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• Diversion Ditch Design Methodology <br />Peak flow through the diversion ditches was calculated for a 25 -year, 24-hour <br />event, using the SCS method described in "Procedures for Determining Peak Flows <br />in Colorado - Technical Release No. 55." Ditches were sized using a typical <br />drainage area length of 1400 feet and varying the vertical spacing from 25 feet <br />to 100 feet. <br />Ditch dimensions were calculated using the modified Manning equation: <br />D = ((Qn)/(KS1/2)J•375 <br />where D = design flow depth (ft) <br />Q - design flow (cfs) <br />n - roughness coefficient <br />K = discharge factor <br />S - ditch gradient (ft/ft) <br />The discharge factor (K) is a function of ditch geometry and is discussed in the <br />"Civil Engineering Handbook" edited by Urquhart, Chapter 4. <br />Peak flow velocity was calculated using the Manning Equation: <br />V = (1.49/n) R2/3S1/2 <br />where V = velocity (fps) <br />n - roughness coefficient <br />R = hydraulic radius (ft) <br />• S - ditch gradient (ft/ft) <br />A summary of diversion ditch design for various vertical spacings is shown in <br />Table 3.6--13. Figure 3.6-6A shows a typical diversion ditch cross-section with <br />all applicable dimensions. <br />HBH/jsb <br />12/7/84 - <br />3-151a <br />