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BENCH DRAIN DESI6l1 FOR CRDA N0. 2 AND N0. 3 <br />References: Procedures for Determining Peak Flow in Colorado. <br />SCS, March 1984 <br />Note: Calculated for largest area draining into a bench drain below the top <br />bench. <br />Area: 1.7 Acres = Aa <br />CN: 90 <br />Precipitation Event: 100 Year - 24 hour <br />Precipitation: 2.2 inches; Figure S-7, Peak Flows <br />Direct Runoff: 1.27 inches; Page 70, Peak Flow <br />Average Slopes: S = 0.25 z (Lc25 + Lc50 + Lc75)/DA <br />z = Elevation Difference. Ft. <br />Lc~ = Contour Lengths, Ft. <br />Da = Drainage Area, Ftz <br />5 = 0.25(30)(800 + 800 + 800)/(1.7)(43560) = 0.24 <br />~, • <br />Peak Flow Calculations <br />1 Slope Length 830'; Exhibit 51 and 52 <br />Ae = Equivalent Area = (1/209) <br />(830/209) = 10 acres <br />qe = Unadjusted Peak Discharge = 18 cfs/in runoff <br />(10 acres, 16X slope, Type II storm, CN 90) <br />Figure S-IA Page 18, Peak Flows <br />Slope Adjustment = 1.06; Table E-1. Peak Flows <br />Qe = (I8 cfs/in runoff)(1.27 in runoff) 1.06 = 24.2 <br />Qa = Actual Peak Flows = Qe (Aa/Ae) = 24.2 (1.7/10) = 4.1 cfs <br />Culvert Size <br />12" CMP with 12" cover will carry 4.5 cfs <br />- • (New 10-31-90) <br />CRDA3 - 11 <br />