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<br />5, There is no overbank flo~. <br /> <br />6. Flow regime criteria are not available and can not be made for <br />a cobble-bed channel. <br /> <br />7. This step is not required because the flow depth was preselected. <br /> <br />Example 2.--Natural Flood Plain <br /> <br />1, Reach A in the natural flood plain shown in figure 2 extends from <br />stations 0 to 570 ft measured along the flood plain, Cross-section <br />1 represents the average conditions in the reach and is located <br />285 ft upstream from the initial reference point. The channel is <br />composed of boulders, and the reach needs an average number of <br />adjustment factors. The left overbank is covered by sparse brush <br />and trees, and the right overbank is overgrown with a dense stand <br />of trees. Lack of channel scour or deposition indicates the <br />channel is stable at most discharges, Existing channel conditions <br />correspond to those for a 100-year discharge of 6,000 ft3/s. The <br />estimated 100-year flood depth of 7 ft is shown on section 1 of <br />figure 3, A sample completed roughness evaluation form for <br />a natural flood plain is shown in figure 148. <br /> <br />2. The cross section is subdivided, based on shape, into the channel <br />and a left and right overbank. Subarea 1, the left overbank, <br />extends from stations 35 to 210 ft; subarea 2 the channel, from <br />stations 210 to 280 ft; and subarea 3, the right overbank, from <br />stations 280 to 400 ft; as shown in figure 3, <br /> <br />3. Moderate flow velocities during onsite inspection did not enable <br />an actual particle size count; however, the average particle <br />diameter of the boulders was estimated to be 12 in. The n for <br />boulders ranges from 0,04 to 0,07 (table 2). Preliminary hydraulic <br />computation indicates the 100-year friction slope is 0.02 and the <br />hydraulic radius is 7 ft, which are within the limits of prediction <br />equation 6 and figure 12. As indicated in figure 12, for a slope <br />of 0.02 and a hydraulic radius of 7 ft, n is equal to 0.065. <br /> <br />4. The channel is average with respect to cross-section irregularities, <br />channel variation, effect of obstructions, channel vegetation, and <br />degree of meandering, so that adjustments to the base n value are <br />not needed because the prediction equation was used. The degree <br />of meandering ratio, L to L equals 580 ft divided by 570 ft <br />or 1.02. No adjustmenf for ffieandering is required. The main <br />channel n value is 0.065. <br /> <br />48 <br />