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<br />Table 1.-~Adjustment factors for the determination of n values for the entire channel <br />(Modified from Aldridge and Garrett, 1973] <br /> <br />Channel conditions <br /> <br />Example <br /> <br />n value <br />adjustment1 <br /> <br />Cross-section <br />irregular~ <br />ities, nl' <br /> <br />Smooth <br /> <br />0.000 <br /> <br />Compares to the smoothest channel attainable in a given bed <br />material. <br /> <br />Compares to carefully dredged channels ip good condition but <br />having slightly eroded or scoured side slopes. <br /> <br />Compares to dredged channels having moderate to considerable <br />bed roughness and moderately sloughed or eroded side slopes. <br /> <br />Badly sloughed or scalloped banks of natural streams; badly <br />eroded or sloughed sides of canals or drainage channels; <br />unshaped, jagged, and irregular surfaces of channels in <br />rock. <br /> <br />Channel varia- <br />tions, n2 <br />(Do not <br />reevaluate <br />channel <br />variation <br />in the <br />hydraulic <br />computa- <br />tions) . <br /> <br />Minor <br /> <br />0.001-0.005 <br /> <br />Size and shape of channel cross sections change gradually. <br /> <br />Large and small cross sections alternate occasionally, or the <br />main flow occasionally shifts from side to side owing to <br />changes in cross-sectional shape. <br /> <br />Large and small cross sections alternate frequently, or the <br />main flow frequently shifts from side to side owing to <br />chsnges in cross-sectionsl shape. <br /> <br />Effect of <br />obstruc~ <br />tions, n3. <br /> <br />Moderate <br /> <br />0.006-0.010 <br /> <br />A few scattered obstructions, which include debris deposits, <br />stumps, exposed roots, logs, piers, or isolated boulders, <br />that occupy less than 5 percent of the cross-sectional ares. <br /> <br />Obstructions occupy less than 15 percent of the cross-sectional <br />area, and the spacing between obstructions is such that the <br />sphere of influence around one obstruction does not extend to <br />the sphere of influence around another obstruction. Smaller <br />adjustments are used for curved smooth-surfaced objects than <br />are used for sharp-edged angular objects. <br /> <br />Obstructions occupy from 15 to 50 percent of the cross-sectional <br />area, or the space between obstructions is small enough to <br />cause the effects of several obstructions to be additive, <br />thereby blocking an equivalent part of a cross section. <br /> <br />Obstructions occupy more than 50 percent of the cross~sectional <br />area, or the space between obstructions is small enough to <br />cause turbulence across most of the cross section. <br /> <br />Severe <br /> <br />0.011-0.020 <br /> <br />Gradual 0.000 <br /> <br />Alternating <br />occasion- 0.001-0.005 <br />ally. <br /> <br />Alternating 0.010-0,015 <br />frequently. <br /> <br />Negligible <br /> <br />0.000-0.004 <br /> <br />Mi~or <br /> <br />0.005-0.015 <br /> <br />Appreciable <br /> <br />0.020-0.030 <br /> <br />Severe <br /> <br />0.040-0.060 <br /> <br />16 <br />