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Spillway Capacity RequirementsSpillway Capacity RequirementsInflow Design Flood (IDF) DeterminationInflow Design Flood (IDF) Determination„„Alternative MethodsAlternative Methods––Site Specific Hydrometeorologic AnalysisSite Specific Hydrometeorologic Analysis»»Current scientific methods to determine the probable intensity aCurrent scientific methods to determine the probable intensity and duration nd duration of an extreme storm for the drainage basin for the dam. of an extreme storm for the drainage basin for the dam. »»Generally results in a 10 to 30 percent reduction in the PMP valGenerally results in a 10 to 30 percent reduction in the PMP value. The ue. The results vary with basin size, elevation and storm duration and iresults vary with basin size, elevation and storm duration and in some cases n some cases the resulting storm is greater than PMP values. the resulting storm is greater than PMP values. »»14 site specific studies have been approved since 1992; three ar14 site specific studies have been approved since 1992; three are currently e currently being reviewed; and one is in progress. being reviewed; and one is in progress. ––Incremental Damage Analysis (IDA) Incremental Damage Analysis (IDA) »»Determination of an IDF less than the minimum requirements assocDetermination of an IDF less than the minimum requirements associated iated with PMP methodology.with PMP methodology.»»A comparison of the downstream damage resulting from design flooA comparison of the downstream damage resulting from design flood d without a dam and damages as a result of a failure of the dam duwithout a dam and damages as a result of a failure of the dam due to e to overtopping during the design flood event. overtopping during the design flood event. »»19 IDA19 IDA’’s have been approved since 1999, six resulted in reduction in ths have been approved since 1999, six resulted in reduction in the e PMP requirements.PMP requirements.