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<br />Guide For Approximate Zone A Areas <br /> <br />Developing BFBs <br /> <br />the ratiooal formula. other hydrograph methods will also be <br />noted but not described in detail due to their complexity. <br /> <br />Discharge-Drainage Area Relationships <br /> <br />This method is suggested for approximate Zone A areas because <br />it is straightforward and the only data needed are drainage <br />areas and corresponding lOO-year flood discharges. These data <br />can be obtained from the Summary of Discharges table in a <br />Flood Insurance Study report. <br /> <br />The relationship between drainage area and discharge is non- <br />linear in most cases; therefore, the drainage areas and <br />corresponding lOO-year flood discharges from the Flood <br />Insurance Study should be plotted on log-log paper as shown in <br />Figure 2D from the example which begins on the following page. <br />The streams plotted may have varying drainage areas; however, <br />other watershed characteristics should be similar. A straight <br />line should be drawn through the plotted points as shown in <br />Figure 21. The lOO-year flood discharge for a particular <br />location can then be estimated based on the drainage area at <br />the location as shown in Figure 21 from the example. <br /> <br />Limitations - If the relationship of plotted points cannot be <br />approximated by a straight line, then this method should not <br />be used. In addition, this method is not appropriate when the <br />stream along which the site is located is regulated by dams, <br />detention ponds, canals, or other flow control structures or <br />diversions. <br /> <br />V-16 <br />