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<br />from field investigations and the data were integrated into <br />appropriate cross section data. <br /> <br />Roughness coefficients (Manning's n values) were determined from <br />field investigations and are on t,he 'average of 0~040 fer the <br />channel and 0.050 to 0.055 in the overbank areas (Reference 8). <br /> <br />Flood profiles were drawn sho~i;~ the computed water-surface <br />elevations for floods of the selected recurrence intervals. <br /> <br />The hydraulic analyses for this study are based on unobstructed <br />flow. The flood elevations showh on the profiles are thus <br />considered valid only if hydraulic structures remain <br />unobstructed, operate properly, and do not fail. <br /> <br />All elevations are referenced to the National Geodetic Vertical <br />Datum (NGVD) of 1929. <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 />includes a flood boundary map designed to assist communities in <br />developing sound floodplain management measures. <br /> <br />4.1 Floodplain 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 floodplain management purposes, The <br />0.2 percent annual chance (500-year) flood is employed to <br />indicate additional areas of flood risk in the community. For <br />each stream studied in detai 1, the boundaries of the 100- and <br />500-year floodplain boundaries have been delineated using the <br />flood elevations determined at each cross section. Between cross <br />sections, floodplain boundaries were interpolated using u.s. <br />Geological Survey topographic maps at a scale of 1:24,000, with a <br />contour interval of 50 feet (Reference 9). <br /> <br />The 100- and 500-year floodplain boundaries are shown on the <br />Flood 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 (Zone AE) and the 500-year floodplain <br />boundary corresponds to the boundary of areas of moderate flood <br />hazards. In cases where the 100- and 500-year floodplain <br />boundaries are close together, only the 100-year floodplain <br />boundary has been shown. Small areas within the floodplain <br />boundaries may lie above the flood elevations but cannot be shown <br />due to limi tat ions of the map scale and! or lack of detailed <br />topographic data. <br /> <br />7 <br />