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<br />Cross section data for the back.water analyses of the Colorado River <br />detailed study reach were obtained from topographic maps compiled <br />from aerial photographs. The photogrammetric subcontractor, Air <br />Photo Surveys, Inc., flew the study area on December 8, 1988 during <br />low flow conditions (Reference 16). Cross sections were digitized <br />from 1:4,800 scale maps with 2-foot contour intervals. <br />Approximately 20 cross sections from previous studies were used, <br />after adjustments based on new maps. <br /> <br />Field surveys were conducted to obtain soundings in the Colorado <br />River that defined the channel geometry below the water line. A <br />total of eleven sections, or one section approximately every 3 <br />miles, were tak.en. <br /> <br />Based on these 11 sections below the water line, the channel bottom <br />along the study reach was defined and the channel geometry on the <br />digitized sections were modified accordingly. <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 />Cross sections for the backwater analyses of Indian Wash and Leach <br />Creek were obtained by field survey and by use of 2-foot contour <br />orthophoto maps (Reference 17). Cross sections for the channel <br />improvements on Indian Wash were obtained from as-built plans from <br />the city. All bridges, dams, and culverts were field surveyed to <br />obtain elevation data and structural geometry. <br /> <br />Cross section data from Horizon Drive Channel were obtained from <br />previous studies conducted by the City of Crand Junction USing <br />orthophoto maps prepared by the u.s. Bureau of Reclamation in 1974 <br />(Reference 17), and supplemented by field surveys conducted by the <br />city at the request of study contractor. <br /> <br />Information on all culverts and hydraulic structures on Horizon <br />Drive Channel were also provided by the City of Grand Junction. <br /> <br />Water-surface elevations of floods of the selected recurrence <br />intervals were computed using the COE HEC-2 step-backwater computer <br />program (Reference 18). The version of the program dated October <br />1972 was used in the computations of the profiles for Leach Creek. <br />Starting water-surface elevations for both the Colorado River and <br />Horizon Drive Channel were determined using critical depth. <br />Starting water-surface elevat ions for Indian Wash were est imated <br />using a rating chart developed for the upstream side of the large <br />culvert passing under the multiple tracks of the Denver & Rio <br />Grande Western Railroad near the south end of the study reach. <br /> <br />Channel and overbank roughness factors (Manning1s "nil) used in the <br />hydraulic computations were chosen by engineering judgment and were <br />based on field observations of the stream and floodplain areas. <br /> <br />18 <br />