Question

Example: Estimate the runoff generated by a 50 yr rainstorm that lasts 30 minutes in watershed near Tuscaloosa. The watershed is 20 acres, mainly hilly with 15% of the area pavements and roofs and the rest are woodland and forest.

Answer 1

I've considered average precipitation of rainfall for 30mis is 1" = 0.083 ft

total area of watershed = 871200 sq ft

area of pavements and roof = 871200 * 0.15

=130680 sft

rest of area(woodland and forest) = 871200-130680

=740520

(note -1)At pavements ( at pavements runoff will be 100%)

   2) at grasslands or forest runoff will be 60%) )

runoff = precipitation of rainfall x area x runoff percentage

Total runoff = runoff at rooftops and pavements + runoff at forest

   =0.083 * 130680 * 1 + 0.083 *740520 * 0.6

   =10846.44 + 36877.8

   =477254.24 cubic feet

NOTE-

I've considered intensity of rainfall for 30min is 1"