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<br />In some cases, a more detailed subdivision may be needed. Verification <br />studies have shown that the proper method of subdividing the cross section <br />based on roughness is as follows (Davidian, 1984; and Chow, 1959). <br /> <br />The channel is not subdivided for changes in bed roughness at the low <br />water edge because of the variation between bed and bank roughness. A cross <br />section of a hypothetical main channel and flood plain showing proper subdivi- <br />sion based on geometry and roughness is shown in figure 4. The channel bed <br />consists of gravel having an n value of 0.030. A dense growth of willows on <br />the sides of the low-water channel has an n value of 0.15. A single composite <br />roughness value needs to be calculated for the main channel (subarea 3) using <br />equation 3, simplified from Chow (1959). <br /> <br />(Plnl+P2n2+...P n ) <br />mm <br />n = <br />c P <br /> <br />(3) <br /> <br />P is the <br />The composite roughness <br />eter, width, or subarea <br /> <br />is the composite n value for the channel; <br />P2, P are the wetted perimeter, in feet, for each <br />m <br />roughness area; <br />nl, n2, nm are the roughness coefficients corresponding <br />to each wetted perimeter; and <br />total wetted perimeter, in feet, for the channel. <br />often is approximated by weighting with wetted perim- <br />rather than conveyance (Chow, 1959). <br /> <br />where <br /> <br />n <br />c <br />Pi' <br /> <br />For channel subarea 3 shown in figure 4, nl = 0.15, Pi <br />0.030, P2 = 165 ft, ns = 0.15, Ps = 22 ft, and P = 202 ft. <br />the composite roughness value for subarea 3 is <br /> <br />= 15 ft, n2 = <br />Using equation 3, <br /> <br />n <br />c <br /> <br />= 0.15(15 ft)+0.030(165 ft)+0.15(22 ft) = 0.052 . <br />202 ft <br /> <br />If the roughness varies with depth, several composite n values may need to be <br />computed and used in the hydraulic analysis. <br /> <br />10 <br />