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<br />'Ihere are several means of compositing the results to obtain an <br />equivalent n value for a stream cross section. These procedures, summa- <br />rized by Chow (1959, p. 136), involve use of each of the following three <br />assumptions: (1) The mean velocity in each subsection of the cross <br />section is the same; (2) The total force resisting the flow is equal to <br />the sum of the forces resisting the flows in the subdivided areas; and <br />(3) The total discharge of the flow is equal to the sum of the discharges <br />of the subdivided areas. It is also assumed that the slope of the energy <br />grade line is the same for each of the subsections. In some cases it is <br />not necessary to compute the equivalent n value. Instead, the subsection <br />conveyances, which are additive, are computed through assumption (3) to <br />obtain the total conveyance for the cross section. <br /> <br />Roughness values for flood plains can be quite different from values <br />for channels. Therefore, roughness values for flood plains should be <br />determined independently from channels. As in the computation of channel <br />roughness, a base roughness (nb) is assigned to the flood plain, and <br />adjustments for various roughness factors are made to determine the total <br />n value for the flood plain. <br /> <br />Seasonal variability of roughness coefficients should be considered. <br />Floods often occur during the winter when there is less vegetation. Thus, <br />the field surveys, including photograf*1s, may not be completed until <br />spring when vegetation growth would be more dense. In these instances, a <br />variable roughness coefficient may be needed to account for seasonal <br />change s. <br /> <br />In developing the ability to assign n values, reliance must be on n <br />values that have been verified. A verified n value is one that has been <br />computed where both discharge and cross-section geometry are known. <br /> <br />METIDD FOR ASSIGNING n VALUE> FOR CHANNELS <br /> <br />Although several factors affect the selection of an n value for a <br />channel, the most important factors are the type and size of the materials <br />that compose the bed and banks of a channel and the shape of the channel. <br />Cowan (1956) developed a procedure for estimating the effects of these <br />factors to determine the value of n for a channel. In this procedure, <br />the value of n may be computed by <br /> <br />n = (nb + nl + n2 + n3 + n4)m <br /> <br />(3) <br /> <br />where: <br /> <br />n3 <br />n4 <br />and m <br /> <br />a base value of n for a straight uniform, smooth channel <br />in natural materials; <br />nl = a value added to correct for the effect of surface <br />irregularities; <br />n2 = a value for variations in shape and size of the channel <br />cross section; <br />= a value for obstructions; <br />= a value for vegetation and flow conditions; <br />= a correction factor for meandering of the channel. <br /> <br />nb = <br /> <br />3 <br />