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<br />Compare the channel with other channels for which n values have been <br />verified or assigned by experienced personnel in order to estimate the <br />possible range in n values. (See photographs in the report by Barnes <br />(1967). ) <br /> <br />4. Determine the factors that cause roughness and how each is to be <br />taken into account. Some factors may be predominant in a particular <br />segment of the channel, or they may affect the entire cross section <br />equally. The manner in which each factor is handled depends on how it <br />combines with other factors. A gently sloping bank may constitute a <br />separate segment of the cross section; whereas, a vertical bank may add <br />roughness either to the adjacent segment or to the entire channel. <br />Obstructions, such as debris, may be concentrated in one segment of the <br />channel. Isolated boulders generally should be considered as obstruc- <br />tions; but if boulders are scattered over the entire reach, it may be <br />necessary to consider them in determining the median~particle size of the <br />bed material. vegetation growing in a distinct segment of the channel <br />may be assigned an n value of its own; whereas, roughness caused by <br />vegetation growing only along steep banks or scattered on the channel <br />bottom will be accounted for by means of an adjustment factor that can be <br />applied to either a segment of the channel or to the entire cross section. <br />If a composite n is being derived from segments, the user should continue <br />with step 5; otherwise step 5 should be omitted. <br /> <br />5. Divide the channel width into segments according to general <br />roughness. If distinct, parallel banks of material of different particle <br />sizes or of different roughness are present, it is fairly easy to define <br />the contact between the types of material. (See fig. 23, section 2.) <br />The dividing line between any two segments should parallel the general <br />flow lines in the stream and should be located so as to represent the <br />average contact between types of material. The dividing line must extend <br />through the entire reach, as defined in step 1, although one of the types <br />of bed material may not be present throughout the reach. If a segment <br />contains more than one type of roughness, it may be necessary to use an <br />average size of bed material. Where sand is mixed with gravel, cobbles, <br />and boulders throughout a channel, it is impractical to divide the main <br />channel. <br /> <br />6. Determine the type of material that occupies and bounds each <br />segment of channel and compute the median-particle size in each segment, <br />using either method a or b (below). If the Limerinos equation (equation <br />5) is used, the size corresponding to the 84th percentile should be used <br />in the computation. <br /> <br />(a) If the particles can be separated according to size by <br />screening, small samples of the bed material should be <br />collected at 8 to 12 sites in the segment of the reach. <br />The samples are combined, and the composite sample is <br />passed through screens that divide it into a minimum of <br />five size ranges. Either the volume or weight of <br />material in each range is measured and converted to a <br />percentage of the total. <br /> <br />48 <br />