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I I I I I I I I I I I I I I I I I I I infonnation. Consequently. adjustments were made to the elevation data associated with these three bridges based on a combination of available survey data and adjacent channel thalweg elevations. Detailed documentation of the adjustments made to the hydraulic modeling data used for these three bridges is included in Appendix E.2. All bridges were represented in the hydraulic model using standard bridge modeling techniques. with specific parameters defined based on field observations and measurements. The bridges were modeled using the HEC.RAS Version 3.1.2 bridge routine and the highest upstream energy solution criteria. .t.5 Boundary Conditions The hydraulic analyses were conducted assuming suhcriticaL steady state flow conditions. Consequently. the only boundary conditions required for the analyses were \..:ater surface elevations tor the South Platte River at the do\\nstream end of each of the three reaches. For the lower detailed study reach near South Platte. starting water surface elevations for the South Platte River were defined based on normal depth using an average channel slope of 0.0043 ftJfl. For the upper detailed study reach near Nighthawk. starting water surface elevations for the South Platte River were defined based on normal depth using an a\"erage channel slope of 0.0022 ft/ft. The 100-year starting water surface elevation for the approximate study reach through the Wigwam area was defined based on the knO\m (computed) water surface elevation of 6439.74 feet. NA YD. obtained from the upstream cross section of the hydraulic model for the upper detailed study reach. 4.6 Floodway Analysis For the I-foot rise floodwav analysis. floodway encroachments were initiallv defined .". . assuming equal conveyance reduction in the overbank areas along both sides of the channel. Adjustments were then made to optimize the floodway using fixed encroachments. while attempting to maintain equal conveyance reduction. Due to relatively confined flow conditions at numerous locations along the study reaches. setting tloodway encroachments inside the limits of the 100-year floodplain would have resulted in negative surcharges. Consequently. at many locations flood way encroachments were defined to coincide with the 100-year floodplain boundary. CDCWCB03_South PlatU'_rtporl.doc 16 ..hdERSO'\ eo'\5LlThG E'\Ghm~... he.