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<br />I <br />I <br />I, <br />., <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I, <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />OC1822 <br /> <br />alternative in-channel n-values of 0.06, which compares well to results found for cross- <br />section AR-20. <br /> <br />7.3.3. Hydraulic Modeling Results <br /> <br />Hydraulic analysis of several variables including water surface slope, velocity, channel <br />shear, and hydraulic depth, are used for incipient motion analysis to help identify general <br />bed material mobility characteristics and potential problem areas within the six study <br />reaches. Ultimately, results from hydraulic analyses are combined with hydrologic <br />analyses, bed-load measurements, and bed-material sampling, to develop an incipient <br />motion analysis. Incipient motion analysis requires estimation of bed shear for <br />calculation of the particle size dianleter moving (mobilized) for a given discharge. Thus, <br />incipient motion analysis determines the maximum bed material size moving at a given <br />discharge. <br /> <br />7.3.3.1. Hydraulic Conditions for Incipient Motion Sites <br /> <br />A steep water surface profile exists at 1M Site 1 (Geomorphic Subreach 2), with an <br />average water surface slope of approximately 0.9% (App. B.l) for the range of discharges <br />from the 1.1- to 1 OO-year return period. The water surface profile shows a break in slope <br />between cross-sections 5 and 6, where slopes approach 2.0%. Average channel velocities <br />for the reach range between 3.0 and 4.7 fps for the annual peak discharges (Fig. 7.4, App. <br />B.2), although velocities locally reach a maximum of 8.2 fps and a minimum of 2.6 fps. <br />Reach average shear stresses range between 0.9 and 1.8 psf(Fig. 7.5, App. B.3). The <br />highest modeled shear stresses is 5.2 psf, and the minimum is 0.6 psffor all modeled <br />discharges. Reach average hydraulic depths range between 1.7 and 3.3 ft (Fig. 7.6, App. <br />B.8); maximum and minimum modeled hydraulic depths are 4.8 ft and 1.1 ft, <br />respectively. <br /> <br />The average water surface slope at 1M Site 2 (Geomorphic Subreach 3) is approximately <br />0.6 to 0.7% for the range of discharges from the 1.1- to 100-year return period (Table 7.6, <br />App. B.5). The water surface profile shows a slight break in slope between cross- <br />sections 3 and 7, where the reach minimum slopes of approximately 0.4% occur. <br />Average channel velocities for the reach range between 2.9 and 4.0 fps for the annual <br />peak discharges; local velocities reach up to 4.8 fps. Minimum velocities range between <br />2.5 and 3.3 fps (App B.6). Reach average shear stresses range between 0.8 and 1.2 psf <br />(Fig. 7.5), although shear stress values locally exceed 1.4 psf for high flows. Minimum <br />shear stresses of approximately 0.6 psf locally occur for all discharges. Reach average <br />hydraulic depths range between 1.8 and 3.2 ft (Fig. 7.6). Maximum hydraulic depths <br />range between 2.4 and 4.4 ft; minimum hydraulic depths range between 1.0 and 2.3 ft <br />(App. B.8). <br /> <br />The average water surface slope at 1M Site 3 (Geomorphic Subreach 3) is approximately <br />0.6 to 0.7% for the range of discharges from the 1.1- to 100-year return period (Table 7.6, <br />App. R5). Average channel velocities for the reach range between 2.6 and 3.6 tps for the <br />annual peak discharges (Fig. 7.4, App. B.! 0). Locally velocities reach a maximum of 4.3 <br />fps and minimum of 2.0 tps (App. RIO). Reach average shear stresses range between 0.8 <br />to 1.3 psf(Fig. 7.5, App. B.l1), and locally reach a maximum of2.1 psf. <br /> <br />May 7. 1999 <br /> <br />Fluvial Geomarphalogical Assessment <br />Upper Arkansas River <br /> <br />Page57 <br />