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<br />PROCEDURE FOR ASSIGNING n VALUES <br /> <br />The procedure outlined in this section will enable the user to systemat- <br />ically evaluate the factors affecting channel and overbank roughness. Because <br />of the interaction of the factors affecting roughness, sound experienced <br />engineering judgment is needed in estimating n values. <br /> <br />The steps outlined are for one discharge or depth of flow. If n values <br />are to encompass a range of flow depths, the procedure would be repeated at <br />selected depths to account for changes in roughness with depth of flow. It is <br />suggested the procedure be applied first to the channel and then to the <br />overbank areas, <br /> <br />Following the procedure, four examples are shown for the hypothetical <br />channel shown in figure 2. A roughness evaluation form similar to figure 14A <br />and photographs of the stream are useful as documentation of stream roughness <br />coefficients. <br /> <br />I. Determine the extent of the reach in which roughness seems uniform <br />and to which n values are to apply. Conditions may not be uniform <br />throughout the reach; however, n values need to be assigned for the <br />average conditions. Use evidence of scour or deposition to <br />determine whether the channel is sand bed, stable, or a combination <br />of both. Determine if existing conditions are representative of <br />flow conditions being considered. <br /> <br />2. Determine how the cross section will be subdivided, Usually, the <br />section is subdivided by geometry into a channel and left and right <br />overbank subareas. If roughness is not uniform across the channel, <br />a composite n value needs to be computed. If distinct changes in <br />roughness occur in the overbanks and are uniform throughout the <br />reach, then separate n values need to be selected for each subarea. <br /> <br />3. Determine the type and size of material in each subarea of the <br />channel. Tables 2 and 5 or the prediction equations 5 or 6 can be <br />used to determine the base n values for each subarea of channel. <br />In many cases, there only will be one subarea to describe the <br />channel, <br /> <br />. <br /> <br />4. Determine the adjustment factors from table 1 that apply to the <br />channel. Consider upstream conditions that may cause disturbances <br />in the reach being studied. Add the adjustment factors to the <br />base n value, Multiply the adjusted values by the meander <br />coefficient. The value obtained is the overall n value for the <br />channel, as selected in step 2. <br /> <br />5. Determine the type of material in each overbank segment. <br />Tables 6 and 7,or the urban method outlined by Hejl (1977) <br />can be used to determine the overbank roughness coefficients. <br />In most cases, there only will be two segments to represent <br />the left and right overbanks. <br /> <br />45 <br />