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Table 4.8-4. Peak flows and ditch design for a typical 10 -acre drainage <br />•� area. <br />Drainage area 10 acres <br />Average slope 14% <br />2 -year, 24-hour rainfall 1.1 inches <br />Curve number 87 <br />Runoff .28 inches <br />Slope adjustment factor .98 <br />Peak flow, 2 -year, 24-hour event 4.8 cfs <br />Ditch configuration: V -ditch, 2H:1V side slopes <br />D = (On/KSl/2) •375 where D = flow depth (ft) <br />Q = peak flow (cfs) <br />n = Manning's n <br />K = discharge factor <br />s = ditch gradient (ft/ft) <br />D = [(4.8)(.035)/(1.74)(.14) 1/2).375 = .60 ft <br />•r Velocity (V) = 1.49/n R2/3 S1/2 where n = Manning's n <br />R = hydraulic radius <br />S = ditch gradient (ft/ft) <br />V = (1.49/.035)(.27)2/3(.14)1/2 = 6.6 fps <br />F=03' D (flow depth) = 0.6' <br />D=0.6' 2 F (freeboard) = 0.3' <br />JMM/jb 11/86 <br />0 4-180 <br />REVISED FEB 13 '87 <br />