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<br />HYDROLOGIC SAFETY <br />The maximum flood the project is designed to handle safely without failure <br />is examined under three different criteria or guidelines. The hydrologic safety <br />performance of each class of alternatives is demonstrated by the Risk versus <br />Downstream Cherry Creek Damage curves on figure 2. <br /> <br />Current CorDS of En~ineers Criteria. High hazard, dams upstream from <br /> <br />urbanized areas are to pass the PMF with adequate freeboard to protect the <br /> <br />integrity of the dam (antecedent conditions dictate routing on a half-full or <br /> <br />full flood control pool). Also, "with project" conditions should not create or <br /> <br />increase a hazard compared to "without project" conditions. On figure 2, all <br /> <br />alternatives except dam raises less than 19 feet meet the Corps criteria in that <br /> <br />they handle 100 percent of the PMF without overtopping and possible dam failure. <br /> <br />The 9-, 15-, and 19-foot dam raises handle 82, 94, and 100 percent of the PMF <br /> <br /> <br />under existing runoff conditions with 5 feet of freeboard. <br /> <br />IWR Guidelines. IWR Draft Report 86-R-7 defines the Base Safety Condition <br /> <br /> <br />(BSC) flood event. This is the smallest inflow design flood where there is no <br /> <br />significant increase in adverse consequences from dam failure compared with the <br /> <br />dam passing the flood safely (safety modification in place). If failure always <br /> <br />results in increased losses, the BSC is the PMF. In general, the lowest cost <br /> <br />alternative should be recommended for implementation. A higher cost alternative <br /> <br /> <br />may be recommended when emergency operation with the lowest cost alternative <br /> <br /> <br />increases upstream or downstream hazard from nonfailure threatening events <br /> <br /> <br />compared to the existing level of safety design. All alternatives except the dam <br /> <br />raises of 3, 9, and 15 feet as shown on figure 2 achieve significant reductions <br /> <br /> <br />in downstream damage at the PMF risk level. <br /> <br />Risk AcceDtance. A letter dated 8 April 1985, Subj ect: <br /> <br /> <br />Evaluating Modifications of Existing Dams Related to Hydrologic <br /> <br /> <br />signed by the Director of Civil Works states in part: <br /> <br />Policy for <br />Deficiencies, <br /> <br />64 <br />