RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 20

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Figure 1-15 Hydrograph of Meeting of the Waters Creek, as affected by impervious
surfaces.

the original stream system replaced by a network of sewer pipes that discharge
to the remnant stream channel (Figure 1-14B).
The increase in runoff volume produced by this increase in impervious cover
is dramatic (Figure 1-15), as illustrated during the 2-year-frequency rainfall. The
increase in runoff volume (the area under the hydrograph) affects the downstream
riparian corridor, eroding stream banks and conveying the pollutant load from
the impervious surfaces as well as the channel to the large Lake Jordan, a future
water supply reservoir, several miles downstream [16].
Impervious surfaces, be they rooftops, pavements, or streets, turn every drop
of rainfall into direct and immediate runoff. In addition, the recharge of ground-
water aquifers is effectively reduced or prevented altogether. Finally, the energy
of runoff scours every pollutant that is dripped, dropped, spilled, or spread on
the impervious (and pervious) land surfaces, and convey this non-point source
pollution to surface waters. In all regions of the country, this impact can be
understood by comparing the runoff volume increase from pre- to postdevelop-
ment. For example, in the mid-Atlantic, where the annual rainfall reaches 45 in.
and natural runoff is on the order of 8 inches, the net increase in runoff from
impervious surfaces is 36 in. (3 ft) per year. One need only consider a typical
parking lot surface as shown in Figure 1-13, and picture it covered with this
depth of increased runoff, to fully understand the underlying cause of stormwater
impacts from land development.

Increased Volume of Runoff

Figure 1-16 represents the same unit area as that shown in Figure 1-2, and
illustrates the net impact of land development and impervious surfaces on the
hydrologic cycle. To describe the process in simple terms, any impervious surface
results in direct rainfall being converted into immediate and almost total runoff.
Our building programs have always tried to transport this runoff away from the