Abstract:
A laboratory study was conducted to observe the particle movements that create a cluster microform, measure the dimensions of the cluster then detect the influence the cluster has on the sediment in the remainder of the bed. Experiments were undertaken in a laboratory flume using well graded rounded river gravels ranging in diameter from 1 - 30 mm. A well defined, stable cluster was created in the test environment. Continuous image recording for the duration of the experiment allowed monitoring of the cluster formation and disintegration. Particle tracking was utilised to give the particle movement within and around the cluster and image analysis was used to record cluster dimensions. These tools showed that the stoss size was similar to the anchor stone size for most of the period where the cluster was formed. The size of the wake fluctuated significantly and the cluster orientation was observed to remain within 6˚ of the streamwise direction while the cluster was fully formed. The particle mechanics leading to the formation and disintegration are highlighted, providing insight into the dependence of the cluster stability on the surrounding bed. Measurements of the sediment transport rates showed a reduction in the localised peak rates while the cluster was formed, and increased again after the cluster disintegrated.
