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~.............-- ~ . ~ 1.1 .. .. .. .. ~ L I l 1-- 1- I 1 1;R. 35 to CR. 33 -- The channel in this reach has limited capacity (less than the 1 a-year event). Once the flows exceed the channel capacity, they follow along the relatively flat topography in a ",ide. shallow floodplain. Along the north overbank in an area approximately l,200 feet dovmstream of CR. 33, the flows split away from the main channel and are conveyed to the north towards the City of Sterling contributing to the Pawnee Creek Overflow. These flows mainly inundate agricultural fields with some potential for flooding individual structures. Along the south overbank, the flood flows are wide until the creek forms a large oxbow. At this location, the flood flows overtop the bank and follow a direct alignment east to CR. 35. In the area of the oxbow, the Pawnee Creek channel contains the flows until they reach CR. 35 extended and at this point combines back with the main channel flows. CR. ,3 to CR. 31 -- As in the lower reaches, the channel has limited capacity and cannot handle flows of even the 1 O-year magnitude. The I aD-year floodplain is generally wide with shallow depths and has flows splitting off from the main channel. In the upstream area adjacent to CR. 31, the floodplain is very wide (approximately 4,600 feet). The existing channel and bridge crossing are undersized, and the road acts as a weir spreading out the flood waters. Along the south overbank, flows split in a southeasterly direction away from Pawnee Creek toward HWY 6 near the Town of Atwood. The highway and railroad redirect the flows northeast towards the Pawnee Creek main channel. A berm has been constructed approximately one mile south of the Pa"'l1ee Creek crossing. This berm blocks some of the flows and forces a portion of the water over the highway and railroad embankment. The remaining flows overtop the berm and continues north to the highway bridge where they combine with the flows along the main stem of Pawnee Creek. In the lower portion of this reach at CR.' 33, the floodplain is confined by the channel banks south of the bridge crossing, as well as by a gravel driveway to the north and CR. 33 to the east. Upstream of CR. 31 -- No floodplain mapping is available upstream of CR. 3 l. However, hydraulic analysis was completed by the SCS for a distance of about 1,000 feet. Based on discussions with the City of Sterling and the Colorado Water Conservation Board, the floodplain is very wide in the upstream reaches. There is the potential for flooding of some local residences. as well as damage to agricultural farmland. No major flow splits have been determined in this area which would impact the City of Sterling. Hvdrology A detailed hydrologic analysis was not completed for this study. Detailed hydrology was completed by the SCS and presented in their April J 992 Report. Flow data for various storm events generated for that report were used as a basis for development of existing floodplain information. as well as for the evaluation of proposed improvements. Final flows were reviewed and approved by the Colorado Water Conservation Board and the City of Sterling for use in this study. A summary of the discharges used for existing conditions is presented in Table 6. 24