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<br />DRAINAGE CRITERIA MANUAL <br /> <br />RUNOFF <br /> <br />Step 4. Determine Cp using Figure 4-3, the value of imperviousness, the . <br />given bas in area, the Ct found inStep 2 and Equati on 4-6. <br />From Figure 4-3: <br />P = 6.21 <br /> <br />Then <br /> <br />C = P C AO .15 <br />P t <br />= 6.21 x 0.091 x (0.38)0.15 <br /> <br />= 0.49 <br /> <br />(Equation 4-6) <br /> <br />Step 5. Determine qp using Equation 4-3. <br />qp = 640 Cp/tp <br />= 640 x 0.49/0.225 <br />= 1394 cfs/sq. mi. <br />Step 6. Determine Qp using Equation 4-4. <br />Qp = qpA <br />= 1394 x 0.38 <br /> <br />. <br /> <br />= 530 cfs <br /> <br />Step 7. Determine Tp using Equation 4-2. <br />Tp = 60 tp + 0.5 tu <br />= (60 x 0.225) + (0.5 x 5.0) <br /> <br />= 16.0 minutes <br /> <br />Step 8. Determine the width of the unit hydrograph and the portion of <br />that width ahead of Q at 50 and 75 percent of Q using Figures <br />p p <br />4-4 and 4-5. <br />W50% = 0.359 hours (21.0 minutes) <br /> <br />W75% = 0.186 hours (11.2 minutes) <br /> <br />W50% ahead of Q = 0.35 x 21 <br />P <br />= 7.4 minutes* <br /> <br />. <br /> <br />5-1-84 <br />