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<br />HYDRAULIC CALCULATIONS <br />Permanent Ditches <br />Peak flows resulting from the 10-year, 24-hour storm event were used to design <br />all permanent diversion ditches. Manning's Equation for open channel flow was <br />used to size the ditches as follows: <br />Q = (1.48&/n)ARZ"S'n <br />Where: Q =peak discharge from 10-year, 24-hour event (cfs) <br />n = bfanning's roughness coefficient (0.035) <br />A =cross sectional area of channel flow (ftz) <br />R =hydraulic radius (ft) <br />S =channel slope (ft/ft) <br />The Manning's roughness coefficient of 0.035 represents an earthen channel <br />• with some weeds and stones (Ref: Barfield et. al., 1981, page 159). <br />For channel sections with flow velocities exceeding 5.0 feet per second, rock <br />riprap will be provided. The following equation is used to determine riprap size. <br /> <br />Where: D,~ = Riprap mean particle size Cnches) <br />Q =Peak discharge from 10-year, 24-hour event (cfs) <br />S =Channel slope (ft/ft) <br />R =Hydraulic radius (ft} <br />P =Wetted perimeter (ft) <br />C, <br />J <br />The diameter of the largest stone size should be 1.5 times the D~ size. The <br />minimum thickness of the riprap layer will be 1.5 times the maximum stone size, <br />but not less than six inches. <br />12 <br />