Velocity and concentration profiles in sheet-flow layer of movable bed

Sheet-flow layer of movable bed in steady currents has been investigated experimentally, using four kinds of sediment. Velocity and concentration profiles as well as flow resistance were measured inside and outside the sheet-flow layer. The range of fall-velocity-to-friction-velocity ratio (w/U_f) achieved in the study is 0.2 < w/U_f < 1.7, which covers both the suspension-mode sediment transport (w/U_f < 0.8-1), and the no-suspension-mode sediment transport (w/U_f > 0.8-1). In the suspension-mode sheet flow, the flow resistance is found to depend not only on the Shields parameter but also on a parameter involving w. Measured velocity profiles are found to follow the logarithmic law near the bed, outside the sheet-flow layer, whereas inside the sheet-flow layer, they satisfy a power law. Measured concentration profiles, on the other hand, indicate a linear variation with the distance from the bed inside the sheet flow-layer near the bed, whereas away from the bed, the familiar Rouse distribution is satisfied. Evidently, sediment transport in the sheet-flow layer is influenced by the turbulent bursting process.