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I I I I I I I I I I I I I I I I I I I Hydraulic Analysis Missouri River Mitigation California Bend River Miles 649,5 to 651.0 Harrison County, Iowa Purpose: Hydraulic analyses were performed in order to evaluate and design flow-through pilot channels and an ultimate chute channel at the California Bend site. One of the mitigation alternatives for this project is the hydraulic re-connection of the California Bend (left overbank) to the Missouri River at both the upstream and downstream ends of the site. Pilot channel widths of 10', 25', and 50' were evaluated for discharge and velocity characteristics using HEC-2 backwater models. An ultimate chute channel bottom width was also estimated from the analysis. The need for grade control structures along with design of inlet and outlet structures were included as part of the chute channel analysis. In addition, a floodplain analysis was performed for each of the pilot channel alternatives to estimate the effects of the proposed improvements on the 100-year flood profile. Study Area: The California Bend project area is located along the left bank of the Missouri River from approximately River Miles 649.5 to 651.0 and lies in Harrison County in the State of Iowa. The project area was part of the Missouri River Main channel before the construction of the Missouri River Navigation and Stabilization projects. The Missouri River and overbank areas were modeled from River Mile 649.0 to 650.9 for the purpose of calibration and stage analysis. Data Sources: The base HEC-2 model used for this analysis was provided by the Corps of Engineers. It was assumed that the model cross-sections were developed using available topographic mapping for the overbank areas, and hydrographic survey information for the main river channel The existing HEC-2 deck provided by the Corps was checked for reasonableness, and adjustments were made to the data in order to have an operable model Model calibration is discussed in the following section. It is highly probable that present-day topographic conditions at the California Bend site are considerably different than the elevations shown on the topographic mapping and in the HEC-2 model It is likely that the river channel has degraded, and the overbank area has aggraded since the time of the mapping. Therefore, it is important to realize that the analyses done for this site are based on 1974 data, and the results may not accurately reflect present-day conditions. The Corps of Engineers plans to obtain updated and detailed topographic information for the California Bend site prior to final design. Model Calibration: For the purposes of this analysis, two separate existing conditions HEC-2 model calihrations were performed. One model was calibrated to the lOO-year flood event (Q=I90.000 cfs) using Corps-provided floodplain mapping, and the other to the Construction Reference Plane (CRP) using Corps-provided CRP elevations for river miles in the vicinity of the project. The starting water surface elevations for both models were calihrated to floodplain or synthesized water surface elevations at the most downstream model cross-sections. The slope-area method was used, and the starting energy slopes were adjusted until the computed starting water surface matched the known water surface. The overbank Manning's "n" values were assumed to be 0.040, and the main channel "n" values were adjusted until the water surface profiles closely matched the published data throughout the entire reach. For both calibrations, a main channel "n" value of 0.022 was determined. The calihrated values determined from the CRP model were used for all subsequent split flow and low flow model runs, while the calibrated values from the IOO-Year flood model were used for all floodplain analyses. The use of high water mark data for the calihration of existing conditions hydraulic models would have been preferable. However, this data was not available at the time of this analysis. High water mark data for gaged discharges of 33,100 cfs and 42,100 cfs was later transmitted by the Corps. The existing conditions HEC-2 calibration model (cb-excrp.dat) was re-run with the gaged discharges in order to compare the computed water surfaces to the