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<br />CR. 35 to CR. 33 -- The channel in this reach has limited '~apacity (less than the 10-ye:ar event). Once <br />the flows exceed the channel capacity, they follow along the relativdy flat topography in a wide, <br />shallow floodplain. Along the north overbank in an area approximately 1,200 feet downstream of CR. <br />33, the flows split away from the main channel and are conveyed to the north towards the City of <br />Sterling contributing to the Pawnee Creek Overflow. These flows mainly inundate agricultural fields <br />with some potential for flooding individual structures. Along the south overbank, the flood flows are <br />wide until the creek forms a large oxbow. At this location, the flood flows overtop the bank and follow <br />a direct alignment east to CR. 35. In the area of the oxbow, the Pawnee Creek channel contains the <br />flows until they reach CR. 35 extended and at this point combines back with the main channel flows. <br /> <br />CR. 33 to CR. 31 -- As in the lower reaches, the channel has limited capacity and cannot handle flows <br />of even the 10-year magnitude. Th<: IOO-year floodplain is generally wide with shallow depths and has <br />flows splitting off from the main channel. In the upstream area adjacent to CR. 31, the floodplain is <br />very wide (approximately 4,600 feet). The existing channel and bridge crossing are undersized, and <br />the road acts as a weir spreading out the flood waters. Along the south overbank, flows split in a <br />southeasterly direction away from Pawnee Creek toward HWY 6 near the Town of Town of Atwood. <br />The highway and railroad redirect the flows northeast towards the Pa"mee Creek main channel. A <br />berm has been constructed approximately one mile south of the Pawnee Creek crossing. This berm <br />blocks some of the flows and forces a portion of the water over the highway and railroad embankment. <br />The remaining flows overtop the berm and continues north to the highway bridge where they combine <br />with the flows along the main stem of Pawnee Creek. In the lower portion of this reach at CR. 33, the <br />floodplain is confined by the charmel 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 />Upstrearn of CR. 31 -- No floodplain mapping is available upstrearn of CR. 31. However, hydraulic <br />analysis was completed by the SCS for a distance of about 1,000 feet. Based on discussions with the <br />City of Sterling and the Colorado Water Conservation Board, the floodplain is very wide in the <br />upstream reaches. There is the potential for flooding of some local residences, as well as damage to <br />agricultural farmland. No major flow splits have been detl~rmined in this area which would impact the <br />City of Sterling. <br /> <br />llydrology <br /> <br />A detailed hydrologic analysis was not completed for this study. Detailed hydrology was completed by <br />the SCS and presented in their April 1992 Report. Flow data for various storm events g'~nerated for <br />that report were used as a basis for development of existing floodplain information, as well as for the <br />evaluation of proposed improvememls. Final flows were reviewed and approved by the Colorado <br />Water 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 th,e charmel for the I a-year, 50- <br />year, and laO-year events. Flows which split from the main channel are subtracted out and are <br />accounted for under existing conditions; therefore, the dis(;harges vary significantly from downstream <br />to upstream. For Pawnee Creek Overflow in the City of Sterling, 100-year flows are estimated to be <br />5,000 cfs. <br /> <br />IIl-19 <br />