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12.1 Hydrologic Impacts of Urbanization 399

Postdevelopment

Discharge rate Predevelopment

Time
Figure 12.2 Pre- and Postdevelopment Hydraulics
(Source: U.S. Environmental Protection Agency)

and evapotranspiration paths of precipitation, thus increasing the amount of precipitation
leaving an area as runoff.
In addition to magnifying the volume of runoff, urban development increases the peak
runoff rate and decreases travel time of the runoff. When mechanisms that delay entry of
runoff into receiving waters (i.e., vegetation) are replaced with systems designed to remove
and convey stormwater from the surface, the stormwater’s travel time to the receiving waters
is greatly reduced, as is the time required to discharge the stormwater generated by a storm.
Figure 12.2 shows an urban area’s typical predevelopment and postdevelopment discharge
rates over time.
The following changes to hydrology might be expected for a developing watershed:
1. Increased peak discharges (by a factor of 2 to 5)
2. Increased volume of storm runoff
3. Decreased time for runoff to reach stream
4. Increased frequency and severity of flooding (see Fig. 12.3)
5. Reduced stream flow during periods of prolonged dry weather (loss of base
flow)
6. Greater runoff and stream velocity during storm events.
Each of these hydrologic changes can lead to increased pollutant transport and loading
to receiving waters. As peak discharge rates increase, erosion and channel scouring be-
come greater problems. Eroded sediments carry nutrients, metals, and other pollutants. In
addition, increases in runoff volume result in greater discharges of pollutants. Pollution
problems, therefore, multiply with increased urbanization.
Changes in hydrology affect receiving waters through channel widening and subse-
quent stream bank erosion and deposition, increased stream elevation due to greater dis-
charge rates, and an increased amount of sedimentary material within a stream due to
stream bank erosion. The decrease in the ground surface’s infiltration capacity and loss of
buffering vegetation undermines a significant mechanism for pollutant removal, thereby
increasing the load entering the receiving waters. Hydrologic changes can result in more
subtle but equally important impacts. Removal or loss of riparian vegetation due to ero-
sion, for example, can increase stream temperature as levels of direct sunlight increase,