Equivalent roughness of gravel-bed rivers

The relation between the equivalent roughness and different grain size percentiles of the sediment in gravel-bed rivers was determined under the hypothesis that the vertical distribution of the flow velocity follows a logarithmic law. A set of 954 data points was selected from rivers with gravel size sediment or larger, with a nonsinuous alignment and free of vegetation or obstacles. According to the results, the k(s) roughness is equivalent to approximately 2.4D(90), 2.8D(84), and 6.1D(50). No correlation was detected between the sediment sorting and the sediment mobility index on one hand and, on the other, the coefficient of proportionality of each grain size percentile.