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<br />Burkham and Dawdy (1976) showed that equation 5 applies for upper- <br />regime flow in sand channels. If a measured dS4 is available or can be <br />estimated, equation 5 may be used to obtain a base n for sand channels in <br />lieu of using table l. <br /> <br />Adjustment Factors for Channels <br /> <br />The nb values selected from table 1 or computed from the Limerinos <br />equation are for straight channels of nearly uniform cross-sectional <br />shape. Channel irregularities, alinement, obstructions, vegetation, and <br />meandering increase the roughness; and the value for n must by adjusted <br />accordingly. This is accomplished by adding increments of roughness to <br />the base value, nb' for each condition that increases the roughness. <br />The adjustments apply to stable and to sand channels. Table 2 from <br />Aldridge and Garrett (1973) gives ranges of adjustments for the factors <br />that affect channel roughness for the prevailing channel conditions. The <br />base values of Benson and Dalrymple (l967) in table 1 and those computed <br />from equation 5 generally apply to conditions that are close to average; <br />therefore, those base values require smaller adjustments than do the base <br />values of Chow (1959). Likewise, the adjustments made (using table 2) to <br />base values of Benson and Dalrymple (1967) should be reduced slightly. <br /> <br />The effects of depth of flow on the selection of n values for <br />channels must be considered. If the depth of flow is shallow in relation <br />to the size of the roughness elements, the n value can be large. The n <br />value generally decreases with increasing depth, except where the channel <br />banks are much rougher than the bed or where dense brush overhangs the <br />low-water channel. <br /> <br />Irregularity (nl) <br /> <br />Where the ratio of width to depth is small, roughness caused by <br />eroded and scalloped banks, projecting points, and exposed tree roots <br />along the banks must be accounted for by fairly large adjustments. Chow <br />(1959) and Benson and Dalrymple (l967) showed that severely eroded and <br />scalloped banks can increase n values by as much as 0.02. Larger <br />adjustments may be required for very large, irregular banks having <br />projecting points. <br /> <br />Variation in Channel Cross Section (n2) <br /> <br />The value of n is not affected significantly by relatively large <br />changes in the shape and size of cross sections if the changes are <br />gradual and uniform. Greater roughness is associated with alternating <br />large and small sections where the changes are abrupt. The degree of the <br />effect of changes in the size of the channel depends primarily on the <br />number of alternations of large and small sections and secondarily on the <br /> <br />8 <br />