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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />CR. 33 to CR. 31 -- As in the lower reaches, the channel has limited capacity and cannot <br />handle flows of even the 10-year magnitude. The 100-year floodplain is generally wide <br />with shallow depths and has flows splitting off from the main channel. In the upstream <br />area adjacent to CR. 31, the floodplain is very wide (approximately 4,600 feet). The <br />existing channel and bridge crossing are undersized, and the road acts as a weir spreading <br />out the flood waters. Along the south overbank, flows split in a southeasterly direction <br />away from Pawnee Creek toward HWY 6 near the Town of Town of Atwood. The <br />highway and railroad redirect the flows northeast towards the Pawnee Creek main <br />channel. A berm has been constructed approximately one mile south of the Pawnee <br />Creek crossing. This berm blocks some of the flows and forces a portion of the water <br />over the highway and railroad embankment. The remaining flows overtop the berm and <br />continues north to the highway bridge where they combine with the flows along the main <br />stem of Pawnee Creek. In the lower portion of this reach at CR. 33, the floodplain is <br />confined by the channel banks south of the bridge crossing, as well as by a gravel <br />driveway to the north and CR. 33 to the east. <br /> <br />Upstream of CR. 31 -- No floodplain mapping is available upstream of CR. 31. <br />However, hydraulic analysis was completed by the SCS for a distance of about 1,000 <br />feet. Based on discussions with the City of Sterling and the Colorado Water <br />Conservation Board, the floodplain is very wide in the upstream reaches. There is the <br />potential for flooding of some local residences, as well as damage to agricultural <br />farmland. No major flow splits have been determined in this area which would impact <br />the City of Sterling. <br /> <br />Hydrolo2}' <br /> <br />A detailed hydrologic analysis was not completed for this study. Detailed hydrology <br />used for this study was completed by the SCS and presented in their April 1992 Report. <br />Flow data for various storm events generated for that report were used as a basis for <br />development of existing floodplain information, as well as for the evaluation of proposed <br />improvements. Final flows were reviewed and approved by the Colorado Water <br />Conservation Board and the City of Sterling for use in this study. <br /> <br />There are several areas where the flood flows exceed the capacity of the channel for the <br />to-year, 50-year, and tOO-year events. Flows, which split from the main channel, are <br />subtracted out and are accounted for under existing conditions; therefore, the discharges <br />vary significantly from downstream to upstream. For Pawnee Creek Overflow in the City <br />of Sterling, 100-year flows are estimated to be 5,000 cfs. <br /> <br />As a result of the improvements proposed in this report, flood flows are partially <br />contained in the areas adjacent to the channel in the north overbank. Therefore, these <br />flows were considered a part of the main channel and were included in the analysis. For <br />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 /> <br />33 <br />