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~. <br />~. <br />;~,_. TOP BENCH DRAIN.DESI6N FOR CRDA N0. 2 AND N0. 3 <br />',::~~ <br />References: Procedures for Determining Peak Flow in Colorado, <br />SCS, March 1984 <br />Note: Caiculation based upon aaxiam area on top of the pile draining into <br />the top bench drain. <br />Area: 9.7 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 />S = 0.25(30)(780 + 800 + 720)/(9.7)(43560) = 0.04 <br />'J <br />,; ~~~ <br />Peak Flow Calculations <br />1 = Slope Length = 1200'; Exhibit 51 and 52 <br />Ae = Equivalent Area = {1/209) 1.667 <br />(1200/209) 1.667 = 18.4 acres <br />qe = Unadjusted Peak Discharge = 22.0 cfs/in runoff <br />(18.4 acres, 4X slope. Type 7I storm, CN 90) <br />Figure 5-lA Page 17, Peak Flows <br />Slope Ajustment = 1.00; Table E-1, Peak Flows <br />Qe = (22 cfs/in runoff)(1.27 in runoff) 1.00 = 27.9 <br />Qa = Actual Peak Flows = Qe (Aa/Ae) = 27.9 (9.7/18.4) = 14.7 cfs. <br />• <br />(New 10-31-90) <br />CRDA2 - 10 <br />