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Ditch Desion <br />., • <br />Roads will hove side ditches with 2:1 slopes and a "V" shaped channel. The <br />ditches can be sized using the Chezy-Manning Equation Kennard, 1961) <br />D = 1.49Ap~2/3 So112 <br />n <br />Where: <br />n =Manning Roughness Coefficient - ft.i/6 <br />A =Area - ft 2 <br />RH= Hydraulic Radius - ft. <br />So =Slope of Channel -Percent <br />A Manning n = .035 Is used (OSM Water Diversion Manual) <br />RH = P <br />Pis the wetted perminter <br />• A is the area of flow <br />Fora "V" Ditch with 2H:IV side slopes ~` b ~ d~ <br />A=2 bxd=24dxd=2d2 <br />P = 2 (2d)2 + d2 = 2 5d2 = 2dV5 <br />_ 2d2 d <br />RH 2d }~ . . <br />D = 1=n9 2d2 ~2/3So I /2 = I_n9 2d8/3 : I X2/3 So I /2 <br />_ 1.74 /3 112 Q n 3/8 <br />p_-de So ,solve ford,d=~ ) <br />1.74 501 2 <br />Using the above equation, the ditch is sized by calculating d for a given peak <br />flow and with the flat slope of the ditch. Velocity for steep sections is <br />. checked using Q = VA rearranged as V = Q = Q Designs for each road <br />dffch are presented below. A 2d2 <br />12-6-3 <br />