Practice Question: Rational Method Application for Calculating Maximum Runoff of a Watershed

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Solution

Time of concentration = 45 minutes 

From the Intensity-Duration-Frequency curves for a 50-year storm and a of duration of 45 minutes, the intensity (I) is 4 inches per hour.

According to the table, the runoff coefficient for a downtown area ranges from 0.70 to 0.95. To calculate the maximum runoff rate, consider the upper value of 0.95.

Using the Rational Method, runoff is given by:

\[Q=CiA=0.95\times 4\times 70=266\ ac.\frac{in}{hr}=268\ ft^3/sec\]

\(1\ ac.\frac{in}{hr}=1.008\frac{ft^3}{sec}\)
 

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