Transport layer structure in intense bed-load

We report laboratory experiments on intense bed‐load driven by turbulent open‐channel flows. Using high‐speed cameras and a laser light sheet, we measured detailed profiles of granular velocity and concentration near the sidewall. The profiles provide new information on transport layer structure and its relation to the applied Shields stress. Contrary to expectations, we find that intense bed‐load layers respond to changes in flow conditions by adjusting their granular concentration at the base, slightly above the bed. Two mechanisms account for the resulting behavior: stresses generated by immersed granular collisions, and equilibration of the otherwise unstable shear layer by density stratification.Without parameter adjustment, the deduced constitutive relations capture the responses of velocity, concentration, and layer thickness to a ten‐fold increase in Shields stress