwater runoff by filtration through grass and infiltration into the soil.
Head —The height of water above any plane or object of reference; also used to express the
energy, either kinetic or potential, measured in feet, possessed by each unit weight of a
liquid.
Hydraulic Engineering Circular 1 (HEC-1) —A rainfall-runoff event simulation computer
model sponsored by the U.S. Corps of Engineers.
Hydric soil —A soil that is saturated, flooded, or ponded long enough during the growing
season to develop anaerobic conditions in the upper part.
Hydrodynamic structure —An engineered flow-through structure that uses gravitational set-
tling to separate sediments and oils from stormwater runoff.
Hydrograph —A plot showing the rate of discharge, depth, or velocity of flow vs. time for a
given point on a stream or drainage system.
Hydrologic cycle —A continuous process by which water is cycled from the oceans to the
atmosphere to the land and back to the oceans.
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