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<br /> <br />IX <br />Spillway Design <br />A. Primary Spillway <br />The primary spillway is a 24" diameter CMP, open at the top, with a series <br />of 6" drain holes in one side to allow self-draining. This structure will <br />allow all expected runoff from a 10 year 24 hour storm to drain from the <br />pond with an average detention time of 25 hours. An inflow-outflow hydro- <br />graph is shown on Exhibit U. <br />B. Emergency Spillway <br />The emergency spillway is designed so that the combination of primary and <br />emergency spillways will pass the expected peak flow resulting from a 25 <br />year, 24 hour storm and still maintain 1 foot of freeboard. The spillway <br />is a wide, shallow channel built on top of the dam. As shown on Exhibit U, <br />the spillway will incorporate a layer of coarse gravel on top of the fill <br />with a layer of fine gravel above. <br />The peak flow calculations are shown below: <br />k Flow - 1980 <br />25 year, 24 hour storm <br />Avg. slope <br />Area <br />Tc <br />CN <br />= 2.1" <br />= 11% <br />= 213 acres <br />= 0.47 hours <br />= 91 <br />• <br />