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<br />001308 <br /> <br />39 <br /> <br />may preclude it from ever being available. However, with adequate <br />verification and calibration data, the detailed physically-based <br />models should, intuitively, be able to better simulate the runoff <br />response. <br />The advantage of this better prediction has been taken for granted. <br />People believe that drainage facility analysis will improve as the <br />accuracy of the response prediction increases. Just how important <br />is the prediction accuracy? It would be embarrassing if hydrologists <br />were struggl ing to get perfect prediction of runoff response when an <br />accuracy of :1:.25% is sufficient. "Sufficient" here means that the <br />cost of the project, the calculated benefits of the project, and the <br />economic analysis of the project change very little within that <br />accuracy range. <br />The writers tested the sensitivity of predicted peak discharge <br />to costs, benefits, benefit cost ratio, and optimal design. These <br />four characteristics were chosen because they are the prevalent cri- <br />teria for evaluating proposed urban drainage and flood control <br />facilities. The sensitivity was related to predicted peak discharge <br />because of analysis convenience and because peak discharge is still <br />a major design characteristic of flood control facilities. <br />Costs - The cost of typical urban drainage and flood control <br />facilities such as pipes and open channels is a function of the peak <br />discharge rate (Q). However, a large portion of the construction <br />cost consists of "move-on" or "job-location" costs, overhead, and <br />project work needed regardless of design discharge. If the design <br />discharge value is changed slightly, the total project cost remains <br />relatively constant. This relative insensitivity can be illustrated <br />for installed storm drain pipe. <br />