Arkansas River Channel Capacity and Riparian Habitat Planning Study

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I 180)' 3.2.3 Hydraulic Modeling Numerical hydraulic models were assembled for each of the problem areas to analyze current conditions. The HEC -2, Water Surface Profiles program (USACE 1990) was used for hydraulic analysis. This program is applicable to steady, gradually varied flow conditions. It was assumed that the controlling hydraulic conditions were represented within the boundaries of the geometry provided, i.e., that conditions downstream of the areas would not substantially influence the profiles within the computer models. The downstream boundary condition was calculated using the Slope -Area method. Cross- section geometry was obtained from Digital Terrain Modeling (DTM) provided by Kucera International, Inc., under contract to CWCB. Reach lengths between cross - sections along the channel were generated when the cross - sections were extracted. The overbank reach lengths were scaled from the orthophotos. The State Highway 109 Bridge was modeled according to dimensions provided by the State. Manning's "n" roughness values were assigned based on a combination of previous studies, field conditions, sediment transport theory, and engineering judgement. The specific "n" values for the individual problem areas are shown in Table 3 -4. Contraction and expansion coefficients used were 0.1 and 0.3, respectively. Table 3 -4. Manning's roughness values. Problem Area Left Overbank Channel Right Overbank 1 0.030 -0.090 0.025 0.030 -0.070 2 0.030 -0.070 0.025 0.030 -0.070 3 0.035 -0.070 0.025 0.035 -0.070 Flows of 100, 200, 300, 500, 750, and 1,000 cfs were modeled to indicate channel capacities and potential problem areas under current conditions. Most areas had a channel capacity of less than 1,000 -cfs. There were numerous problems with the lack of topographic detail between cross - sections in determining the location of the channel banks as well as the overbank areas. Therefore, we estimated bank locations using the "Edith2" program and measuring the approximate channel widths on the orthophotos. 3.2.4 Sediment Analysis Sedimentation was analyzed using Hydraulic Design Package for Channels (USACE 1997), also known as SAM. This is an integrated system of programs developed to aid engineers in analyses associated with designing, operating, and maintaining flood control channels and stream restoration projects. SAM provides the computational capability to include erosion, entrainment, transportation, and deposition of sediment in channels. The hydraulic, sediment transport and sediment yield modules of the program were used in this study. 31