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ILLUSTRATIVE EXAMPLE �1 <br />Area of site 1.37 acres <br />ROC before development 0.15 sue! <br />ROC after development 0.90 ''^'• <br />Design storm - predevelopment freq 1 in 10 years>: <br />Post development freq 1 in 10 years <br />Duration 20 mins <ti_Z <br />intensity 1.3 ins <br />/hr ,.�.• <br />Predevelopment runoff <br />0.15 1.2 x 1 x 1.37 x 4356 cubic feet " 7 <br />3 12 <br />0.15 x 2155 cubic feet <br />323 cubic feet <br />Post development runoff = 0.90 x 2155 cubic feet <br />= 1940 cubic feet <br />Runoff differential 1617 cubic feet <br />Additional storage due to lag time equals (time of concentration)X(mean rate <br />of discharge over the period). <br />Assume time of concentration - 5 min. <br />Rate of discharge = 0.27 cubic ft. per see <br />Mean rate of discharge = 0.135 cfs'.. ' <br />Additional storage required = 0.135 x 300 <br />- 40 cft <br />Detention storage proposed - 1657 cft <br />Orifice opening 2-1,4 inches- <br />• discharge =[.15 x 1.3 x 1.3Z7cfs <br />- 0.27 cfs <br />head = 3.76 feet <br />Outlet pipe <br />Diameter 12 inches <br />Slope 1 ft. in 200 <br />- •-- <br />Capacity <br />Q�0.463 D�3 •S����! <br />N <br />-0.463x1x.07 <br />.012 <br />w 2.73 cfs-'u <br />From nortograph Q = 2.7 cfs <br />