Environmental Engineering Reference
In-Depth Information
2
STORMWATER
HYDROLOGY
AND QUALITY
2.1 OVERLAND FLOW: THE BEGINNING OF RUNOFF
Many excellent textbooks explain and explore the subject of hydrology [1, 2],
and other references discuss the chemistry of pollutants conveyed in stormwa-
ter runoff [3, 4]. The suspension, dissolution, and transport of pollutants in
stormwater are complex processes that are best understood by considering a
few simple questions. The first question is: How does surface runoff begin?
This process is considered the “hydrologic response” in a given catchment,
watershed, or drainage basin, and the transport of surface pollutants which are
scoured from that drainage area and conveyed downstream starts with surface
runoff.
The physical model of this process under natural conditions was illustrated in
Figure 1-2 with a surface that is vegetated in woodland on a natural soil mantle
of well-drained material overlaying bedrock with a fairly shallow water table (10
to 15 ft below the surface). At the initiation of precipitation, the rate of rain-
fall follows the temporal pattern shown in Figure 1-9. When the rate of rainfall
that reaches the soil surface exceeds the rate of infiltration, the surface becomes
saturated. Then the pull of gravity begins to move the surface moisture down-
gradient across the land surface, following the path of least resistance. This fluid
movement is by no means uniform and is discontinuous, with numerous “nooks
and crannies” serving as intermediate storage areas. In effect, the flow of rainfall
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