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<br />Cross sections for the backwater analysis of the White River at <br />Meeker were field surveyed at close intervals above llnd below <br />bridges and culverts in order to compute the significant backwater <br />effects of these structures. Additional digitized cross sections <br />were secured when the topographic maps were compiled. <br /> <br />Channel roughness factors (Manning's "n" values) for these <br />computations were assigned on the basis of field inspection of the <br />floodplain areas. The values for Manning 1 s "n" range as follows <br />(Reference 2): Main channel--White River, 0.04 to 0.05; clear <br />overbank areas, 0.04 to 0.06; obstructed overbank areas, 0.06 to <br />0.1. <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 Ceodetic Vertical <br />Datum of 1929 (NGVD). <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 provide a national standard without regional discrimination, 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 (SOD-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 SOD-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 />1:1,200, with a contour interval of 2 feet (Reference 6). <br />Topographic information for other areas was takzn from USGS <br />Quadrangle maps (Reference 7). <br /> <br />The 100- and SOD-year floodplain loundaries 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 and AE) and the SOD-year floodplain <br />boundary corresponds to the boundary of areas of moderate flood <br />hazards. In cases where the 100- and SOO-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 /> <br />7 <br />