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• HYDRAULIC CALCULATTONS <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 = (1.486/n)ARx~sSi/z <br />Where: Q =peak discharge from 10-year, 24-hour event (cfs) <br />n = Manning's roughness coefficient (0.035) <br />A =cross sectional area of flow channel (ft~) <br />R =hydraulic radius (ft) <br />S =channel slope (ft/ft) <br />The Manning's roughness coefficient of 0.035 represents an earthen channel with <br />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 />D~ = 12(118QS21(R/P))o.a <br />Where: D~ = Riprap mean particle size (inches) <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 />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 />C~ <br />16 <br />