VELOCITY AND CONCENTRATION PROFILES IN SHEET-FLOW LAYER OF MOVABLE BED

Sheet-flow layer of movable bed in steady currents has been investigat ed experimentally, using four kinds of sediment. Velocity and concentr ation profiles as well as flow resistance were measured inside and out side the sheet-flow layer. The range of fall-velocity-to-friction-velo city ratio (w/U-f) achieved in the study is 0.2 < w/U-f < 1.7, which c overs 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 sus pension-mode sheet flow, the flow resistance is found to depend not on ly on the Shields parameter but also on a parameter involving w. Measu red velocity profiles are found to follow the logarithmic law near the bed, outside the sheet-flow layer, whereas inside the sheet-flow laye r, they satisfy a power law. Measured concentration profiles, on the o ther hand, indicate a linear variation with the distance from the bed inside the sheet flow-layer near the bed, whereas away from the bed, t he familiar Rouse distribution is satisfied. Evidently, sediment trans port in the sheet-flow layer is influenced by the turbulent bursting p rocess.