Abstract

Cluster bedforms, closely nested groups of clasts aligned parallel to flow, are the prevalent type of bed microtopography in poorly sorted gravel-bed streams. They have been studied in natural channels with a wide range of clast shape and size; relationships between cluster bedform geometry and properties of the channel bed material are identified. Clusters are found to range in length from 0.1–1.2 m in a streamwise direction. Using flume experiments, it has been possible to simulate the formation of clusters resembling those found in natural channels. Observations of cluster formation indicate that these bedforms are developed during falling stage from flood discharges as particles are deposited around exceptionally large clasts which act as obstacles in the flow. Cluster bedforms play a significant role in sediment transport: they delay incipient motion and limit the availability of bed material for transportation.