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Calculations <br />2 -Year, 24 -Hour Precipitation Event = 1.1" <br />Inches of Runoff @ CN = 87 = 0.28" <br />CN = 87 = 0.12" <br />CN = 75 = .05" <br />(Source - Procedures for Determining Peak Flows in Colorado, pp 52, 57, 64; <br />attached for reference). <br />CFS/in of runoff - from Figure II -2, Peak Rates of Dischjarge for Small <br />Watersheds, Procedures for Determining Peak Flows in Colorado, p17; attached <br />for reference. <br />Peak Flow = (CFS/in)(inches of runoff)(Slope Adjustment factor) <br />Slope = 14%. <br />Slope adjustment factor = .98 where area < 50 acres. <br />.97 where 50 < area < 100. <br />.96 where area > 100 acres. <br />(Source - Table II -6, Slope adjustment factors by drainage areas, Procedures <br />for Determining Peak Flows in Colorado; attached for reference). <br />Flow Depth (D) = (Qn/KS1/2).375 where Q = peak flow. <br />n = roughness coefficient = .035. <br />K = discharge factor = 14.8 for D/b = <br />• .1, sideslopes at 1:1. <br />S = .05 (ft/ft) <br />Ditch -Bottom Width (B) = D x 10. <br />Velocity (V) = 1.49/n R 2/3 S 1/2 where,n_= roughness....___._ _. .. . <br />coefficient = .035. <br />R = hydraulic radius. <br />5 = ditch gradient (ft/ft). <br />REVISED FEB 13 '87 <br />