Stormwater Calculations - Mathematics and Statistics for the Environment - page 762

Point source —The discernible, confined and discrete conveyance, including but not limited
to, any pipe, ditch, channel, tunnel, conduit, well, container, concentrated animal feeding
operation, or landfill leachate collection system from which pollutants may be discharged.
This term does not include return flows from irrigated agriculture or agricultural storm
water runoff.
Porosity —The ratio of pore or open space volume to total solids volume.
Principal spillway —The primary spillway or conduit for the discharge of water from an
impoundment facility; generally constructed of permanent material and designed to regu-
late the rate of discharge.
Rational method —Means of computing peak storm drainage flow rates based on average
percent imperviousness of the site, mean rainfall intensity, and drainage area.
Recharge —Replenishment of groundwater reservoirs by infiltration and transmission of
water through permeable soils.
Redevelopment —Any construction, alteration, or improvement on existing development.
Retention —Permanent storage of stormwater.
Retention basin —A stormwater management facility which includes a permanent impound-
ment, or normal pool of water, for the purpose of enhancing water quality and, therefore, is
normally wet, even during nonrainfall periods. Storm runoff inflows may be temporarily
stored above this permanent impoundment for the purpose of reducing flooding or stream
channel erosion.
Rip-rap —Broken rock, cobbles, or boulders placed on earth surfaces such as the face of a dam or
the bank of a stream for the protection against erosive forces such as flow velocity and waves.
Riser —A vertical structure which extends from the bottom of an impoundment facility and
houses the control devices (weirs/orifices) to achieve the desired rates of discharge for
specific designs.
Roughness coefficient. —A factor in velocity and discharge formulas representing the effect of
channel roughness on energy losses in flowing water. Manning's ' n ' is a commonly used
roughness coefficient.
Routing —A method of measuring the inflow and outflow from an impoundment structure
while considering the change in storage volume over time.
Runoff —The portion of precipitation, snow melt, or irrigation water that runs off the land into
surface waters.
Runoff coeficient —The fraction of total rainfall that appears as runoff. Represented as C in
the rational method formula.
Safety bench —A flat area above the permanent pool and surrounding a stormwater pond
designed to provide a separation to adjacent slopes.
Sand filter —A contained bed of sand which acts to filter the first flush of runoff. The runoff
is then collected beneath the sand bed and conveyed to an adequate discharge point or
infiltrated into the in-situ soils.
SCS —Soil Conservation Service (now called Natural Resource Conservation Service,
NRCS), a branch of the U.S. Department of Agriculture.
Sediment forebay —A settling basin or plunge pool constructed at the incoming discharge
points of a stormwater facility.
Soil test —Chemical analysis of soil to determine the need for fertilizers or amendments for
species of plants being grown.
Stage —Water surface elevation above any chosen datum.
Storm sewer —A system of pipes, separate from sanitary sewers, that only carries runoff from
buildings and land surfaces.
Stormwater filtering (or iltration) —A pollutant removal method to treat stormwater runoff
in which stormwater is passed through a filter media such as sand, peat, grass, compost, or
other materials to strain or filter pollutants out of the stormwater.