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<br />PREDICTION EQUATION FOR MANNING'S ROUGHNESS COEFFICIENT <br /> <br />Most equations used to predict channel roughness, such as those <br /> <br />reported by Carter and others (1963), Chow (1959), Limerinos (1970), <br />Simons and Senturk (1977), and Bathurst (1978) require streambed particle- <br />size information. Studies by Golubstov (1969), Riggs (1976), and <br />Ayvazyan (1979) indicate that channel roughness is directly related to <br />channel gradient in natural stable channels. Ayvazyan (1979) evaluated <br />a number of formulas used worldwide to evaluate channel roughness in <br />ear~hen canals and found that the equations yield basically equivalent <br />results, but do not truly reflect the nature of hydraulic resistance. <br />Ayvazyan (1979) showed the reason for disagreement was the failure to <br />allow for the effect of slope. This relation of resistance and slope is <br />due, in part, to the interrelation between channel slope and particle <br />size of the bed material. As slope increases, finer material is removed <br />and larger particles remain in the channel. The effect of increased <br />turbulence and resistance results in increased friction slope. The <br /> <br /> <br />correlation coefficients for selected hydraulic characteristics of the <br /> <br /> <br />data in table 1 are shown in table 3. The coefficient/for Manning's n <br /> <br /> <br />is higher for friction slope (0.71) than for dS4 particles size (0.64). <br /> <br /> <br />This supports the idea that slope has a strong influence on roughness. <br /> <br /> <br />For similar bed-material size, channels with low gradients have much <br /> <br /> <br />lower n values than channels with steep gradients. Values of n as small <br /> <br /> <br />as 0.032 have been obtained for channels having very low gradients, <br /> <br /> <br />shal low depths, and large boulders (Barnes, 1967). <br /> <br /> <br />30 <br /> <br />