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<br />on the Colorado River and Tributaries were used as starting water- <br />surface elevations for Big and Little Salt Washes. For Plateau <br /> <br />ab;ulaL~d asing the slup~ a-r-ea-methud. <br /> <br />The approximate flood elevations for Buzzard Creek were determined <br />using normal-depth calculations and computing the backwater <br />elevation from the bridge upstream of its confluence with Plateau <br />Creek. <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles (Exhibit 1). For stream segments <br />for which a floodway was computed (Section 4.2), selected cross <br />section locations are also shown on the Flood Insurance Rate Map <br />(Exhibit 2). <br /> <br />~ <br /> <br />The hydraulic analyses for this study were based on unobstructed <br />flow. The flood elevations shown on the profiles are thus <br />considered valid only if hydraulic structures remain unobstructed, <br />operate properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum of 1929 (NGVD). Elevation reference marks and the <br />descriptions of the marks used in this study are shown on the maps. <br /> <br />4.0 FLOODPLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and local governments to adopt sound <br />floodplain management programs. Therefore, each Flood Insurance Study <br />provides 100-year flood elevations and delineations of the 100- and 500- <br />year floodplain boundaries and 100-year floodway to assist communities <br />in developing floodplain management measures. <br /> <br />4.1 Floodplain Boundaries <br /> <br />TO'l'rovide anational.'S.tandard wi thout-regional discrimina.tion, the <br />1 percent annual chance (lOO-year) flood has been adopted by FEMA <br />as the base flood for floodplain management purposes. The 0.2 <br />percent annual chance (500-year) flood is employed to indicate <br />additional areas of flood risk in the community. For each stream <br />studied by detailed methods, the 100- and 500-year floodplain <br />boundaries have been delineated using the flood elevations <br />determined at each cross section. Between cross sections, the <br />boundaries were interpolated using topographic maps at a scale of <br />1:2,400, 1:4,800, and 1:24,000, with a contour interval of 2 feet <br />and 20 feet (References 17, 18, 19, 20, 21, 22, and 23). <br /> <br />. <br /> <br />The 100- and 500-year floodplain boundaries are shown on the Flood <br />Insurance Rate Map (Exhibit 2). On this map, the 100-year <br />floodplain boundary corresponds to the boundary of the areas of <br />special flood hazards (Zones A, AE, and AO); and the 500-year <br />floodplain boundary corresponds to the boundary of areas of <br />moderate flood hazards. In cases where the 100- and 500-year <br /> <br />24 <br />