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<br />Cross section data for the backwater analyses of the entire <br />Colorado River detailed study reach in Mesa County were obtained <br />from topographic maps compiled from aerial photographs. The <br />photogrammetric subcontractor, Air Photo Surveys, Inc., flew the <br />study area on December 8, 1988, during low flow conditions. Cross <br />sections were digitized from 1:4,800 scale maps with a 2 foot <br />contour interval (Reference 12). Approximately 20 cros s sect ions <br />from previous studies were used, after adjustments based on new <br />maps. <br /> <br />Field surveys were conducted to obtain soundings in the Colorado <br />River that defined the channel geometry below the waterline. A <br />total of eleven sections, or one section approximately every <br />3 miles, was taken. <br /> <br />Based on these 11 sections below the waterline, the channel bottom <br />along the study reach was defined and the channel geometry on the <br />digitized sections was modified accordingly. <br /> <br />Information on Colorado River bridge geometry and elevation was <br />obtained from the Colorado Department of Highways, County records, <br />and field surveys. <br /> <br />Approximately 3.3 miles of flooding of the Colorado River out of 35 <br />miles studied affect the Town of Fruita. <br /> <br />Cross sections used in the backwater analyses for the <br />portion of Little Salt Wash were obtained from the <br />remainder of the cross sections for Little Salt Wash and <br />Salt Wash were digitized from USBR topographic maps at <br />1:2,400 with a 2 foot contour interval of (Reference 13). <br /> <br />downstream <br />COE. The <br />all of Big <br />a scale of <br /> <br />Water-surface elevations of <br />intervals were computed using <br />program (Reference 14). <br /> <br />floods of the selected recurrence <br />the ,COE HEC-2 step-backwater computer <br /> <br />Starting water-surface elevations for the Colorado River were <br />determined using critical depth. For Big and Little Salt Washes, <br />known elevations derived from the 1976 COE Flood Hazard Information <br />report (Reference 10) were used in the profile computations. <br /> <br />Roughness factors {Manning's "n"} used in the hydraulic <br />computations for Big Salt Wash, Little Salt Wash, and the Colorado <br />River were chosen by engineering judgment and based on field <br />observations of the flooding sources and floodplain areas. The <br />ranges of roughness values assigned are shown in the following <br />table: <br /> <br />Channel <br /> <br />Over banks <br /> <br />Colorado River <br />Big Sal t Wash <br />Little Salt Wash <br /> <br />0.035 <br />0.020 0.035 <br />0.020 - 0.045 <br /> <br />0.060 - 0.073 <br />0.045 - 0.060 <br />0.030 - 0.060 <br /> <br />14 <br />