Calculate the design surface runoff from a 450 acre agricultural watershed in Pr

Question

Calculate the design surface runoff from a 450 acre agricultural watershed in Prairie View. Texas, The area consists of 150 acres of flat, cultivated open sandy loam soil and 300 acres of rolling (5 -10%) clay loam woodland. The maximum length of flow is 6000 feet, and has 2% slope. The return frequency for the storm is 50 years. Given: During a 60-min storm the following amounts of rain fell during successive 12-min intervals: 1.99 in, 1.86 in. 1.65 in. 0.73 in. and 0.07 in. The watershed is located near Prairie View. Texas, and totals 1200 acres with a runoff coefficient. C = 0.32. Questions: What is the maximum rainfall intensity for 12 minutes? What is the average rainfall intensity? What is the peak runoff rate from the storm?

Explanation / Answer

2. a).1.99 in is the maximum rainfall intensity for 12 minutes.

    b).1.99+1.86+1.65+0.73+0.07= 6.3in 6.3/5 = 1.26in is the average rainfall intensity.

    c).   Q=ciA     : where Q is the runoff , c is the runoff coefficient , i is the intensity , A is the area .

Q=0.32*6.3*1200 = 2419.2 ft3/s (cubic foot/second)

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