Practice Question: Runoff Analysis

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Solution

Answer: B) 52 cfs

Refer to PE Civil Reference Handbook Section 6.5.2.1

Rational Method Equation:

\[Q=CIA\]

where:

\(Q= Peak \ discharge\ (ft^3/sec)\\\)

\(I = Rainfall \ Intensity \ (in/hr)\)
\(C= Runoff\ Coefficient\)

\(A = Area \ (acre)\)

Refer to the Runoff Coefficients for Rational Formula Table in Section 6.5.2.1.

For single-family area:

\[C = 0.3\]
For undeveloped area:

\[C = 0.1\]

Calculate the Weighted Average:

\[C_{w} = \frac{( A_{1} C_{1} + A_{2} C_{2})}{A_{total}}=(0.6\times0.3+0.4\times0.1) = 0.22\]
From the given storm frequency/intensity figure:

\[{t_{c}} \text{= 40 min, I = 3.9 in/hr}\\\]

\[A = 60\ acre\]

\[Q=CIA = 0.22 \times 3.9 \times 60 = 51.5\ ft^3/sec\]

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