Sediment Ripples and Dunes

It is a general observation that some of the most appealing and fruitful research fields are found at the borderlines of traditional research areas. One example of this is the study of sediment transport and the behavior of alluvial channels, where important contributions have been made by geologists, oceanographers, and civil engineers. One of the basic problems is that when a flow is confined by boundaries composed of noncohesive sediment, the interaction between flow and boundary molds the geometry of the channel and, hence, determines the hydraulic roughness. Further, the rate of sediment transport, which is another quantity of fundamental importance, depends to a large extent on the hydraulic resistance developed by the bed configuration. The dual problem of predicting flow resistance and sediment transport as well as attempting to explain the general behavior of alluvial channels is the subject of this review. Many of the aspects are far from being clarified in a satisfactory way, and it has not been possible to avoid controversial issues. In such cases the authors have tried to state the situation objectively, but without trying to hide their own point of view. To restrict the length of the paper, pneumatic transport of sediment is not mentioned. Neither do we treat important special subjects such as gravel-bed streams and rivers with very graded sediment or very high content of wash load.