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<br />Table 4.-~tline and example of procedures for determining n values <br />for a hypothetical channel and flood plain--Continued <br /> <br />Channel Roughness (steps 3-13) --Continued <br /> <br />Step <br /> <br />Item to be determined or <br />operation to be performed <br /> <br />Factors on which decisions are <br />based and the results <br /> <br />(2) Divide into segments according <br />to type of material. <br /> <br />(3) Boulder at head of reach--add <br />as an adjustment factor to <br />composite n. <br /> <br />5 Divide into segments <br /> <br />The channel has three basic tyPes <br />of roughness caused by parallel <br />bands of bedrock, sand, and gravel <br />and cobbles. E3ch band is a <br />segment. <br /> <br />6 Type of material and <br />grain size <br /> <br />(1) Bedrock--slightly irregular <br />with fairly sharp projections hav- <br />ing a maximum height of about 3 in. <br /> <br />(2) Sand--determined by sieve <br />analysis, median particle size is <br />0.8 rom. <br /> <br />(3) Gravel and cobbles--as deter- <br />mined by examination, the material <br />is from 2 to 10 in. in diameter. <br />As determined from 100-point grid <br />system, the median particle size <br />is 6 in. <br /> <br />7 <br /> <br />Base nb <br /> <br />(1) Bedrock--table 1 shows that nb <br />for jagged and irregular rock cut <br />is from 0.035 to 0.050. Assume that <br />the projections have an average cut, <br />nb for this segment is 0.040. <br /> <br />(2) Sand--table 1 gives an nb <br />value of 0.025. <br /> <br />(3) Gravel and cobbles--table 1 <br />shows that the base nb for <br />cobbles ranges from 0.030 to 0.050. <br />The median diameter is small for <br />the size range. Use a base nb <br />value of 0.030. <br /> <br />54 <br />