Effect of Bed Sand Content on the Turbulent Flows Associated with Clusters on an Armored Gravel Bed Surface

As a gravel bed river armors, a bed surface often develops clusters as part of its structure. The influence of sand on armoring and the impact of clusters on the surrounding flow processes were investigated through flume experiments. Armored beds were created from four different sediments, which progressively increased from 1-38% sand in the bed sediment, extending from gravel framework to sand matrix beds. Turbulent kinetic energy (TKE) and xz-direction Reynolds stress calculated from velocity profiles showed the clusters increased turbulence and induced formation of recirculation cells. Similar recirculation cells formed downstream of every cluster, but the strength and orientation of these cells varied with bed condition. The increase in sediment sand content and change in bed condition from gravel framework to sand matrix correlated to an increase in the energy and momentum in the recirculation cell but a reduction in cluster influence across the flow profile. Results show that the sand content of the bed sediment during armoring was a primary factor affecting the influence of a cluster on the local flow field and the overall bed surface stability.