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the selected alternative, improvements were not proposed in the upper reaches of Pawnee <br /> Creek along the south overbank since they do not directly contribute to the Pawnee Creek <br /> Overflow in the City of Sterling. Subsequently, the areas where flows split out of the <br /> channel and travel southwest towards the Town of Atwood were considered valid for <br /> proposed conditions and were subtracted out of the main channel at these locations. A <br /> portion of these flows eventually drains back to the main channel at HWY 6 and the <br /> Pawnee Creek crossing. <br /> A field survey was completed by the Colorado Water Conservation Board in the area <br /> along HWY 6 south of Pawnee Creek to determine the amount of flows, which gets back <br /> to the main.channel. The flows in this area are controlled by an existing berm, which <br /> blocks the flow path. Estimates were made by the Colorado Water Conservation Board <br /> as to how much of the flow overtops the berm and how much would be forced over HWY <br /> 6 and the UPRR embankment at Town of Atwood prior to the July 1997 flood event. <br /> Hydraulics <br /> Hydraulic analyses were completed for both the existing channel conditions and for <br /> proposed improvements shown in the alternatives. The most recent version (4.6.2; May <br /> 1991) of the U.S. Army Corps of Engineers' HEC-2 computer program was used to <br /> determine water surface profiles for the 10-year, 50-year, and 100-year flood events. The <br /> model used for this study was the same model developed by the SCS as presented in their <br /> April 1992 Report. For existing channel conditions, this model was duplicated and the <br /> resulting water surface profiles matches those presented by the SCS. For proposed <br /> improvements, some cross-sections and split flow areas were modified in order to <br /> accurately depict proposed bridge, channel, and levee improvements. High water marks <br /> from the July 1997 flood were also used in the hydraulic analysis. <br /> Comparisons of the 100-year water surface elevations were made for existing and <br /> proposed conditions in order to evaluate potential impacts of the proposed improvements. <br /> The proposed improvements will not increase the floodway elevations, except for the <br /> area immediately downstream of the proposed improved HWY 6 crossing. This can be <br /> expected since the proposed improvements will allow more flows to follow the historic <br /> Pawnee Creek drainage way rather than being diverted along the highway. <br /> The selected alternative was analyzed using the U.S. Army Corps of Engineers' HEC- <br /> RAS Version 3.0 computer program. The 5000 cfs, 100-year Pawnee Creek Overflow <br /> flow rate was used for the analysis and to determine the appropriate channel size. No <br /> existing condition analysis was prepared for the selected alternative since this area is not <br /> a natural drainage collection point. <br /> Project Formulation Sc <br /> Based on an evaluation of existing studies and input from the project sponsors and land <br /> owners, it was determined that a channel in the vicinity of the Riverside Cemetery is the <br /> most economical approach to solving the Pawnee Creek Overflow problems. The project <br /> 30 <br />