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3.7) at a discharge of 300 cfs to nearly 17 fps in the riffle at the head of the primary bar (XS 12.1 -Figure 3.1) at a discharge of 32,300 cfs (Figure 3.8). Four tertiary bars were identified in the study reach. The first is a bank-attached bar along the right side of the channel between XS 5 and XS 6 (Figure 3.2). The second is a bank-attached bar on the left side of the channel between XS 9 and XS 10. The third is a separation bar created by a scallop in the bank along the right side of the channel just upstream of XS 10, and the fourth is a bank attached bar on the left side of the channel near the head of the riffle, just downstream of XS 12. The results of the HEC-2 hydraulic modeling were used to evaluate incipient motion conditions in the vicinity of these bars, as well as other key locations within the study reach. The variation in incipient motion conditions, based on the cross sectionally averaged grain shear stress and a dimensionless critical shear ('f *c) of 0.03 is shown in Figure 3.9. The ordinate of the figure is the dimensionless grain shear stress ('f:) which Is the ratio of the grain shear stress ('t 1 to the critical shear stress ('t c) or. T' T ' = . = yy'S T .c(ys-y)Dso (3.7) Tc , , When 't * < 1, there is insufficient shear stress to mobUize the bed material; when 't * > 1, particle mobilization is indicated. F"lgure 3.9 shows that, using the cross sectionally averaged hydraulics and average gradation of the primary bar surface (050- 69mm), critical conditions ~.e. particle motion) are exceeded in the reach downstream of the bend (XS 1-3) at discharges of greater than about 10,000 cfs. Between the bend and the base of the riffle at the head of the primary bar (XS 5-10), critical conditions are not exceeded, even for the highest modeled flow of 32,300 cis. In the riffle, critical conditions are exceeded at about 2,000 cfs, and in the pool upstream of the primary bar (XS 13-14), critical conditions are only exceeded at discharges greater than about 30,000 cis. The results shown in Figure 3.9 consider the average grain shear stress across the entire active channel. These results provide a good picture of the dynamics of the study reach that lead to formation of the primary bar. The low shear stress conditions in the reach between the bend and the head of the bar indicate that sediment transported to the reach from upstream at high discharges will deposit on the bar. 3.18 Resource Consultants & Engineers, Inc.