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6.A(4) The inflow design flood (IDF) requirements for existing dams shall be determined in accordance with the principles of Rule 5.A.(5)(c) and (d), except those structures whose spillways were designed and approved in accordance with the methods published in the U.S, Department of the Interior, Bureau of Reclamation, "Design of Small Dams", Second Edition, 1973, shall be considered adequate for the classification (formerly referred to as hazard rating) for which they were designed. If the classification has changed, then the provisions of Rules 5.A.(5)(c) and (d) apply. 6.A.(4)(a) Whenever the methods of Rule 5.A.(5)(c) and (d) apply, the requirements for determining the spillway capacity are summarized in the following table. These requirements may be waived for good cause shown. Spillways designed in accordance with these rules will not be required to enlarge them due to subsequent revisions in PMP estimates, unless, in the opinion of the State Engineer, there is a substantial threat to public safety. INFLOW DESIGN FLOOD REQUIREMENTS Dam Class I II III IV Dam Size Large .75 PMP .50 PMP 100 YR 50 YR Intermediate .75 PMP .50 PMP 100 YR 50 YR Sma 11 .75 PMP .50 PMP 100 YR 25 YR Minor .50 PMP 100 YR 50 YR 25 YR 6.A.(4)(a)(I) Existing Large, Intermediate, and Small Class I dams shall have spillways capable of passing, as a mlnlmum, the IDF generated by 75 percent of the probable maximum precipitation, unless an incremental damage analysis demonstrates a lesser flood is applicable. 6.A.(4)(a)(II) Existing, Minor Class I dams, and Large, Intermediate, and Small Class II dams shall have spillways capable of passing as a minimum, the IDF generated by 50 percent of the probable maximum precipitation, unless an incremental damage analysis demonstrates a lesser flood is applicable. - 28 -