Page 438 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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Chapter 12

Urban Runoff and Combined

Sewer Overflow Management

12.1 HYDROLOGIC IMPACTS OF URBANIZATION
Because stormwater flow quantities are high, reaching many orders of magnitude greater
than dry-weather flows, control, whether through flow balancing, multiple uses of facilities,
runoff retardation, or combinations thereof, is the focus of cost-effective planning for
runoff management.
When precipitation contacts the ground surface, it can take several paths. These include
returning to the atmosphere by evaporation; evapotranspiration, which includes direct
evaporation and transpiration from plant surfaces; infiltration into the ground surface; reten-
tion on the ground surface (ponding); and traveling over the ground surface (runoff).
Altering the surface that precipitation contacts alters the fate and transport of the runoff.
Urbanization replaces permeable surfaces with impervious surfaces (e.g., roof tops, roads,
sidewalks, and parking lots), which typically are designed to remove rainfall as quickly as pos-
sible. As seen in Fig. 12.1, increasing the proportion of paved areas decreases the infiltration

100%

80

60

40

20

0
Natural 10 20% 35 50% 75 100%
cover paved paved paved
Legend
Evapotranspiration Runoff Infiltration
Figure 12.1 Typical Changes in Runoff Flow Resulting
from Paved Surfaces
(Source: U.S. Environmental Protection Agency)

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