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I I I I I I I I I I I I I I I I I I I 11 analysis, Such a screening is an ideal time to involve the decision makers in the analysis process. It should be emphasized very strongly at this point that the screening of alternatives should involve a detailed discussion between the analyst and the decision makers. If the alternatives are not properly formulated and screened, the analyst will be required to undertake a great deal of unnecessary evaluation work. With the experience available in the UDFCD, the number of alternatives to be evaluated should be screened to approximately three, This will normally involve a consolidation of combinations of approaches into alternatives which can be evaluated as units. The analyst should arrange the necessary coordination meetings to ensure that the screen- ing process is completed. He should try to avoid uncertainty at this stage over which alternatives are to be evaluated in detail. 5. Study of Variation of Costs and Benefits with Design Frequency within Each Decision Unit. This study is the key to finding the damage reduction optimum for each Decision Unit. This optimum will not always be the preferred choice because of other considerations to be introduced in the tradeoff analysis, but it is usually of interest to the decision makers to know the economic performance functions of their flood control investments. UDFCD policy is established with the objective of managing at least the laO-year floodplain. The design flood used to size convey- ance facilities or detention storage reservoirs may under certain circumstances be less than the lOa-year and result in portions of the lOa-year flood levels falling into the non-structural measures category. In some cases, the entire floodplain could be covered by nonstructural measures and no structural measures used at all.