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<br />24 <br /> <br />Decision Units, then decisions regarding the dams can be made <br />based on total damages prevented by the dams themselves, <br />4. Screening to Identify Most Promising Alternatives in Each <br />Decision Unit, Table 11-7 displayed seven Decision Units, each of <br />which required evaluation under one, two or three combinations of <br />upstream dams, If four flow frequencies are. considered for design, <br />there will still be an excessive number of calculations required for <br />complete evaluation, <br />The screening of alternatives is very much an art and requires <br />engineering judgment in consultation with decision makers. In this <br />case, because of the possibility of using dams, the screening is more <br />complex, <br />An exact screening prescription cannot be given because of the <br />complexity. It is suggested, however, that the alternatives for the <br />Decision Units be screened first under the No Dams case, After <br />evaluation of this situation is completed, then the 2 Dams and 6 Dams <br />cases should be evaluated for comparison, Only a step-by-step <br />methodical process will find the best solution in every case. <br />5. Study of Variation of Costs and Benefits with Design <br />Frequency. By now it is apparent to the reader that the formulation <br />of alternatives is a complex task, For purposes of this example, we <br />will focus on a single Decision Unit, The Unit selected is reach <br />103-105 in the City of Englewood. Floodplain regulation is in effect <br />and upstream detention reservoirs will determine the flow-frequency <br />relationships. <br />The basic classes of alternatives to be considered are: <br />