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Step 3. Selection of modifying value for variations in the shape and size of <br />cross sections. In considering this factor, judge the approximate magnitude of <br />increase and decrease in successive cross sections as compared to the average. <br />Gradual and uniform changes do not cause significant turbulence. Turbulence <br />increases with the frequency and abruptness of alternation from large to small <br />channel sections. <br />Shape changes causing the greatest turbulence are those for which flow shifts <br />from side to side in the channel. Select modifying values based on Table <br />8.04c_ <br />Modifying value: channel <br />cross section Gradual (0.000) 1 0 Table 8.04c . <br />Step 4. Selection of modifying value for obstructions. This factor is based <br />on the presence and characteristics of obstructions such as debris deposits, <br />stumps, exposed roots, boulders, and fallen and lodged logs. Take care that <br />conditions considered in other steps not be double-counted in this step. <br />In judging the relative effect of obstructions, consider the degree to which <br />the obstructions reduce the average cross-sectional area at various depths and <br />the characteristics of the obstructions. Sharp-edged or angular objects induce <br />more turbulence than curved, smooth-surfaced objects. Also consider the <br />transverse and longitudinal position and spacing of obstructions in the reach. <br />Select modifying values based on Table 8.04d. <br />Modifying value: effect of r <br />obstructions Negligible 0.000. 0 Table 8.04d <br />Step 5. Selection of modifying value for vegetation. The retarding effect <br />of vegetation is due primarily to turbulence induced as the water flows around <br />and between limbs, stems, and foliage and secondarily to reduction in cross <br />section. As depth and velocity increase, the force of flowing water tends to <br />bend the vegetation. Therefore, the ability of vegetation to cause turbulence is <br />related to its resistance to bending. Note that the amount and characteristics of <br />foliage vary seasonally. In judging the retarding effect of vegetation, consider <br />the following: height of vegetation in relation to depth of flow, its resistance to <br />bending, the degree to which the cross section is occupied or blocked, and the <br />transverse and longitudinal distribution of densities and heights of vegetation <br />in the reach. Use Table 8.04e as a guide. <br />Modifying value: vegetation Low 0.0050.010 0 Table 8.04e