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0018J!J HYDRAULIC EVALUATIONS The hydraul i c ana lys i s of the Ri 0 Grande through the study reach was performed using the U.S. Army Corps of Engineers HEC-2 Water Surface Profile Computer program (Ref. 5). The hydraulic model used in the flood insurance study for community (Ref. 1) was provided by the U.S. Army Corps of Engineers Albuquerque office (Ref. 6). Supplemental river cross sections and levee profiles were provided by the A 1 amosa County Surveyor in coordi nati on with the Colorado Water Conservation Board. The surveyed cross sections are shown in Figure 3. The hydraul ic model provided by the COE was modified to reflect the surveyed cross sections and the new bridge at State Avenue. The model considered that the total flow would be confined between the levees. The river was then modeled for four conditions, as follows: 1. 100-year flood flow under existing channel conditions, inclUding sandbars and vegetation (roughness parameter 'n' = 0.04) , I 2. 100-year flood flow for channel cleared of vegetation, but sandbars remaining (roughness parameter 'n' = 0.03) 3. 100-year vegetation (roughness revised to flood flow for improved channe 1 wi th cleared and select sandbars removed parameter 'n' = 0.03, with cross sections reflect sandbar removal) I 4. 500-year flood flow for improved channel as described in 3. above. The water surface profiles resulting from the hydraulic analyses are shown in Figures 4-7, included at the end of the report. As anticipated, differences existed between cross section data in the COE model and fi e 1 d data provi ded by the survey. These differences were attributed to ten years of aggradation and degradation since the COE surveys for the Flood Insurance Study, as well as local development. Adjustments were made to the COE model cross sections, as needed, based upon the new survey, to establish the hydraulic model used for this evaluation. The water surface profiles shown are considered to be reasonable and accurate within the scope of this reconnaissance study. The 1985 profiles compare reasonably well with the reported flood elevations in the Flood Insurance Study at the downstream and upstream study limits. l ~ ~ -17-