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<br />a generalized procedure for evaluating the regulatory effects of possible <br />reservoir projects and for assessing flood damages for natural and regulated <br />conditions. The generalized procedure was used in the screening analysis <br />of possible reservoir projects and systems to determine the better alter- <br />native plans of basin development for flood control. <br /> <br />Data Requirements: <br /> <br />a. Hypothetical storm representative of large-area storms of <br />intermediate magnitude <br /> <br />b. Generalized unit hydrograph and rainfall loss-rate criteria <br /> <br />c. Channel flood-routing criteria <br /> <br />d. Location of index stations, stage-discharge-damage relations, <br />and discharge-frequency relations <br /> <br />e. Location and size of existing and proposed reservoir projects <br /> <br />Study Procedure and Results. Each major river basin was divided <br />into subareas averaging about 300 square miles in size and generalized <br />unit hydro graph and rainfall loss-rate critera were developed. Historical <br />storm data were studied to determine criteria for a well-balanced hypo- <br />thetical storm of intermediate magnitude. Using the above data, a <br />hypothetical flood was computed. Ratios of the hypothetical flood were <br />used for computing other floods of various magnitudes. The flood hydro- <br />graph package program, HEC-ll, was adapted to compute, route, and combine <br />flood hydrographs simultaneously for many flood sizes. The regulatory <br />effects of reservoir projects on downstream flooding were determined for <br />each flood size and the damage reduction was computed. Generalized <br />relations for damage reduction were developed as a function of the percent <br />area controlled, the level of development, and the relative location <br />of the control. These relationships are shown in Figures 1 and 2. <br /> <br />Tibbee River - Mobile District <br /> <br />Objectives. The objectives of the Tibbee River study were to determine <br />the effect of alternative channel improvement schemes on flood runoff in <br />the Tibbee River basin (Figure 3), to determine the reductions in flood <br />damage associated with each scheme, and to select that plan that would <br />most economically mitigate the flood damages. The proposed channel improve- <br />ments will lower the stage for a given flow rate, but the improved channels <br />can collect water faster and cause higher flow rates for given rainfall <br />conditions. To accomplish the study objectives, a hydrologic-economic <br /> <br />41 <br />