Subdivision constitutes a drainage basin with an area of 231 acres. The subdivis

Question

Subdivision constitutes a drainage basin with an area of 231 acres. The subdivision is drained by a
concrete (smoothed concrete) drainage canal that is 6 ft 2 in wide, 1 ft 9 in deep, has vertical walls, and has a
bottom slope of 0.009. The 25-year rainfall event (i.e. the most intense rainfall event that would on average occur
once in 25 years) has a precipitation rate of 1.8 inches per hour.
Using a runoff coefficient of 0.35, find the peak runoff rate than use the Manning eqn to determine if the drainage canal will accommodate runoff due to the 25-year event.

Base flow recession: Q=Qoe^(-at)

Manning Eqn: V=(1.49R^(2/3)*S^(1/2))/n

Explanation / Answer

1) Peak discharge Qp= 1/36 x K x pc x A

Here K is runoff coeffcient, pc rain intensity and A is area

Area of drainage basin A = 6'2"x1'9"

                                     = 7'11" (2.43m2)

There, Qp= 1/36 x 0.35 x 0.0457 x 2.43

               = 0.00107 m3/s

V= (1.49 R2/3xS1/2)/n

Assume the mannig's coefficient to be 0.15

R is hydraulic mean depth R =A/P

Area is 2.43 m2 and perimeter is P= (6'2"+1'9"+1'9")

                                                  = 1.87+0.533+0.533

                                                  = 2.936

Therefore, R = 2.43/2.936 = 0.827

V = (1.49 x R2/3x S1/2)/n

   = (1.49 x 0.8272/3 x 0.0091/2)/0.15

   = (1.49 x 0.88 x 0.99)/0.15

   = 8.65 m/sec

Hence, the drainage canal will accomodate runoff due to the 25 year event

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