Quantification of intense transport of fractions of stratified bimodal bed load based on measured distributions of velocity and concentration

We present the results of laboratory experiments investigating the intense transport of bimodal bed load under high bed shear conditions in a tilting flume. Particles of two lightweight sediment fractions, differing in size, tend to separate during transport above the plane surface of an eroded mobile bed. Coarser fraction particles are predominantly present in the collisional layer above the bed, while finer fraction particles are primarily concentrated in the interfacial layer, which develops between the eroded bed and the collisional layer. This observed stratification of transported fractions influences their respective contributions to the total bed load discharge through the flume. Vertical distributions of local velocity and volumetric concentration were measured across the flow depth for each fraction separately, allowing the determination of each fraction's proportion in the total discharge. The experimental results were combined with a previously collected dataset to compare the discharges of bimodal and unimodal sediments under hydraulically similar conditions. Additionally, the experimentally determined discharges were evaluated against predictions from transport models designed for intense unimodal and bimodal bed loads.