Stormwater Calculations - Mathematics and Statistics for the Environment - page 761

Hydrologic soil group (HSG) —SCS classification system of soils based on the permeability
and infiltration rates of the soils. A-type soils are primarily sandy in nature with a high
permeability while D-type soils are primarily clayey in nature with a low permeability.
Hyetograph —A graph of the time distribution of rainfall over a watershed.
Impervious cover —A surface composed of any material that significantly impedes or pre-
vents natural infiltration of water into soil. Impervious surfaces include, but are not limited
to, roofs, buildings, streets, parking areas, and any concrete, asphalt, or compacted gravel
surface.
Impoundment —An artificial collection or storage of water, such as a reservoir, pit, dugout,
sump, etc.
Industrial stormwater permit —NPDES permit issued to a commercial industry for regulat-
ing the pollutant levels associated with industrial stormwater discharges. The permit may
specify on-site pollution control strategies.
Iniltration facility —A stormwater management facility which temporarily impounds runoff
and discharges it via infiltration through the surrounding soil. While an infiltration facil-
ity may also be equipped with an outlet structure to discharge impounded runoff, such
discharge is normally reserved for overflow and other emergency conditions. Because an
infiltration facility impounds runoff only temporarily, it is normally dry during nonrain-
fall periods. Infiltration trench, infiltration dry well, and porous pavement are considered
infiltration facilities.
Initial abstraction —The maximum amount of rainfall that can be absorbed under specific
conditions without producing runoff. It is also called initial losses .
Intensity —The depth of rainfall divided by duration.
Invert —The lowest flow line elevation in any component of a conveyance system, including
storm sewers, channels, weirs, etc.
Kjeldahl nitrogen —A measure of the ammonia and organic nitrogen present in a water
sample.
Lag time —The interval between the center of mass of the storm precipitation and the peak
flow of the resultant runoff.
Low impact development (LID) —Hydrologically functional site design with pollution preven-
tion measures to reduce impacts and compensates for development impacts on hydrology
and water quality.
Manning's formula —Equation used to predict the velocity of water flow in an open channel
or pipeline.
Micropool —A smaller permanent pool which is incorporated into the design of larger storm-
water ponds to avoid resuspension of particles, provide varying depth zones, and minimize
impacts to adjacent natural features.
Modified rational method —A variation of the rational method used to calculate the critical
storage volume whereby the storm duration can vary and does not necessarily equal the
time of concentration.
Nonpoint source pollution —Contaminants such as sediment, nitrogen and phosphorous,
hydrocarbons, heavy metals, and toxins whose sources cannot be pinpointed but rather are
washed from the land surface in a diffuse manner by stormwater runoff.
Normal depth —Depth of flow in an open conduit during uniform flow for the given conditions.
Ofline —Stormwater management system designed to manage a portion of the stormwater
that has been diverted from a stream or storm drain. A flow splitter is typically used to
divert the desired portion of the flow.
Online —Stormwater management system designed to manage stormwater in its original
stream or drainage channel.
Peak discharge —The maximum rate of flow associated with a given rainfall event or channel.
Percolation rate —The velocity at which water moves through saturated, granular material.