Using the rational formula, determine the runoff rate for a storm of 100-year frequency. Use Table 16.2 for runoff coefficients. Assume that the rainfall intensity curves in Figure 16.2 are applicable to this drainage area and the following land characteristics apply. Use Figure 16.4 to calculate average velocity using "fallow or minimum tillage cultivation" ground cover. Overland flow length = 0.5 miles. Average slope of overland area = 2%
A 196-acre rural drainage area consists of four different watershed areas as follows:
Steep grass covered area = 35%
Cultivated area = 15%
Forested area = 40%
Turf meadows = 10%
What will be an ideal response?
Determine the runoff rate for a 100-year storm.
Determine the runoff coefficients for each of the four groundcover-type areas
from Table 16.2.
Calculate the weighted run-off coefficient using Equation 16.1.
Cw = 196 [(0.35)(0.60) + (0.15)(0.30) + (0.40)(0.20) + (0.10)(0.25)] / 196
Cw = 0.36
Estimate the average velocity using Figure 16.4. This area can be considered as
Fallow or minimum tillage cultivation while average slope is 2%.
v = 0.65 ft/sec
Determine travel time using Equation 16.2,
Ti = L / 3600 V
Ti = (0.5)(5280) / (3600)(0.65)
Ti = 1.128 h
Since only one segment is being investigated, Ti = Tc.
Determine the rainfall intensity, using Figure 16.2.
I = 2.75 in/h
Determine the runoff flow rate using the rational formula (Equation 16.4).
Q = CIA
Q = (0.36)(2.75)(196)
Q = 194.04 ft3/sec
Therefore, the runoff flow rate for this watershed will be 194 ft3/sec.
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