Compute rate of runoff using the rational formula for a 256-acre rural drainage area consisting of three different watershed areas as follows:
Steep grass area = 15%
Forested area = 30%
Cultivated fields = 55%
If the time of concentration for this area is 2.4 hours, determine the runoff rate for a
storm of 50-year frequency. Use the rainfall intensity curves in Figure
16.2. Use Table 16.2 for runoff coefficients.
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Determine the runoff coefficients for each of the three groundcover-type areas from Table 16.2.
Calculate the weighted run-off coefficient using Equation 16.1,
Cw = 256 [(0.15)(0.60) + (0.30)(0.20) + (0.55)(0.30)] / 256
Cw = 0.315
Determine the rainfall intensity, using Figure 16.2.
I = 1.45 in/h
Determine the runoff flow rate using the rational formula (Equation 16.4).
Q = CIA
Q = (0.315)(1.45)(256)
Q = 116.9 ft3/sec
Therefore, the runoff flow rate for this watershed will be 117 ft3/sec.
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