Understanding braiding processes in gravel-bed rivers: progress and unsolved problems

Abstract

Abstract This paper is a review, from a geomorphologist’s point of view, of recent progress in understanding the interrelationships between channel form, flow, bed texture, and bedload transport in braided gravel-bed rivers. The fundamental geomorphological units in such rivers are not bars alone but bar-pool units, usually associated with confluences and bifurcations. Braiding can develop not only by midstream deposition but by bar dissection and partial avulsion; the hydraulic conditions favouring different mechanisms deserve study. Braiding is highly dynamic, with rapid feedback between channel configuration, flow, and sediment transport. These interactions can profitably be observed in Froudescaled flume models, and can to some extent be quantified by intensive field measurements, but numerical modelling is potentially the best approach. Sediment is sorted both laterally and longitudinally within pool-bar units, but some of the possible mechanisms have not been adequately studied. The downstream fining which occurs along entire braidplains can be explained by selective transport but associated changes in bar types may not follow any single sequence.