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<br />6. Check the flow regime for sand-bed channels or overbanks (if they <br />act as a movable bed). Use the n value estimated in step 4 or <br />5 in the Manning equation (eq. 1) to compute the velocity that then <br />is used to compute stream power, The flow regime is evaluated from <br />figure 5, <br /> <br />7. Make the hydraulic analyses and check the computed depth <br />against the estimated flood in step 1. If the computed <br />depth is substantially different from the estimated depth <br />used to estimate the n values, the n values need to be <br />reevaluated using the computed depth as the next estimated <br />depth in step 1, <br /> <br />Example 1.--Natural Channel <br /> <br />Compute the roughness coefficient for reach A of figure 2 for low-flow <br />conditions with a flow depth of 2 ft. <br /> <br />1. Cross section 1 represents the average conditions in the reach <br />and is located 300 ft upstream from the initial reference point <br />as measured along the main channel thalweg. The channel is <br />stable and is composed of boulders, and the reach needs an <br />average number of adjustment factors. There is no flow in the <br />left or right overbanks (fig. 3). The flow depth of 2 ft is <br />approximately equal to the hydraulic radius, <br /> <br />2. No subdivision is needed. <br /> <br />3. The channel is composed of boulders computed to have an <br />intermediate diameter of 10 in. by the Wolman method (Wolman, <br />1954). The n values for boulders range from 0.04 to 0.07 <br />(table 2), Surveyed water-surface levels indicate the water <br />slope is 0.02 and is assumed equal to Lhe friction slope. The <br />slope and hydraulic radius are within the limits of the <br />prediction equation 6 and figure 12. Using equation 6: <br /> <br />then <br /> <br />n = 0.39(0.02)0.38 (2)-0.16 = 0.079 <br /> <br />Use the n value from the prediction equation because it is based <br />on data similar to the example problem. <br /> <br />I' <br />I <br />I <br />!- <br /> <br />4. The channel is average with respect to cross-section <br />irregularities, channel variation, effect of obstructions, <br />channel vegetation, and degrees of meandering, so that <br />adjustments to the base n value are not needed because the <br />prediction equation was used. The degree of meandering <br />ratio (m) L to L equals 1,00 as the flow is within the <br />main channeP. Th~ main channel n value is 0.079. <br /> <br />47 <br />