Flow resistance in alluvial channels

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There have been numerous fluvial studies of flow resistance in alluvial channels during the last few decades. Significant progress has been made towards predicting flow resistance (and therefore velocity) for a given discharge. These past applications rely heavily on the characterization of particle sizes and the effects of changing relative submergence on flow resistance estimates. Different types of equations have been shown to provide reasonably good estimates in specific environments. Major difficulties arise from characterizing mobile beds, bed topography and its evolution and how these factors control rates of change of average velocity as discharge rises along a given river reach. Different issues can be recognized as a function of the spatial and temporal scales of investigation. A case can made that more emphasis should be placed upon reach-scale investigations. Detailed studies of bed topography, its maintenance, its evolution (at the reach scale) and its interactions with macroturbulence structure and sediment transport would ultimately provide valuable information and improved knowledge on both flow resistance processes and applications (predictions). Moreover, technological means now allow detailed characterization of bed topography and flow fields of large river systems. Such promising avenues should be further pursued with the goal of providing not only a better understanding of flow-bed-sediment transport interactions in large river systems but also a better understanding of flow stage variations, flood hazards, flow resistance estimates and therefore partitioning of depth and velocity as discharge rises along major river systems.