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<br />Cross section data for the backwater analysis of the Colorado River <br />detailed-study reach were obtained from topographic maps compiled <br />from aerial photographs taken on December 8, 1988 (Reference 7). <br />Cross sections were digitized from 1:4,800 scale maps with 2-foot <br />contour intervals. <br /> <br />Water-surface elevations of floods of the selected recurrence <br />intervals were determined through use of the COE HEC-2 step- <br />backwater computer program (Reference 8). Starting water-surface <br />elevations for the Colorado River were determined using critical <br />depth. <br /> <br />Roughness factors (Manning's Un") used in the hydraulic <br />computations were chosen by engineering judgment and based on field <br />observations of the streams and floodplain areas. The channel Un" <br />value for the Colorado River was 0.03S and the overbank Un" values <br />ranged from 0.060 to 0.073. <br /> <br />Information on Colorado River bridge geometry and elevation was <br />obtained from the Colorado Department of Highways, Mesa County <br />records, and field surveys. <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 descriptions <br />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 <br />SOO-year floodplain boundaries and 100-year floodway to assist <br />communities 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 (SOO-year) flood is employed to indicate <br />additional areas of flood risk in the conununity. For each stream <br />studied by detailed methods, the 100- and SOO-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 />1:4,800, with a contour interval of 2 feet (Reference 7). <br /> <br />8 <br />