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<br />12 <br /> <br />A, Procedure of Analysis <br /> <br />a. Development of a Cost versus Return Period Curve <br /> <br />Each of the alternatives considered for a Decision Unit <br />has associated with it a cost function, showing the <br />variation of project cost with design frequency, The <br />analyst should have available at least an approximation <br />of this relationship so that he can compare the costs, <br />The analyst should be able to identify the lowest cost <br />combination of measures to contain each of the design <br />flows considered. The plot of these lowest costs against <br />design frequency becomes the supply curve for providing <br />flood protection, For th~ development of the underlying <br />theory behind this approach, see James (4), <br /> <br />b. Development of the Benefit versus Return period Curve <br /> <br />The demand curve for flood protection is more complicated <br />to develop. For each design frequency, there will be <br />certain "residual" average annual damages. For example, <br />if a channel is designed to convey the ten-year flood, <br />there will be damaging floods occurring for return periods <br />exceeding ten years, These damages, averaged over time, <br />become th~ damages "residual" to the ten-year facilities, <br />For any Decision Unit, the flood control benefit of <br />a project will be the reduction in average annual damages <br />due to the project, The details for calculation of this <br />reduction are given in Appendix A, <br />