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r <br />,' <br />1 <br />1 <br /> <br />DIVERSION DITCH DESIGNS <br />(Cont'd) <br />Ditch P-1 <br />Tributary area = 46 ac. disturbed <br />Weighted CN = 87 <br />Total runoff @ CN(87) = 0.75 (in) / 12 in/ft x 46 ac. = 2.88 ac-ft <br />Peak flow (steep slope) 58 cfs/in. runoff x 0.75 in runoff = 43.5 cfs uncorrected <br />Basin slope factor (actual slope 10%) = 0.890 <br />Basin shape factor (hydraulic length 1,500 ft., equivalent area 27.5 ac.) = 46/27.5 = <br />1.673 <br />Corrected peak flow = 43.5 cfs x 0.890 x 1.673 = 64.8 cfs <br />Design: trapezoidal ditch, 3.5 ft. deep, bottom width = 2.0 ft., side slopes 2:1, <br />gradient = 0.005 ft/ft, n = 0.03 <br />Flow conditions: depth = 2.36 ft., velocity = 4.10 fps, area = 15.80 sq. ft. <br />Check calculation: Q = VA = 4.10 x 15.80 = 64.8 cfs <br />11 <br />