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<br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVO). <br /> <br />4.0 FLOOD PlAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and local governments to adopt sound flood <br />plain management programs. Therefore, each Flood Insurance Study produces <br />maps designed to assist communities in developing flood plain management <br />measures. <br /> <br />4.1 Flood Boundaries <br /> <br />TO provide a national standard without regional discrimination, <br />the 1 percent annual chance (lOO-year) flood has been adopted by <br />FEMA as the base flood for flood plain management purposes. The <br />0.2 percent annual chance (500-year) flood is employed to indicate <br />additional areas of flood risk in the community. For each stream <br />studied in detail, the 100- and 500-year flood plain boundaries <br />have been delineated using the flood elevations determined at each <br />cross section. Between cross sections, the boundaries were inter- <br />polated using topographic maps at scales of 1:2,400 and 1:4,800, <br />with contour intervals of 2 and 5 feet (References 8 and 9). <br /> <br />The 100- and 500-year flood plain boundaries are shown on the Flood <br />Boundary and Floodway Map (Exhibit 2). In caSeS where the 100- <br />and 500-year flood plain boundaries are close together, only the <br />lOO-year flood plain boundary has been shown. Small areas within <br />the flood plain boundaries may lie above the flood elevations but <br />cannot be shown due to limitations of the map scale and/or lack of <br />detailed topographic data. <br /> <br />4.2 Floodways <br /> <br />The floodway is the channel of a stream, plus any adjacent flood <br />plain areas, that must be kept free of encroachment so that the <br />100-year flood can be carried without substantial increases in <br />flood heights. <br /> <br />No floodays were computed in the Flood Plain Information Report <br />for Yuma County (Reference 1). Therefore, no floodways are shown <br />in this study. Regulatory elevations computed at selected cross <br />sections used in the hydraulic analyses are shown in Table 3. <br /> <br />5.0 INSURANCE APPLICATION <br /> <br />To establish actuarial insurance rates, data from the engineering study <br />must be transformed into flood insurance criteria. This process includes <br />the determination of reaches, Flood Hazard Factors, and flood insurance <br />zone designations for each flooding source studied in detail affecting <br />Yuma County. <br /> <br />11 <br />