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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 /> <br />0~1815 <br /> <br />7. Hydraulics and Sediment Transport <br /> <br />This chapter addresses hydraulic and sediment transport conditions within the Arkansas <br />River project area. Hydraulic conditions were evaluated in detail for six representative <br />sites from just below the California Gulch confluence downstream to the area located <br />approximately 1.5 miles south of Kobe, Colorado, on Highway 24 and County Road <br />Bridge 55 (Figure 1.2). Additionally, general hydraulic conditions were provided in the <br />URS (URS, 1998) HEC-RAS model for the entire study reach (US ACE, 1997). The <br />URS analysis was based on cross-sections derived from aerial photography, whereas <br />cross-sections used in this study were derived from actual channel surveys. Parameters <br />evaluated in the hydraulic analysis included velocity, depth, slope, water surface <br />elevations, flow area, top width, and shear stress. Hydraulic conditions were determined <br />for a range of flows from less than bankfull to the estimated I OO-year flood. In addition, <br />bankfull discharges and their associated return periods also have been determined for <br />each representative reach. <br /> <br />Reaches used in hydraulic and sediment transport analyses should not be confused with <br />project sub-reaches described in Chapter 4. The project sub-reaches are a continuous <br />representation of the Arkansas River with each reach abutting the next downstream reach. <br />The hydraulic and sediment transport sites are short segments of the channel within the <br />project area that were chosen to represent the variety of typical conditions observed in the <br />study area. For clarity, these latter reaches are designated with the prefix "1M," <br />representing incipient motion, one of the main properties analyzed for these sites. <br /> <br />Sediment transport analyses focused on determination of sediment sizes that would be <br />mobilized at various flows within the main channel. Relating size of bed material <br />mobilized, percentage of material in the bed, and frequency of associated flows allows <br />for assessment of relative mobility of the channel boundary for each of the selected <br />reaches. Incipient motion analyses were calibrated utilizing bedload measurements made <br />during the 1998 snowmelt runoff season. Sediment transport analysis used for this study <br />does not address the rate of sediment transport for various particle sizes. Estimates of <br />sediment transport rates would require additional analysis and should be considered for <br />subsequent design phases. <br /> <br />7.1. Site Selection <br /> <br />Six reaches in the project area were chosen for detailed field investigations and analyses. <br />Sites were selected to represent typical range of conditions encountered within the project <br />area. Use of the 1M reaches allows for collection of detailed information required to <br />perform quantitative analyses that would not be feasible for the entire study reach. <br />However, similar detailed analyses should be conducted in any areas identified for design <br />of channel alterations. <br /> <br />Reach lengths range between 1,010 and 2,150 teet (Table 7.1). Within each reach, cross- <br />section end points were located using a combination of a Global Positioning System <br />(GPS) and total station surveys. Channel cross-sections were obtained through level and <br /> <br />May 7. 1999 <br /> <br />Fluvial Geomorphological Assessment <br />Upper Arkansas River <br /> <br />Page 47 <br />