Question

Apply the Modified Rational Method to a 7 acre watershed with a time of concentration of 0.80 hours and a runoff coefficient of 0.7.

a.Determine and plot the direct runoff hydrograph for a 1-hour storm that drops 2 inches of rain. What is the peak flow, time to peak, and base time?

b.Determine and plot the direct runoff hydrograph for a 10-minute storm that drops 0.8 inches of rain. What is the peak flow, time to peak, and base time?

Answer 1

Drainage area in acres = A = 7 acres

Runoff coefficient = C = 0.7

Time of concentration = Tc = 0.8 hours

Tp Time to peak = the time from the center of mass of rainfall excess to the peak of the hydrograph.

Base time Tb = Total duration of hydrograph.

a)

Rainfall intensity in inches/hr = i = 2 inches

Q (Peak flow) = C * i * A

    = 0.7 * 2 * 7 = 9.8 cubic feet per second

Tp (time to peak) = 0.4 Hours

Base time = Tb = 1.7 Hours

b) Rainfall intensity in inches/hr = i = 0.8*6 = 4.8 inches (convert 10 min to 1 hour)

Q (Peak flow) = C * i * A

    = 0.7 * 4.8 * 7 = 23.52 cubic feet per second

Highest Q will be the reached in 10 min. = 23.52/6 = 3.92

Tp (Time to peak) = 0.05 Hours

Base time = Tb = 0.1 Hours ( as it takes almost same time for water to decrease)