Stormwater Calculations - Mathematics and Statistics for the Environment - page 763

Stormwater hot spot —An area where the land use or activities are considered to generate
runoff with concentrations of pollutants in excess of those typically found in stormwater.
Stream buffers —The zones of variable width, which are located along both sides of a stream
and are designed to provide a protective natural area along a stream corridor.
Surcharge —Flow condition occurring in closed conduits when the hydraulic grade line is
above the crown of the sewer. This condition usually results in localized flooding or storm-
water flowing out the top of inlet structures and manholes.
SWMM (Storm Water Management Model) —Rainfall-runoff event simulation model spon-
sored by the USEPA.
Technical Release No. 20 (TR-20), Project Formulation: Hydrology —SCS watershed hydrol-
ogy computer model that is used to compute runoff volumes and route storm events through
stream valleys and/or impoundments.
Technical Release No. 55 (TR-55), Urban Hydrology for Small Watersheds —SCS watershed
hydrology computation model that is used to calculate runoff volumes and provide a sim-
plified routing for storm events through stream valleys and/or ponds.
Time of concentration —The time required for water to flow from the hydrologic most distant
point (in time of flow) of the drainage area to the point of analysis (outlet). This time will
vary, generally depending on the slope and character of the surfaces.
Trash rack —A structural device used to prevent debris from entering a spillway or other
hydraulic structure.
Travel time —The time required for water to flow from the outlet of a drainage sub-basin to
the outlet of the entire drainage basin being analyzed. Travel time is normally concentrated
flow through an open or closed channel.
Ultimate condition —Full watershed build-out based on existing zoning.
Ultra-urban —Densely developed urban areas in which little pervious surface exists.
Urban runoff —Stormwater from city streets and adjacent domestic or commercial properties
that carries nonpoint source pollutants into the sewer systems and receiving waters.
Water quality window —The volume equal to the first 1/2 inch of runoff multiplied by the
impervious surface of the land development project.
Water surface profile —Longitudinal profile assumed by the surface of a stream flowing in an
open channel; hydraulic grade line.
Water table —Upper surface of the free groundwater in a zone of saturation.
Watershed —A defined land area drained by a river, stream, or drainage way, or system of
connecting rivers, streams, or drainage ways such that all surface water within the area
flows through a single outlet.
Wet weather low —Combination of dry weather flows and stormwater runoff.
Wetted perimeter —The length of the wetted surface of a natural or artificial channel.
25.3 HYDROLOGIC METHODS
Hydrology is the study of the properties, distribution, and effects of water on the earth's surface, and
in the soils, underlying rocks, and atmosphere. The hydrologic cycle (see Figure 25.1) is the closed
loop through which water travels as it moves from one phase, or surface, to another. Water, lost
from the Earth's surface to the atmosphere either by evaporation from the surface of lakes, rivers,
and oceans or through the transpiration of plants, forms clouds that condense to deposit moisture on
the land and sea. A drop of water may travel thousands of miles between the time it evaporates and
the time it falls to Earth again as rain, sleet, or snow. The water that collects on land flows to the
ocean in streams and rivers of seeps into the earth, joining groundwater. Even groundwater eventu-
ally flows toward the ocean for recycling (see Figure 25.1). When humans intervene in the natural
water cycle, they generate artificial water cycles or urban water cycles (local subsystems of the water
cycle, or integrated water cycles; see Figure 25.2) (Spellman and Drinan, 2000).