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. . . Table 2. Peak Discharges Along Pawnee Creek Main Channel Under ExistiDg Conditions 100~Year (cm)i .... 2226 2226 6645 8611 (ilfs) *Mouth (South Platte River 100- Y ear Floodplain) 1517 *Upstream HWY 6 1517 *Upstream CR. 33 2945 *Upstream CR. 31 3264 * Split flows cause reduced discharge in the main channel 50- Year. . (em) 2026 2026 5434 6729 Ilydraulics The hydraulic analysis for this report was completed using the U.S. Army Corps of Engineers HEC-2 computer program, Version 4.6.2. Existing HEC-2 models from previous studies were used as a basis for the hydraulic analysis and modified to study the proposed improvements in more detail. The source of the base HEC-2 models for Pawnee Creek is the 1992 SCS Report. The base models for Pawnee Creek Overflow, within the City of Sterling, came from the 1983 Leaf/RCI Report. Flow measurements taken by the CWCB from high water marks during the 1997 flood were used to help calibrate and refine the hydraulic model. Flow characteristics for proposed future improvements were modeled using HEC-2 in conjunction with weir overflow equations. Hydraulic analysis was made to determine the carrying capacity of Pawnee Creek itself and to help define where and how much out-of-channel flow occurs. Significant diversion of flows occurs at several locations. Overflow spills out of the main channel occur near CR. 29 and 31, and along the HWY 6/Railroad embankments. Alternative Improvements Based on a review of the existing reports and discussions with the project sponsors and local citizens, six alternative improvement projects were selected for further study. A conceptual design of all of the proposed projects was completed in sufficient detail to determine relative construction quantities, cost estimates, and expected benefits. Approximate locations of the alternatives are shown on Figure 4. Each Alternative, with the exception of the "No Action" Alternative, protects varying lengths of the area surrounding the lower portions of Pawnee Creek, and the Pawnee Creek overflow. The alternatives studied are as follows: Alternative No.1: "No Action" Alternative. This alternative would not result in any construction of flood prevention improvements. In the event of future floods, high damage to cropland, farm houses, residential homes in the City of Sterling, commercial buildings, roads and railroads would very likely occur. Over $10 million in damage occurred from the 1997 flood, in the lower reaches of Pawnee Creek. This alternative was not acceptable to the City of Sterling, Logan County and the majority of Citizens. 1-7