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<br />3. Spillway and Flood Outlet Selection and Design <br /> <br />a. General. - Spillways and flood outlets should be designed to safely <br /> <br /> <br />convay major floods to tha watercourse downstream from the dam and to <br /> <br /> <br />prevent overtopping of the dam. They are selected for a specific <br /> <br /> <br />dam and reservoir on the basis of release requirements, topography, <br /> <br /> <br />geology, dam safety, and project economics. <br /> <br />b. Considerations in selection of gated or ungated spillways. - An <br />ungated spillway releases water whenever the reservoir elevation <br />exceeds the crest level. A gated spillway can regulate releases <br />over a broad range of water levels. <br /> <br />Operation of ungated spillways is more rel iable than gated spillways. <br /> <br /> <br />Gated spillways provide greater operational flexibility and large <br /> <br /> <br />discharge capacity per unit length. Design of gated spillways and/or <br /> <br /> <br />their regulating procedures should generally ensure that peak flood <br /> <br /> <br />outflow does not exceed natural downstream flow that would occur <br /> <br /> <br />wi thout the dam. <br /> <br />The selection of a gated or ungated type of spillway for a specific dam <br />depends upon site conditions, project purposes, economic factors, costs <br />of operation and maintenance, and other considerations. <br /> <br />The following paragraphs focus on considerations that influence the <br />choice between gated and ungated spillways: <br /> <br />(1) Discharge capacity. - For a given spillway crest length and <br /> <br /> <br />maximum allowable water surface elevation, a gated spillway can <br /> <br /> <br />be designed to release higher discharges than an ungated spillway <br /> <br />19 <br />