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<br />Roughness Coefficients (Manning's "n") <br /> <br />Stream <br /> <br />Channel <br /> <br />Overbank.s <br /> <br />Colorado River <br />Indian Wash <br />Leach Creek. <br />Horizon Drive Channel <br /> <br />0.035 <br />0.022 to 0.060 <br />0.035 to 0.070 <br />0.016 to 0.035 <br /> <br />0.060 to 0.073 <br />0.030 to 0.090 <br />0.050 to 0.180 <br />0.024 to 0.060 <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 mark.s and descriptions <br />used in this study are shown on the maps. <br /> <br />4.0 FLOODPLAIN ~AGEMENT APPLICATIONS <br /> <br />The NFIP encourages State and local governments to adopt sound <br />floodplain management programs. Therefore, each Flood Insurance Study <br />provides laO-year flood elevations and delineations of the 100- and SOO- <br />year floodplain boundaries and lOO-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 (lOa-year) flood has been adopted by FEMA <br />as the base flood for floodplain management purposes. The 0.2 <br />percent annual chance (SOO-year) flood lS 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 and 1:4,800, with a contour interval of 2 feet. <br /> <br />The 100- and SOO-year floodplain boundaries are shown on the E'lood <br />Insurance Rate Map (Exhibit 2). On this map, the lOa-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 limitations of the map scale and/or lack. of detailed <br />topographic data. <br /> <br />19 <br />