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(di)i/6 (13) <br />ci do <br />where, <br />c. = roughness modifier for each channel segment <br />da = median diameter of the particle size which is larger than <br />75% of the bed material, in the segment where Manning's n <br />is known. This may be related to the segment where n is <br />lowest,. or to the mean value of n for the whole cross-section- <br />di = the median diameter of the particle size which is larger <br />than 75% of the bed material, for-the channel segment in <br />question. <br />In order to simplify data collection to satisfy the changing nature <br />of n across a channel, many investigators base their initial channel <br />segmentation on substrate changes or "breaks" across the channel. A <br />problem with equation 12 is that form roughness (resistance caused by <br />bed configuration) and vegetation roughness are not included in the <br />estimation. In these cases, estimates of roughness for different types <br />of channels may be made using tables, diagrams, and pictures from Chow <br />(1959) or Barnes (1967). <br />MANNING'S EQUATION WITH ONE SET OF VELOCITY MEASUREMENTS <br />Referring to the cross section shown in Figure 5, suppose that in <br />addition to the slope, width, depth, and discharge, a measurement of the <br />velocity was made at each vertical column separating each channel <br />segment- Such velocity measurements would be repeated for each cross <br />section. Each channel segment would then have an average velocity, <br />calculated from the measured velocities on either-side of the segment. <br />In this case, the roughness for each channel segment may be calibrated <br />using the Manning equation for each channel subdivision: <br />_ 486 di 2/3 Sit (14) <br />ni Vi <br />This approach is applicable where the flow for which velocities are <br />being predicted is wholly within the bounds of the channel segments <br />containing the calibration flow. If the new flow-is higher than the <br />calibration flow, the roughness in an uncalibrated segment (i.e. segment <br />8 in Figure 5) may be estimated by comparison with an adjacent cali- <br />brated segment. This estimation may be made by visual observation and <br />judgement, or by the method presented previously. In this case: <br />ni 2 ci mnm (15) <br />where, <br /> <br />ni = the roughness of the uncalibrated channel segment <br />.20