Submitted to: International Conference on Fluvial Sedimentology
Publication Type: Abstract Only
Publication Acceptance Date: 10/15/1997
Publication Date: N/A
Technical Abstract: To understand more fully the fluid and sediment dynamics of upper-stage plane beds, laboratory experiments were conducted in sediment-laden and clear-water flows where turbulent motions of both fluid and sediment were measured using laser and phase Doppler anemometry. Bed-elevation fluctuations on mobile upper-stage plane beds ranged from a fraction of a millimeter to several millimeters. Vertical profiles of time-averaged flow velocity, mixing length and eddy viscosity (spatially averaged over the bed waves) are represented well by the law of the wall for the entire thickness of the boundary layer. Sediment diffusivities Es calculated in a similar way to fluid diffusivities (eddy viscosities, E) indicate that Es=E. The Rouse equation for equilibrium suspended-sediment concentration fits the data well near the bed, but underpredicts concentration higher in the flow because Es is underestimated. Mobile-bed turbulence intensities are greater than those for sediment-free fixed beds due to enhanced wake formation and eddy shedding from the lee-side of near-bed grains and low-relief bed waves. Analysis of upward and downward velocity fluctuations reveals a net upward momentum flux throughout the boundary layer. High concentrations of suspended sediment apparently decrease the magnitude of ejections relative to sweeps higher in the flow.