A new approach for predicting alluvial-channel resistance in the framework of the familiar Manning equation is proposed. The new method proposes a relation for the Manning coefficient that incorporates directly the role of bed configuration in determining alluvial-bed resistance. A relation for bed-form height is developed, which is then used as an independent variable to account for the resistance due to bed-form drag. The method was applied to a large body (969 flows) of river and flume data with satisfactory agreement with the observed values. Mean normalized errors for predicted flow depths and velocities were about 10% for all 969 flows. The procedure can be applied to natural streams with compound channels, since variable roughness for the main channel and overbank sections can be computed separately in this approach. Prediction accuracy was generally better for flows that were clearly in the lower regime (ripple or dune) or in the upper regime (flat bed or antidune), but larger errors were associated with flows that were in or close to the transitional bed regime. It is hoped that future research will focus on flow processes and bed-form geometry during transition regime.
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