Cluster evolution and flow-frictional characteristics under different sediment availabilities and specific gravity

The formation of cluster structures in gravel bed streams is a naturally occurring process that characterizes the response of sediment to the erosive power of flowing water. Cluster structures are formed when two or more particles group together. Their size depends on sediment availability, flow intensity, and sediment material properties such as sediment specific gravity. The scope of the present study was to (1) quantitatively describe the evolution and disintegration processes of cluster microforms under different sediment availabilities and sediment specific gravities; and to (2) examine the flow-frictional characteristics around clusters and compare them with those around a single particle that is fully exposed to the flow. Overall, 14 experimental runs were carried out in a laboratory flume for three sediment availabilities, namely the 2%, 35%, and 50%. Six of these tests were performed by using glass spheres (Specific gravity, SG = 2.58), the other six by employing an equal combination of glass and Teflon spheres (SG = 2.12) of the same diameter to evaluate the role of specific gravity on cluster evolution; the remaining two runs were performed for the 50% sediment availability using heavier lead palettes (spheres) of the same size with the glass and Teflon spheres to facilitate near-bed flow measurements around a cluster and a single particle without the occurrence sediment movement. The results of this study showed that: (1) sediment availability and specific gravity affects the architecture and size of cluster microforms; and (2) if we use the critical stress for the lowest sediment availability (2% availability), tau(cr2%)*, as the reference stress, then particle members of clusters disintegrate at twice the tau(cr2%)* for the 2% availability case, at nine times more than the tau(cr2%)* for the 35% case and at eleven times more than the tau(cr2%)* for the 50% case. Finally, it was determined for the 50% case that computed form resistance comprises 98% of the total when clusters form, which agrees with findings that have been reported in literature.