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378-8 Pond <br />Emergency spillways. Emergency <br />spillways convey large flood flows <br />safely past earth embankments. <br />An emergency spillway must be ' <br />provided for each dam, unless the <br />principal spillway is large enough <br />to pass the peak discharge from <br />the routed design hydrograph and <br />the trash that comes to it without <br />overtopping the dam. <br />The minimum capacity of a natural <br />or constructed emergency spillway <br />snail be that required to pass the <br />peak flow expected from a design <br />storm of the frequency and <br />duration shown in table 4, less <br />any reduction creditable to <br />conduit discharge and detention <br />storage. <br />The emeroency spillway shall <br />safely pass the peak flow, or the <br />storm runoff shall be routed <br />through the reservoir. The <br />routing shall start either with <br />the water surface at the elevation <br />of the crest of the principal <br />spillway or at the water surface <br />after 10-days drawdown, whichever <br />is higher. The 10-day drawdown <br />shall be computed from the crest <br />of the emergency spiliway or from <br />the elevation that would oe <br />a~tained if the entire design <br />storm were impounded, whichever is <br />lower. Emergency spillways shall <br />provide for passing the design <br />flow at a sa=e velocity to a point <br />downstream where the dam will not <br />be endangered. <br />Constructed emergency spillways <br />are open channels that consist of <br />an inlet channel, a control <br />section, and an exit channel. <br />They shall be trapezoida: and <br />shall be located in undisturbed or <br />compacted earth. The side s~ooes <br />shall be stable for the material <br />in which the spillway is to be <br />constructed. For dams having an <br />effective height exceedingLU ft~ <br />the emergency spillway shall have <br />a bottom width of not less than <br />10 ft. <br />Upstream from the control section, <br />the inlet channel shall be level <br />for the distance needed to protect <br />and maintain the crest elevation <br />of the spillway. The channel <br />above the control section should <br />have a slope of not less than 2.0 <br />percent towards the reservoir. <br />The inlet channel may be curved to <br />fit existing topography. The <br />grade of the ex i t chan_ne I of a <br />constructed emergency spillway <br />must be not greater than that <br />which will result in maximum <br />permissible velocities for the <br />soil type or the planned grass <br />cover. <br />Structural emergency spillways. <br />If chutes or drops are used for <br />principal spillways or principal <br />emergency or emergency spillways, <br />they shall be desioned according <br />to the principles set forth in the <br />Engineering Field Manual for <br />Conservation Practices and the <br />National Engineering HandbooL,-- <br />Section 5, Hydraulics; Section 11, <br />Drop Spillways; and Section 1S, <br />Chute Spillways. The minimum <br />capacity of a structural spillway <br />shall be that recuired to pass the <br />peak flow expected from a oesign <br />storm of the frequency and <br />duration shown in table 4, less <br />any reduction creditable to <br />conduit discharge and detention <br />storage. <br />Visual resource design. The <br />visual design of ponds shall be <br />carefully considered in areas of <br />high public visibility and those <br />associated with recreation. The <br />underlying criterion for all <br />visual oesign is apDropriaieness. <br />The shape and form of ponds, <br />SCS, Colorado, January 1989 <br />