Arkansas River Channel Capacity and Riparian Habitat Planning Study

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I . 1808 The resulting sediment transport values for the chosen stable design discharges were also I used to generate tables of stable channel dimensions for the channel improvements. The improved channel models were similarly used to generate new sediment transport rating curves. 1 Sediment yields for the existing conditions are not presented in this report. Annual yields are typically calculated by integrating the sediment transport rating curve with the flow - duration curve for the area. Because of the deterioration in channel capacity within the problem areas, reliable sediment transport curves for the existing conditions could not be obtained. The limited range of flows modeled hydraulically (up to 1,000 cfs) does not provide an adequate representation of the river hydrology. Based on the discharge frequency peaks presented in Table 3.2, 1,000 cfs does not cover enough of a range of flows to provide meaningful yield calculations. Stable channel geometry "templates" were generated for the three problem areas. These are conceptual floodway cross - sectional configurations designed to balance sediment inflow and outflow. The target values for inflowing bed sediment load were estimated from the most efficient channel segments with at least 1,000 cfs capacity and other data sources. The channel configurations are compound. That is, they have more than one prism. The sediment transport values for the chosen stable design discharges were used to generate tables of stable channel dimensions for the channel improvements. A low -flow trapezoid was designed to convey the current channel capacity, estimated to be about 800 cfs. A second trapezoid with a capacity of 2,500 cfs was included to carry higher flows. The 100 to 1,000 cfs sediment transport rating curve was extended through extrapolation and integrated with the flow- duration curve for the pre -darn condition from the Lower Arkansas River Planning Assistance to States Study (1999a). This indicated an effective discharge of around 2,500 cfs. These two channel sizes were combined together, with the Iow -flow (800 cfs) channel cut within the 2,500 cfs channel. Both of the channels used 1V:2.5H sideslopes. The widths varied with each problem area. The channel template dimensions are discussed below under the specific Problem Areas. The resulting compound channels were inset into a simple, 10-point cross - section, idealized as a 700 -foot wide, 10-foot deep floodplain. A 400 -foot length segment of this idealized- configuration for each problem area was modeled in HEC -2 and the improved channel models were used to generate new sediment transport rating curves and calculate sediment yield and average concentrations. The results are listed in Table 3.7 and shown graphically in Figure 5. The variables listed in Table 3.7 are reasonably uniform, with a generally increasing trend in the downstream direction. This promotes transport through the study reach, transporting sediment to John Martin Reservoir, and allows for some increase in load from tributaries. Table 3.7. Improved channel template sedimentation results. Sediment Variable Problem Area 3 Problem Area 2 Prob. Area 1 Mean Daily Load (ton/da 2584 2673 2164 Mean Daily Concentration (TaU 1717 1776 1438 Annual Sediment Yield ton 942,902 975,619 789 Annual Sediment Yield cu. ds.) 751,017 1 777,076 628 33