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<br />(b) If the material is too large to be screened, a grid <br />system having 50 to 100 intersecting points or nodes per <br />segment is laid out. The width, or intermediate diameter <br />of each particle that falls directly under a node is <br />measured and recorded. The sizes are grouped into a <br />minimum of five ranges. The number of particles in each <br />range is recorded and converted to a percentage of the <br />total sample. <br /> <br />In both of the above sampling methods, the size that corresponds to the <br />50th percentile (using table 1) or the 84th percentile (using the <br />Limerinos method) is obtained from a distribution curve derived by <br />plotting particle size versus the percentage of sample smaller than the <br />indicated size. Experienced personnel generally can make a fairly <br />?ccurate estimate of the median-particle size by inspection of the <br />bhannel, if the range in partiCle size is small. <br /> <br />7. Determine the base n for each segment of channel using table 1 <br />or equation 5, or the comparison given in step 3. Chow's (1959) base <br />values (table 1) are for the smoothest condition possible for a given <br />material. The values (table 1) of Benson and Dalrymple (1967) are for a <br />straight, uniform channel of the indicated material and are closer to <br />actual field values than are those of Chow. If a composite n is being <br />derived from segments, the user should proceed with step 8. If n is <br />being assigned for the channel as a whole, the user should go to step ll. <br /> <br />8. Add the adjustment factors from table 2 that apply only to <br />individual segments of the channel. <br /> <br />9. Select the basis for weighting n for the channel segments. <br />Wetted perimeter should be used for trapezoidal and V-Shaped channels <br />having banks of one material and beds of another material; wetted <br />perimeter also should be used where the depth across the channel is fairly <br />uniform. Area should be used where the depth varies considerably or where <br />dense brush occupies a large and distinct segment of the channel. <br /> <br />10. Estimate the wetted perimeter or area for each segment, and <br />assign a weighting factor to each segment that is proportional to the <br />total wetted perimeter or area. Weight n by multiplying the n for each <br />segment by its weighting factor. <br /> <br />11. Select the adjustment factors from table 2 for conditions that <br />influence n for the entire channel. Do not include adjustment factors <br />for any items used in steps 7 and 8. Consider upstream conditions that <br />may cause a disturbance in the reach being studied. If Chow's (1959) <br />base values are used, the adjustment factors in table 2 may be used <br />directly. If base values are computed from the Limerinos equation or are <br />taken from Benson and Dalrymple (1967), the adjustment factors should be <br />from one-half to three-fourths as large as those given in table 2. If n <br />is assigned on the basis of a comparison with other streams, the adjust- <br />ment factors will depend on the relative amounts of roughness in the two <br />streams. Add the adjustment factors to the weighted n from step 10 to <br /> <br />49 <br />