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<br />exhibit marginally safe performance characteristics for the IDF. <br />However. they should exhibit excellent performance characteristics <br />for frequent and sustained flows such as up to the 1 percent chance <br />flood event. Other physical limitations may also exist which have an <br />effect on spillway sizing. <br /> <br />Auxiliary spillways designed for infrequent use may sustain limited <br /> <br /> <br />damage during passage of the IDF. Selection is based on economic <br /> <br /> <br />considerations. These designs should be subject to the following <br /> <br /> <br />requirements: <br /> <br />. The auxiliary spillway should discharge into a watercourse <br /> <br />sufficiently separated from the dam abutment to preclude abutment <br /> <br /> <br />damage and should discharge into the main stream a sufficient <br /> <br /> <br />di stance downstream from the toe of the dam so that f1 ows wi 11 not <br /> <br /> <br />endanger the dam structural integrity or usefulness of the service <br /> <br /> <br />spillway. <br /> <br />. The auxiliary spillway channel should either be founded in <br /> <br /> <br />competent rock or adequate length and protective surfacing be <br /> <br /> <br />provided to prevent the spillway crest control from degrading to <br /> <br /> <br />the extent it results in an unacceptable loss of conservation <br /> <br /> <br />storage or large, uncontrolled discharges which exceed peak <br /> <br /> <br />inflow. <br /> <br />Emergency spillways may be used to obtain a high degree of hydrologic <br />safety with minimal additional cost. Because of their infrequent use- <br />they may be permitted to sustain significant damage when used and may <br />be designed to lower structural standard~ than auxiliary spillways. <br /> <br />22 <br />