A 20 acre plot of gently sloping (o.008 f/ft pasture (short grass) land is being

Question

A 20 acre plot of gently sloping (o.008 f/ft pasture (short grass) land is being developed into a typical St Louis subdivision on acre lots. The peak runoff takes 5 minutes to travel from the roof of the house to the street gutter. The velocity in the gutter is 2 for 300 feet. The velocity in the storm sewer system is 5 ft/sec for a distance of 1000 feet. The existing velocity in the natural flow path is 2 ft/sec for 3000 feet with an ini travel time of minutes. Use the IDF curves attached for St Louis. (a) (10 pts) determine the time of concentration for pre and post development (b) (6 pts) determine the runoff coefficients for pre and post development for the 2-year, 10 year and events (c) (18 pts) Compute the peak discharge for pre and post development discharge for the 2 year, 10-year and 100-year events with the Rational formula

Explanation / Answer

a)The time of concentration for pre and post development is 4 and 16 minutes.

b)The Run off coefficient for 2,10,and 100 years would be 5,10 and 200(nearly double the times).

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