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<br />Bedforms and bars also can change flow direl::tion in a <br />channel. At low-flow, bars can be residual causing high <br />velocity flow along or at a pier or other structures, with <br />potentially deeper scour occurring. <br /> <br />In design of a bridge or countermeasure, it is good <br />engineering practice to assume a dune bed (Iargl~ n value) <br />for water surface profile analysis, and plane bed (small n <br />value) for evaluation of channel stability or scour. <br /> <br />3, Resistance to flow <br /> <br />a, General approach <br /> <br />The general approach for estimating the resistanCE! to flow in <br />a stream channel is to select a base value for materials in <br />the channel boundary, and then to make corrections to <br />account for channel irregularities, sinuosity, and other factors: <br /> <br />n = (nb + n1 + n2 + nS + nJ m <br /> <br />where <br /> <br />nb = Base value for straight, uniform, smooth channels <br />n1 = Value for surface irregularities in the cross SElction <br />n2 = Value for variations in shape and size of the channel <br />n3 = Value for obstructions <br />n4 = Value for vegetation and flow conditions <br />m = Correction factor for sinuosity of the channel <br /> <br />See HEC-20, Section 3.2.3 or HIRE for further information on <br />the use of this equation and its application to floodplains. <br /> <br />3.14 <br />