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<br /> <br />PRO::EDURES FOR ASSIGNING n VALUES <br /> <br />The procedure given in the following paragraf*1s is designed to <br />determine n values for a cross section. Parts of the procedure apply <br />only to roughness of channels, and other parts deal with roughness of <br />flood plains. <br /> <br />The procedure involves a series of decisions that are based on the <br />interaction of roughness-causing factors. A flow chart (fig. 21) <br />illustrates the steps in the procedure. A description of each step <br />follows. A form is also provided to help in the computation of the n <br />values. After using the procedure a few times, the user may wish to <br />combine steps or to change the order of the steps. Experienced personnel <br />may perform the entire operation mentally, but the inexperienced user may <br />find the form in figure 22 useful. Steps 3 through 13 apply to channel <br />roughness, and steps 14 through 23 apply to flood-plain roughness. The <br />procedure is adapted from the report by Aldridge and Garrett (1973), but <br />it is extended to include assigning n values for flood plains. <br /> <br />Description of Steps for Assigning n Values <br /> <br />l. Determine the extent of stream reach to which the roughness <br />factor will apply. Although n may be applied to an individual cross <br />section that is typical of a reach, it must take into account the <br />roughness in the reach that encompasses the section. When two or more <br />cross sections are being considered, the reach that applies to anyone <br />section is considered to extend halfway to the next section. For example <br />in figure 23, the n value for section 1 represents the roughness in reach <br />A, and the n value for section 2 represents the roughness in reach B. If <br />the roughness is not uniform throughout the reach being considered, n <br />should be assigned for average conditions. <br /> <br />2. If the roughness is not uniform across the width of the cross <br />section, determine where subdivision of the cross section should occur. <br />Determine whether sutdivision between channel and flood plain is <br />necessary and whether subdivision of the channel or flood plain is also <br />necessary. If the roughness is not uniform across the width of the <br />channel, determine whether a base n should be assigned to the entire <br />channel cross section or whether a composite n should be derived by <br />weighting values for individual segments of the channel having different <br />amounts of roughness. (See steps 4-10.) When the base value is assigned <br />to the entire channel, the channel constitutes the one segment being <br />considered; and steps 5, 8, 9, and 10 do not apply. <br /> <br />Channel Roughness (Steps 3-13) <br /> <br />3. Determine the channel type--stable channel, sand channel, or a <br />cornbination--and whether the conditions are representative of those that <br />may exist during the design event being considered. Look especially for <br />evidence of bed movement and excessive amounts of bank scour. If the <br />conditions do not appear to be the same as those that would exist during <br />the flow event, attempt to visualize the conditions that would occur. <br /> <br />39 <br />