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<br />Channel Gradient <br /> <br />Studies by Golubtsov (1969), Riggs (1976), and Ayvazyan (1979) indicate <br />that base n values are directly related to channel gradient in natural stable <br />channels. This relation is due, in part, to the interrelation between channel <br />slope and particle size. The effect of increased turbulence and resistance <br />results in increased friction slope. For similar bed-material size, channels <br />having low gradients have much smaller n values than stable channels having <br />high gradients. Values of n as small as 0.032 have been obtained for stable <br />channels having very low gradients, shallow flow depths, and large boulders. <br /> <br />Depth of Flow <br /> <br />Many hydraulic studies involve evaluating flow resistance over a range of <br />depth from low to high. In many cases, n values need to be selected to <br />reflect the change in flow resistance with depth of flow. <br /> <br />Based on a review of verified channel roughness data, the base n value in <br />a uniform channel does not vary with depth of flow if the ratio of the mean <br />depth (usually hydraulic radius) of flow to bed-material size (usually the <br />median diameter) is greater than 5 and less than 276. This can be expressed <br />as <br /> <br />276 >~> 5 <br />dso <br /> <br />It is assumed the channel widths are large relative to depth of flow, and the <br />bed and bank materials are the same. This condition generally exists for <br />sand- and gravel-bed channels, but not for cobble- and boulder-bed channels. <br /> <br />Ratios less than 5 generally apply to mountain streams having relatively <br />large median bed-particle sizes. In these streams, n values vary signifi- <br />cantly with depth of flow. Ratios of greater than 276 generally are found in <br />sandbed streams where significant variations in n values are due to changes in <br />bed-form configuration. <br /> <br />Prediction equations provided later in this report aid <br />change of roughness with depth of flow in natural channels. <br />judgment is needed for other types of channels and overbank <br /> <br />in evaluating <br />Subjective <br />areas. <br /> <br />As the depth of flow increases, the effect of streambed particle size <br />generally decreases, and channel roughness decreases. Channels not fitting <br />this relation consist of those whose banks are much rougher than the bed <br />material, channels in which a dense growth of vegetation impedes the main <br />channel flow, and channels whose cross sections are extremely irregular. <br /> <br />14 <br />