Vertical Sorting in Intense Transport of Bimodal Contact Load: Measured Distributions in Collisional Layer and Effects on Bed Friction and Flow Condition

In the intense transport of bimodal contact load under high bed shear conditions, particles of the two fractions, which differ in size, tend to separate. The finer particles form an interfacial layer at the top of the mobile bed, while the coarser particles predominantly occupy the collisional layer above the interface. We report on tilting-flume experiments focusing on measurements of the local velocity and concentration distributions of particles for each fraction within the collisional layer. The results reveal that, due to vertical sorting, the presence of finer fraction particles rapidly diminishes with increasing elevation above the interfacial layer, whereas the local concentration of the coarser fraction is higher within the collisional layer and reaches a maximum near the center of the layer. The local velocity of the sediment is lower than the local velocity of water at the same elevations, indicating the expected local slip between the phases within the collisional layer. Furthermore, we utilize the measured integral quantities along with a previously collected dataset to compare similar flows with unimodal and bimodal transport. The comparison indicates that bimodal transport offers advantages in terms of reduced channel resistance and enhanced transport capacity.