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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Hydrologic Analysis <br /> <br />Missouri River Mitigation <br />California Bend <br />River Miles 649.5 to 651.0 <br />Harrison County, Iowa <br /> <br />Purpose: Hydrologic analyses were performed in order to evaluate the potential inflow from the alternate water <br />source at the north end of the California Bend site. Three of the mitigation alternatives for this project include the <br />use of the alternate water source in order to supplement the supply of water the to existing andlor proposed <br />backwater areas at the southern and eastern portions of the site. In addition, peak flow values for several flood <br />frequencies for the Missouri River near California Bend were estimated and confirmed with the Corps. <br /> <br />Study Area: The California Bend project area is located along the left bank of the Missouri River from <br />approximately River Miles 649.5 to 651.0 and lies in Harrison County in the State ofIowa. The project area was <br />part of the Missouri River Main channel before the construction of the Missouri River Navigation and Stabilization <br />projects. The Missouri River and overbank areas were modeled from River Mile 649.0 to 650.9 for the purpose of <br />calibration and stage analysis. <br /> <br />Data Sources: The drainage basin for the alternate water supply was delineated on USGS 7.5 minute quadrangle <br />mapping. Soil survey information was obtained from the Harrison County Soil Conservation Service, and USGS <br />regression equation manuals were provided by both the USGS and the Harrison County drainage engineer. Other <br />helpful information about the site and local drainage characteristics was obtained from the Iowa Department of <br />Transportation, the Iowa Department of Natural Resources, and Sundquist Engineering. Figure 1 shows a sketch <br />of the Old Soldier River (alternate water supply) drainage basin. <br /> <br />Methodologies: For the purposes of this analysis, three separate hydrologic analyses were performed for the <br />Soldier River (alternate water supply) assuming existing basin conditions. Peak flow values for the 2-, 5-, 10-, 25- <br />, 50-, and 100-year flood events were estimated using the three methods described below. The Missouri River peak <br />flood flows were interpolated from the flood-frequency rating curve for the Missouri River gaging station at <br />Omaha, Nebraska. <br /> <br />The first hydrologic method used was the "Method for Estimating the Magnitude and Frequency of Floods at <br />Ungaged Sites on Unregulated Rural Streams in Iowa," U.S. Geological Survey, WRI Report 87-4132. Regional <br />regression equations are used to relate the size of the drainage area to the probable flood magnitude for five <br />geographic regions within Iowa. The regression equations are applicable to sites having drainage areas ranging <br />from 0.04 to 5,150 square miles providing that the streams are not significantly affected by regulation and the <br />drainage areas are mostly rural. For this site, the combination of two regression equations was used in order to <br />determine the peak flows for various flood frequencies. A portion of the Old Soldier River watershed lies in an <br />area which is best represented by equations developed for region 1, while a majority of the watershed lies in an <br />area suitable for the region 4 equations. The mixed landform analysis was performed following the recommended <br />procedure presented by the Iowa Department of Transportation. <br /> <br />The second hydrologic method used was "Estimating Design-Flood Discharges for Streams in Iowa using Drainage <br />Drainage-Basin and Channel-Geometry Characteristics," U.S, Geological Survey. WRI Report 93-4062. Statewide <br />drainage-basin equations were developed from analyses of 164 streamfIow-gaging stations. The significant <br />characteristics identified for the drainage-basin equations included contributing drainage area, relative relief, <br />drainage frequency, and 2-year, 24-hour precipitation intensity. The equations are applicable to unregulated rura1 <br />drainage areas less than 1,060 square miles. The drainage-basin characteristics, rather than the channel-geometry <br />characteristics, were determined to be appropriate for this site. <br />