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I I I I I I I I I I I I I I I I I I I the main channel due to the chute channel split flows was minimal. For low to moderate Missouri River flows, the chute channel flows are relatively small, and therefore the split flow determination followed a slightly different procedure. For each pilot channel width, the energy elevation of the Missouri River was plotted for each of the four flows modeled, and then the chute channel energy curve was plotted on the same graph. The point at which the two curves cross (i.e. equal energy) was used to find the corresponding chute discharge. The ultimate chute channel hydraulic design followed the same split flow analysis procedure as described above. The ultimate bottom width was developed by trial and error keeping in mind the constraints on chute channel velocities and impacts on navigation. Assumed criteria for the chute channel specify that water surface elevations in the main channel should not be decreased by more than 1. 0 foot under any flow condition. The average velocities in the chute channel should be in the range of 1.0 to 5.0 feet/second, which is a rough guide for channel stability purposes given the soil types found at California Bend. Through the modeling process, it was determined that a 150' bottom width and 3H to IV side slopes would be a reasonable geometric design for the ultimate chute channel. Figure I presents a tabular summary of the split flow analysis. while Figure 2 displays chute channel discharge vs. total river discharge for the 150' wide ultimate chute channel. Figure 3 shows average channel velocities along the 150' wide ultimate chute channel for Missouri River discharges of 31,000 and 40,000 cfs. Rating curves developed for each of the split flow models are also included in this section. HEC-2 output files for the 150' wide chute channel split flow runs (Q=40,000 cfs) are also included in this section. As a revision to the draft DPR, the HEC-2 models for the calibration and floodplain runs were extended downstream to river mile 647.4 at the request of the Corps. The computed water surfaces for the "revised" existing conditions models were within 0.02 feet of the previous (draft) models. Therefore, cross-sections for the split flow models for the chute analysis were not extended downstream based upon the results of the existing conditions model. The split flow analysis for this report is identical to the split flow analysis in the draft report. Floodplain Analysis: Impacts to the 100-year water surface elevations in the Missouri River were evaluated for the proposed 10', 25', and 50' wide pilot channels using HEC-2 backwater models. The excavated pilot channels for each width were modeled by adjusting the OR data in the left overbank along the proposed channel alignment. In addition, the excavated material was coded into the cross-sections as a benn with a top width equaI to 2 times the pilot channel bottom width, and side slopes of 2H to I V. For each case, the maximum change in water surface from existing conditions was much less than 1. 0 foot. The computed water surface elevations for proposed conditions upstream of the chute outlet were actually higher than the existing conditions water surfaces. This can probably be attributed to the disposal berms along the pilot channels. Impacts to the 100-year water surface elevations in the Missouri River were also evaluated for the proposed backwater/wetland complex alternatives. Flood profile impacts would be maiuly due to the proposed backwater areas at the southern and eastern portions of the site. Proposed open water areas would have a water surface elevation of approximately 9%.0 feet Oow sill elevation). The areas of standing water were considered to be ineffective flow areas, and the left overbank was therefore modeled with a minimum elevation of 996.0 feet. The computed water surface elevations for the backwater/wetland option were easily within one foot of the existing water surface elevations. Floodplain impacts from the proposed alternative water supply system were assumed to be negligible. Figure 4 presents a tabular summary of the computed loo-year water surface profiles for existing and proposed conditions. Figure 5 shows the same information in graphical format. HEC-2 output files for the proposed 50' wide pilot channel and the backwater/sill loo-year floodplain runs are also included in this section. Summary: If the flow-through chute option is selected, the recommended pilot channel is trapezoidal in geometry with a 50' bottom width and 2H to I V side slopes. This geometry would require more excavation than the 10' or 25' pilot channels, but the smaller pilot channel widths may not enlarge rapidly enough, or possibly not at all due to the lower velocities. The estimated ultimate channel would be a chute with a 150' bottom width and 3H to IV side slopes.