The proportion of the total rainfall that will reach the drainage system depends on
the surface vegetation condition, soil type, imperviousness of the surface, land slope
and ponding characteristics of the area. Impervious surfaces, such as asphalt pavements
and roofs of buildings, will be subject to approximately 100 percent runoff (regardless
of the slope). On-site inspections and aerial photographs may prove valuable in estimating
the nature of the surfaces within the drainage area.
It should be noted that the runoff coefficient "C" is the Rational Method variable
which is least amenable to precise determination. A reasonable coefficient must be
chosen to represent the integrated effects of infiltration, surface ponding, evaporation,
flow routing and interception, all of which affect the time distribution and peak
rate of runoff.
It is often desirable to develop a composite runoff coefficient based upon the percentages
of different types of surfaces in the drainage area. This procedure is often applied
to typical "sample blocks" as a guide to selection of reasonable values of the coefficient
for an entire area. Suggested coefficients with respect to specific surface types
are given in Table 2-1. "C" values for developed conditions should be based on maximum
allowable impervious cover as listed in the City's zoning and watershed ordinances.
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