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It has long been recognized that urban development has a pronounced effect on the
rate of runoff from a given rainfall event. The hydraulic efficiency of a drainage
area is generally increased as a byproduct of urbanization which in effect reduces
the storage capacity of a watershed. This reduction of a watershed's storage capacity
is a direct result of the elimination of pervious surfaces, small ponds, and holding
areas. This comes about by the grading and paving of building sites, streets, drives,
parking lots, and sidewalks and by construction of buildings and other facilities
characteristic of urban development. The result of the improved hydraulic efficiency
is illustrated graphically in Figure 2-1 in Appendix D of this manual, which is a
plot of the runoff rate versus time for the same storm with two different stages of
watershed development.
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