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• Once peak flow values are calculated by SEDCAD+~ Mannings Equation for <br />open channel flow was utilized to size the ditches. <br />Q = 1.486 AR2/351/2 <br />n <br />Where Q ^ peak discharge for desired storm event (cfs) <br />n = Mannings roughness coefficient <br />A = cross sectional area of channel flow (ft~) <br />R hydraulic radius (ft) <br />S = channel slope (ft/ft) <br />A Mannings roughness coefficient of 0.04 was used corresponding to an <br />earthen channel with some weeds and stones (Barfield et. al.. 1981. page <br />159). <br />For channel sections with flow velocities exceeding 5.0 feet per second <br />• rock riprap will be installed. The following equation is used to <br />determine riprap size. <br />D50 = 12(118 Q 52.17 (R/P))0.4 <br />Mhere D50 = riprap mean particle size <br />Q =Flow rate (cfs) <br />S =Channel bottom slope (ft/ft) <br />R =Hydraulic Radius <br />P Metted Perimeter <br />The diameter of the largest stone size should be 1.5 times the 050 size. <br />The minimum thickness of the riprap layer will be 1.5 times the maximum <br />stone size. but not less than six (6) inches. <br />-8- <br />