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P'120N'1'IEP. I3ANIC <br />ST012M ll12AINnG1, SYSTBM RUNOP'1•' CFILCULn'PIONS <br />In all storm drain�ye systems the Rational Method for computinci <br />peak discharge shall be used. The Rati.onal Method employs :-he <br />formula: Q= C A I <br />(� = Runofi in cubic feet per second (cis) <br />C == Coefficient of Runof-f <br />n= Contributing drainage basin in acres <br />T= Rainfall intensity in inches per hour <br />Compute Q = CAI <br />Predevelopment Itunoff <br />C = 0.90 <br />7, = 0.82G acres <br />I= 1.3 inches/hour (20 minute duration) <br />(0.9)(0.626)(1.3 inches/hour) = 0.966 CFS <br />Post deVelopment RunofS <br />C = 0.82 <br />A = 0.826 acr.es <br />I= 1.3 inches/hour (20 minute duration) <br />(0.82�(0.8'_'G)(1.3 inches/hour) = 0.391 CPS <br />Predevelopn;ent ltunoff = D•966 cfs <br />Post development Runoff = 0.881 cis <br />Runoff due to development = 0.0 cis <br />Therefore, cn site rete:ntion wi11 not i�e re��_iir.;d. A restrict- <br />ing orifice will not be installed since the post �unoff caill be <br />less tnan i�.he predeveloped runoff. <br />The existing 12° storm sewer is capable oi carrying f3.87 CPS <br />at 0.0518 feet/foot slope. <br />� _ 1.4(36 ��, ?�3 S 1/2 <br />' n <br />� = 38.9G x 0.2275�G <br />n = II.87 CI'S <br />