Bimodal fluvial bed sediments: Origin, spatial extent and processes

Historically fluvial channels have been broadly classified as either gravel or sand bedded. However, a third type, bimodal, also exists. In this case significant quantities of both gravel and sand are present. Studies into bimodal sediments have concentrated on their origin, spatial extent and, more recently, the processes occurring within them. Given the advances in our knowledge of these complex processes it is appropriate to review the developments of research into bimodal sediments. Based on a synthesis of this research it is shown that bimodality may occur readily and be spatially widespread. The effects of the surface sorting prevalent within bimodal sediments on entrainment and transport processes are also explored. From this it is concluded that bimodal beds may represent a distinct threshold between gravel- and sand-bed states. This manifests itself as an abrupt downstream change from a gravel bed to a sand bed.