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I I I I I I I I I I I I I I I I I I I branch. A critical flow section occurs at the head of the central chute channel, allowing the HEC-2 split flow option for weir flow to be used for this portion of the model. Separate HEC-2 models were developed for each of the two primary branches, with the chute channel being part of the left branch, Cross Sections 0.0 and 0.5, at the downstream end, and Cross Sections 6 through 8 at the upstream end were common to both models. The energy balance was performed for each discharge considered in the analysis by iteratively adjusting the discharge in each branch until the predicted energy gradeline at Cross Section 6 (the first cross section upstream of the bar that is common to both models) matched within a reasonable tolerance. For each iteration, the left branch was run first so that the discharge in the chute channel, and thus the increase in flow at right branch Cross Sections 0,8, 1.0 and 1,2 associated with the cross flow could be determined. Downstream control for the model was established using the HEC-2 slope-area method, For low flows (Le" flows less than 1 ,200 cfs), the energy gradient at the downstream cross section was adjusted until the predicted water-surface elevations matched those measured at the time of the surveys, Downstream highwater marks that could be clearly associated with a specific discharge were not available, The variation in energy gradient with discharge at higher flows was, therefore, estimated by approximating the energy loss that would occur through the bend at the downstream of end of the project reach using relationships between energy loss, bend geometry and velocity discussed by Henderson (1966). The combination of channel narrowing and energy loss through the bend will cause the energy gradient at the bend entrance to decrease with increasing discharge, creating a backwater condition in the upstream channel. The energy gradient at the downstream cross section necessary to match the measured water-surface elevations varied from 0.25 percent at flows less than 520 cfs to 0.2 percent at about 1200 cfs. Based on the energy loss relationships, the downstream energy gradient was assumed to reduce progressively to about 0.14 percent at the highest modeled discharge of 32,300 cfs. Based on the relatively coarse bed material, main channel Manning n values used in the model varied from 0.04 in the split flow reach around the primary bar at flows less than 1,200 cfs to 0.03 in the single channel reaches up- and downstream of the bar at flows greater than 20,000 cfs. Water-surface elevations predicted by the final model were in good agreement with the elevations that were measured during the survey on August 24 and 25, 1995, when the discharge in the river was 516 cfs (Figures 3.2 and 3.3). Additionally, the modeled flow distribution for a total discharge of 516 cfs was 289 cfs (left branch) and 227 cfs (right branch), which is very close to the flows that were measured on August 25, 1995 of 293 cfs and 224 cfs, respectively. The model results for discharges ranging from 300 cfs to 32,300 cfs indicate that the left branch carries 50 percent to 60 percent of the total flow (Table 3.1). The chute channel begins to flow when the total discharge in the river is about 900 cfs, and the entire bar is inundated at about 10,000 cfs. Although there will be at least some connection between the flows on either side of the bar, the split flow and energy balance model was also used to evaluate the higher discharges. This was accomplished by simulating an artificial boundary between the two models along the crest of the primary bar, The energy 3.3 Mussetter Engineering, Inc.