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<br />derive the overall n for the channel reach being considered. When a <br />multiplying factor for meander is used, it is applied after the other <br />adjustments have been added to the base n. Round the n value as desired. <br />The value obtained is the coop:>site or overall n for the channel reach <br />selected in step l. When more than one reach is used, repeat steps 1-13 <br />for each reach. <br /> <br />12. Compare the study reach with f*1otographs of other channels <br />found in Barnes (1967) and Chow (1959) to determine if the final values <br />of n obtained in step 11 appear reasonable. <br /> <br />13. Check the flow regime for all sand channels. Use the n from <br />step 11 in the Manning'S equation to compute the velocity, which is then <br />used to compute :;;tream power. The flow regime is determined from figure <br />l. The assigned value of n is not reliable unless the stream power is <br />sufficient to cause upper-regime flow. <br /> <br />Flood-Plain Roughness (Steps 14-23) <br /> <br />14. As in step 1, the n value selected must be representative of <br />the average conditions of the reach being considered. Determine if the <br />flOOd-plain conditions are representative of those that may exist during <br />the design event being considered. Compare the flood plain with other <br />flood plains for which n values have been determined, or assigned by <br />experienced personnel, in order to estimate the possible range in n <br />values. Compare with f*1otograf*1s in this design guide and in other <br />references. <br /> <br />15. The n value for the flood plain may be determined using the <br />measurement of vegetation density or resistivity; there may be cases <br />where the roughness is determined by a qualitative evaluation of the <br />roughness, using equation 6 and the adjustment factors in table 3. A <br />decision must be made as to which method will be used. <br /> <br />16. If there are abrupt changes in roughness on the flood plain, it <br />will need to be subdivided. A representative sampling area is selected <br />for each subarea of the flood plain. <br /> <br />17. Determine the factors that cause roughness and how each is to <br />be taken into account. Such factors as surface irregularities and <br />obstructions may be accounted for in the boundary roughness, whereas <br />vegetation may be accounted for in the boundary roughness or by using the <br />quantitative method. <br /> <br />18. A base value, nb' for the flood plain's surface (bare soil <br />without anything protruding on the surface) must be chosen. A value for <br />nb is chosen from table 1. <br /> <br />19. Select the adjustment factors from table 3 for conditions that <br />influence roughness of the flOOd-plain subsection. <br /> <br />50 <br />