Geoscience Reference
In-Depth Information
Cradle —A structure usually made of concrete shaped to fit around the bottom and sides of a
conduit to support the conduit, increase its strength and, in dams, to fill all voids between
the underside of the conduit and soil.
Crest —The top of a dam, dike, spillway or weir, frequently restricted to the overflow portion.
Curve number (CN) —A numerical representation of a given area's hydrologic soil group,
plant cover, impervious cover, interception and surface storage derived in accordance with
Natural Resource Conservation Service methods. This number is used to convert rainfall
depth into runoff volume. It is sometimes referred to as the runoff curve number (RCN) .
Cut —A reference to an area or material that has been excavated in the process of a grading
operation.
Design storm —A selected rainfall hyetograph of specified amount, intensity, duration, and
frequency that is used as a basis for design.
Detention basin —A stormwater management facility which temporarily impounds runoff
and discharges it through a hydraulic outlet structure to a downstream conveyance system.
While a certain amount of outflow may also occur via infiltration through the surrounding
soil, such amounts are negligible when compared to the outlet structure discharge rates
and, therefore, are not considered in the facility's design. Since an extended detention basin
impounds runoff only temporarily, it is normally dry during nonrainfall periods.
Disturbed area —An area in which the natural vegetative soil cover or existing surface treat-
ment has been removed or altered and, therefore, is susceptible to erosion.
Diversion —A channel or dike constructed to direct water to areas where it can be used,
treated, or disposed of safely.
Drainage basin —An area of land that contributes stormwater runoff to a designated point.
Also called a drainage area or, on a larger scale, a watershed.
Drop structure —A device constructed to transition water to a lower elevation.
Duration —The length of time over which precipitation occurs.
Embankment —A manmade deposit of soil, rock or other material used to form an
impoundment.
Energy dissipator —A device used to reduce the velocity or turbulence of flowing water.
Erosion —The wearing away of the land surface by running water, wind, ice, or other geologi-
cal agent.
Accelerated erosion —Erosion in excess of what is presumed or estimated to be natu-
rally occurring levels and which is a direct result of human activities.
Gully erosion —Erosion process whereby water accumulates in narrow channels and
removes the soil to depths ranging from a few inches to 1 or 2 feet to as much as 75 to
100 feet.
Rill erosion —Erosion process in which numerous small channels only several inches
deep are formed.
Sheet erosion —Spattering of small soil particles caused by the impact of raindrops
on wet soils. The loosened and spattered particles may subsequently be removed by
surface runoff.
Exiltration —The downward movement of runoff through the bottom of a stormwater facility
and into the soil.
Extended detention basin —A stormwater management facility that temporarily impounds
runoff and discharges it through a hydraulic outlet structure over a specified period of
time to a downstream conveyance system for the purpose of water quality enhancement or
stream channel erosion control. While a certain amount of outflow may also occur via infil-
tration through the surrounding soil, such amounts are negligible when compared to the
outlet structure discharge rates and, therefore, are not considered in the facility's design.
Since an extended detention basin impounds runoff only temporarily, it is normally dry
during nonrainfall periods.
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