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This paper explores the underlying mechanism of flow resistance in a wide alluvial channel with bed forms. On the basis of published data, it is shown that the grain roughness can be taken as equal to 2 times the median diameter of the bed sediment. An empirical equation for the bed form roughness has been proposed, and it depends on the bed form height and bed form steepness. The influence of the bed form length and height on the total bed shear stress and energy slope is deliberated, and empirical expressions for the length of the separation zone behind the bed forms are also proposed. The study proposed an equation to compute the total bed shear stress as a function of the grain and bed form roughness as well as the important role of the bed form geometry in the overall flow resistance in alluvial channels. The model is tested and verified against 670 flume measurements and 1540 field observations. The computed and measured energy slopes are in good agreement with close to 71% of all data sets within the ±20% error band.
Yang et al. (Thu,) studied this question.