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indicating the backwater condition at higher flows (Figure 3.5). To account for the loss of energy through the bend, the contraction and expansion loss coefficients applied to the bend cross sections were increased until the computed water surface and energy profiles were consistent with the measured high-water marks. The computed water-surface elevations at the discharges used to verify the model compare well with the measured elevations, including the steep profile (indicating significant energy loss) through the bend for the higher flows. After performing the above adjustments to the model, a multiple-profile run was made for several discharges between 300 cfs and 32,300 cfs. The results were then used to evaluate the variation in bed shear stress and incipient motion of the bed material with discharge at specific locations within the reach. Two significant hydraulic controls exist within the study reach. As previously discussed, the bend and constriction of the main channel between XS 3 and XS 5 creates significant energy loss. Additionally, the riffle at the head of the primary bar controls the water-surface profile upstream of the study reach, and creates very high velocities through the riffle. These conditions are illustrated in figure 3.6; where it can be seen that the water surface slope and the energy gradient increase with increasing discharge in the bend, and a significant drop in the water surface and energy gradeline occurs in the riffle between XS 10 and XS 12.1. The multiple profile analysis indicates that the primary bar is entirely inundated at a discharge of about 22,000 cfs. The hydraulic analysis is based on a one-dimensional solution to a multi-dimensional problem, and as such, local velocities and shear stress can vary significantly from the cross-section averages predicted by the HEC-2 model. However, the present analysis clearly defines the variation in hydraulic energy that occurs over a range of discharges at various locations at the site. Incipient Motion Analysis The critical particle size (Dc; particle size that is on the verge of motion) for the range of discharges modeled was estimated using the Shields' (1936) relation: T = T (y -y) D c .c s (3.2) where 't c is the critical shear stress, 't *c is the dimensionless critical shear stress, y s is the unit weight of sediment (-165lb/ft~, y is the unit weight of water (62.4Ib/ft~, and D is the particle size. Dc is obtained by substituting the grain shear stress ('t') for 't c and Dc for D in Equation (3.2), and rearranging into the following form: 3.13 Resource Consultants & Engineers, Inc.