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<br />I <br />I <br />I <br />I <br />: I <br />II <br />I <br />I <br />I <br />I <br />I <br />II <br />I <br />I <br />I <br />I <br />I <br />II <br />I <br /> <br />measured water surfaces. The purpose of these final HEC,2 runs was to provide a comparison and to check the <br />validity of the existing conditions model that was calibrated to the CRP. The results indicated that maximum <br />difference between measured and computed water surfaces was about 0.6 feet along the stUdy reach (river miles <br />649.0 to 650.9). Figure 6 of this Appendix presents a comparison of computed and measured water surfaces. <br />Adjustments to the split-flow and floodplain analysis models based on the high water mark data were not made, <br />and therefore hydraulic design of the chute channel should be verified during final design if it is chosen as the <br />selected plan. <br /> <br />Design Parameters: The proposed chute was assumed to have an iulet elevation set at 5 feet below the CRP. The <br />CRP elevation at the proposed inlet location (River Mile 650.3) was interpolated from the known CRP elevations at <br />River Miles 649.0 and 650.0. The chute inlet invert was determined to be 989.6 feet. The chute channel outlet <br />invert was determined by setting the chute channel slope just slightly steeper than the average Missouri River <br />slope, and then calculating the outlet elevation based on chute slope times chute length. The chute outlet invert is <br />at elevation 988.5 feet and is located at River Mile 649.6. Due to the fact that the project site is located on the <br />outside of the river bend, the chute channel is forced to take a longer flow path than the main channel (during <br />lower river discharges when the chute flow is hydraulicaUy disconnected from the main channel). If the chute <br />outlet were to also be set at 5 feet below the CRP, the slope of the chute would be flatter than the average main <br />channel slope. Intuitively, this situation would not be conducive to dredging a pilot channel into an ultimate chute <br />channel through natural hydraulic processes. This situation is contrary to the chute channel designs through other <br />mitigation sites, such as Boyer or Hamburg, where the chute channels flow through the inside of the bends. The <br />California Bend site presents a challenge for designing a flow-through pilot channel that would eventually scour <br />into a larger chute channel. <br /> <br />The Manning's "n" values assumed for the proposed pilot channels were 0.035 for the main channel and 0.40 for <br />the overbank areas. The overbank "n" value is consistent with the existing conditions overbank "n" value. <br />Contraction and expansion coefficients used were 0.1 and 0.3, respectively. In addition, the proposed flow-through <br />chute channel must not have an adverse impact on the I QO-year floodplain or on the navigation channel in terms of <br />stages or velocities. <br /> <br />Determination of Discharge Through Chute Channels: Previous analyses of similar nature revealed that the <br />best way to determine flow through a chute area is to treat the split flow as flow around an island, treating the <br />chute as a flow area separate from the main channel. This split flow analysis involves balancing the energy <br />elevations of the Missouri River and the chute channel at the upstream flow split location (i.e. chute inlet location) <br />using graphical methods. Two separate HEC-2 models were used to represent the main channel and chute <br />channel. The main channel model included encroachments which excluded flow in the chute channel (overbank <br />area), while the chute channel model included encroachments which excluded flow in the main channel. <br /> <br />Prior to the split flow analysis, it was decided that the best use of a flow-through chute would be for the benefit of <br />large-river fish habitat. Based upon this objective, a single long chute may be more effective than two or more <br />shorter chutes. In addition, a single chute would be much more cost effective than two or more chutes. The <br />remaining analysis. therefore, focused on a single flow-through chute. Split flow discharges for pilot channel <br />bottom widths of 10', 25', and 50' were determined for Missouri River low flow discharges of 18,000, 25,000, <br />31,000. and 40,000 cfs. All of the pilot channels were assumed to have side slopes of2H to IV. Various amounts <br />of flow were excluded from the main channel model between the chute outlet and inlet. and a series of flows were <br />modeled for the various pilot channel widths in the chute channel models. The starting water surface elevation at <br />the chute outlet location was the equal for both of the split flow models. For each pilot channel width and Missouri <br />River flow stated above, a rating curve was developed for both the main channel and chute channel at the location <br />of the chute inlet (River Mile 650.3). The rating curves were plots of the energy elevation vs. discharge for the <br />range of flows that were modeled. <br /> <br />NormaUy, the chute rating curves would be graphically added to the Missouri River rating curves to obtain a <br />combined flow rating curve. The energy elevation at which the combined flow curve passes through the correct <br />total flow would then be used to determine the chute flow. However, for this analysis, the reduction in energy of <br />