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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 />SECTION 3 <br />HYDRAULIC ANALYSIS <br /> <br />3.1 PURPOSE <br /> <br />A hydraulic model was developed for the lower 0,9 miles of the Elk River to determine <br />hydraulic parameters pertinent to stream stabilization, The results of the hydraulic model <br />provide the basis for the channel stabilization investigation and stability improvement <br />recommendations, The hydraulic model was not intended for floodplain delineation, or to <br />address other flood control issues, <br /> <br />3.2 METHODOLOGY <br /> <br />The U.S, Army Corps of Engineers River Analysis System computer program (HEC-RAS, <br />version 2,2) was used to develop the hydraulic model. Water surface profiles were computed <br />for a range of discharges that include the 5-year, 100-year, and 500-year events. In addition, <br />water surface profiles were computed for the June 1997 flood event discharge and a low flow <br />discharge of 500 cfs, <br /> <br />The best available topographic mapping for this project consists of USGS 7,5-minute <br />quadrangle mapping. The mapping scale is I inch = 2,000 feet, with a contour interval of 40 <br />feet, To provide more accurate data for the hydraulic analysis, cross-sections for the hydraulic <br />models were field surveyed by the CWCB during a field reconnaissance visit with ICON on <br />September I and 2, 1999 (Appendix F), Distances between cross-sections were measured from <br />the available 7,5-minute quadrangle mapping, Locations of these cross-sections are shown on <br />Figure 3,1. <br /> <br />Manning's 'n' values were initially estimated from a combination of published information and <br />field observation. These values were later adjusted based on calibration to high water marks <br />from the June 1997 flood event provided by CWCB, <br /> <br />Starting water surface elevations are based on the slope-area method, A starting slope of 0,002 <br />feet per foot was computed from surveyed channel invert elevations downstream of the <br />Highway 40 bridge, <br /> <br />3.3 FLOWS TO THE WEST FORK OF THE ELK RIVER <br /> <br />It was observed from both the field reconnaissance visit and an aerial photograph taken during <br />the June 1997 flood event that several areas exist along the west bank of the Elk River at which <br />flow may "breakout" of the main channel at higher discharges and either flow to the West Fork <br />of the Elk River (West Fork) or return to the Elk River main stem, There is a formal diversion <br />structure for flows to the West Fork located approximately 1.2 miles upstream of the Highway <br />40 bridge, This structure consists of four 24-inch corrugated metal pipes, Head gates control <br />the amount of flow that leaves the Elk River and enters the West Fork through these pipes, A <br />sill exists on the river just downstream of the pipe inlets that provides a pool at the diversion, <br />This condition would appear to maintain flow through the pipes during low flow periods on the <br />Elk River. <br /> <br />13 <br />