RAINWATER AS THE RESOURCE - Low Impact Development and Sustainable Stormwater Management - page 21

2 ″
Evaporative
loss from
impervious
surfaces

Rainfall
38 ″ /yr

Reduced infiltration
through regraded and
compacted soils in
grasses

36 ″ Runoff from
impervious cover

0” of infiltration
under impervious
surfaces

Reduction in base
flow by 10.5

/yr
under impervious
surfaces

″

Figure 1-16 The impact of land development on the hydrologic cycle.

built environment to the remnant surface flow pathways and streams as quickly
and directly as possible and to assure that both buildings and roadways can be
utilized in the most severe rainfalls. Although this makes a great deal of sense
from the building and transportation perspective, the net result was and is a
dramatic and rapid increase in the rate and volume of runoff from a developed
landscape. Observing this effect from a position downstream of the developed
land, Figure 1-17 illustrates the rise in stream flow produced by a sudden increase
in runoff from the upland drainage. This hydrograph of flow rate versus time
defines a volume of runoff produced within a river basin, watershed, or catchment
as the area contained within the figure, and is the most important aspect of the
hydrograph measure.
Regardless of where land development occurs, the increases in imperviousness,
the changes in vegetation, and the soil compaction associated with that devel-
opment result in significant increases in runoff volume. The relative increase
in runoff volume varies with event magnitude (return period). For example, the
2-year rainfall of 3.27 in. each 24 hours in southeastern Pennsylvania will result
in an increase in runoff volume of 2.6 in. from every square foot of impervious
surface placed on well-drained HSG B soil in woodland cover (Figure 1-18).
For larger events, as the total rainfall increases, the net runoff also increases, but
less than proportionately. For example, total rainfall for the 100-year storm is
twice the rainfall for the 2-year storm (7.5 in. vs. 3.27 in.); however, the increase
in runoff is only 1.7 in. (4 . 3 − 2 . 6 in.). This pattern holds true throughout the
United States.