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<br />As a result of the improvements proposed in this report, flood flows ar,~ partially contained in the areas <br />adjacent to the channel in the north overbank. Therefore, these flows were considcred a part of the <br />main channel and were included in the analysis. For the selected alternative, improvements were not <br />proposed in the upper reaches of Pawnee Creek along 1Jle south overbank since they do not directly <br />contribute to the Pawnee Creek Overflow in the City of Sterling. Subsequently, the areas where flows <br />split out of the channel and travel southwest towards the Town of Town of Atwood were considered <br />valid for proposed conditions and were subtracted out of the main channel at these locations. A portion <br />of these flows eventually drain back to the main channel at HWY 6 and the Pawnee Creek crossing. <br /> <br />A field survey was completed by the Colorado Water Conservation Board in the ar(:a along HWY 6 <br />south of Pawnee Creek to determine the amount of flows which gets back to the main channel. The <br />flows in this area are controlled by an existing berm whi<:h blocks the flow path. Estimates were made <br />by the Colorado Water Conservation Board as to how much of the flow overtops the berm and how <br />much would be forced over HWY 6 and the UPRR embankment at Town of Atwood prior to the July <br />1997 flood event. <br /> <br />llydraulics <br /> <br />Hydraulic Analyses were compkted for both the exu:tmg channel conditions and for proposed <br />improvements. The most recent version (4.6.2; May 199]) of the U.S. Army Corps of Engineers' <br />HEC-2 computer prograrn was us~:d to determine water surface profiles for the 10-year, 50-year, and <br />] OO-year flood events. The model used for this study was the same model developed by the SCS as <br />presented in their April 1992 Report. For existing channel conditions, this model was duplicated and <br />the resulting water surface profiles matches those presented by the SCS. For proposed improvements, <br />some cross-sections and split flow areas were modified in order to accurately depict proposed bridge, <br />channel, and levee improvements. A more detailed discussion of these modifications is included in the <br />Project Formulation Section of this report. High water marks from the July 1997 flood were also used <br />in the hydraulic analysis. <br /> <br />Comparisons of the 100-year water surface elevations were made for existing and proposed conditions <br />in order to evaluate potential impacl~ of the proposed improvements. The proposed improvements will <br />not increase the floodway elevations, except t<)r the arm immediately downstream of the proposed <br />improved HWY 6 crossing. This '~an be expected since the proposed improvements will allow more <br />flows to follow the historic Pawnee Creek drainageway rather than being diverted along the highway. <br /> <br />hoject Formulation <br /> <br />Based on an evaluation of existing :;tudies and input from the project sponsors, it was determined that a <br />system of levees and channel improvements in the vicinity of the Riverside Cemetery, in combination <br />with widening the existing bridges at HWY 6 and the UPRR is the most economical approach to <br />solving the Pawnee Creek Overflow problems, Development of the Pawnee Creek improvements <br />began with modifications to the existing HEC-2 model to analyze the inlpacts of proposed levees, <br />bridge, and channel improvements. The following describes the modifications made to the existing <br />HEC-2 model: <br /> <br />II1-20 <br />