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<br />Sediment load <br /> <br />Limited laboratory and field data indicate that small suspended-sediment <br />concentrations result in appreciable reduction in turbulence and roughness in <br />stable channels. The relative effect is hard to assess because information on <br />particle size, settling velocity, and amount of sediment present in the flow <br />are needed. In addition, if sufficient sand is available, large changes in <br />roughness result from changes in bed configuration associated with sand-bed <br />channels. As the flow increases, larger bed material, as bed load, is moved <br />by rolling or sliding along or temporary suspension above the streambed; this <br />consumes energy, and roughness increases. These problems have not been stud- <br />ied sufficiently for adequate guidelines to be developed. <br /> <br />Adjustment Factors <br /> <br />Base n values may need to be adjusted to reflect other factors that <br />increase flow resistance. The base n value for channel roughness selected <br />from tables or computed from regression equations may need adjustments for <br />cross-section irregularities, channel variations, effects of obstructions, <br />channel vegetation, degree of meandering, and other factors. <br /> <br />The adjustments for base n values for channels to determine the entire <br />channel n value are shown in table 1 and discussed below. Examples of apply- <br />ing these adjustment factors are given in the section "Procedure for Assigning <br />n Values." <br /> <br />Chow's (1959) adjustments (table 1) are applicable when the base n is <br />selected for the smoothest reach attainable for a given bed material. The <br />base n values of Benson and Dalrymple (1967) generally apply to conditions <br />that are.closer to the average and, therefore, their base values need smaller <br />adjustments than do the base values of Chow (1959). Aldridge and Garrett <br />(1973) suggest an adjustment of about two-thirds of the adjustment factors <br />given in table 1. The base n values of equations 5 and 6 generally would need <br />adjustment only if the respective channel adjustment was for the severe <br />channel condition. For this condition, the adjustment needs to be about half <br />of the respective maximum value given in table 1. <br /> <br />Extremely rough conditions may need larger adjustments than the largest <br />values given in table 1. The conditions may occur in steep mountain channels <br />or during extreme flood conditions. <br /> <br />Cross-Section Irregularities, nl <br /> <br />Surface irregularities such as eroded and scalloped stream banks, exposed <br />tree roots, and rock outcrops increase the wetted perimeter, create turbu- <br />lence, and increase roughness. Generally, the effect of these irregularities <br />increases with depth of flow. Where the ratio of channel width to depth is <br />small, larger adjustments are needed. <br /> <br />15 <br />