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<br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles (Exhibit 1). Channel roughness <br />values (Manning's "n") used in the hydraulic computations were <br />chosen on the basis of engineering judgment and field observat ion <br />of floodplain areas. After consideration of movable bed conditions <br />on the channel bottom, indicating the potential for sediment <br />movement, a conservative value of 0.035 was adopted for the main <br />drainageway. Roughness values for the main channel ranged from <br />0.015 to 0.035, and the values for the overbank areas ranged from <br />0.030 to 0.100. Starting water-surface elevation for Upper Sloans <br />Lake Drainageway on the Sloans Lake water-surface elevation <br />developed from the 100-year hydrograph volumes of various <br />subdrainages and the stage-versus-storage relationships of the <br />Lake. <br /> <br />Flood profiles were drawn showing 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 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). <br /> <br />4.0 FLOODPLAIN MANAGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and local governments <br />floodplain management programs. Therefore, each Flood <br />includes a flood boundary map designed to assist <br />developing sound floodplain management measures. <br /> <br />to adopt sound <br />Insurance Study <br />communities in <br /> <br />4.1 Floodplain Boundaries <br /> <br />To provide a national standard without regional discrimination, the <br />I-percent annual chance (lOO-year) flood has been adopted by FEMA <br />as the base flood for floodplain management purposes. For each <br />stream studied by detailed methods, the boundaries of the 100-year <br />floodplain 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 />1:100, with a contour interval of 1 foot (References 1 and 7). <br /> <br />The 100-year floodplain boundaries are shown on the Flood Insurance <br />Rate Map (FIRM). On this map, the 100-year floodplain boundary <br />corresponds to the boundary of the area of special flood hazards <br />(Zone AE). Small areas within the floodplain boundaries may lie <br />above the flood elevations but cannot be shown due to limitations <br />of the map scale and/or lack of detailed topographic data. <br /> <br />7 <br />