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<br />HYDRAULICS <br /> <br />In this study, the hydraulic analysis along Elm Creek consisted of <br />developing a hydraulic model of the Elm Creek flood plain using the <br />Corps' HEC-2 step backwater program that was updated in September <br />1990. The hydraulic analysis along Shell Creek was performed by HDR <br />Inc. in October 1990, also using the 1990 version of the Corps' HEC-2 <br />step backwater program. <br /> <br />STARTING YATER SURFACE <br /> <br />" <br />" <br />., <br /> <br />In the Elm Creek hydraulic model, the slope area method of HEC-2 <br /> <br /> <br />Was used to establish the starting water surface elevation at the <br /> <br />extreme downstream end of the model IlC normal depch. <br /> <br />Several <br /> <br />duplicate sections were added', below the downstream study limit to <br />ensure that the computed water surface profiles at the downstream <br />limit of the study reach were not being affected by starting water <br />surface elevations. The impacts of Shell Creek floodwater on Elm <br />Creek were tested using the known wate.r s.urface elevation of Shell <br />Cree~ generated by HDR's hydraulic model.:c Results indicat?d that <br />flOOding from Shell Creek had no impact - an" Elm Creek floodwater <br />elevations in the defined study reach. In addition, sensitivity of <br />the model was, tested by adj~sting the starting water surface e1evacion <br />up and down 1 foot. The results of the sensitivity analysis indicated <br />that there was still no impact on Elm Creek water surface elevations <br />in the study reach. Consequently, the starting water surface for the <br />tlm Creek hydraulic model was maintained ;;.at normal depth for the <br />... <br />remainder of the analysis. <br /> <br />7 <br />