Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 10/1/1999
Publication Date: N/A
Technical Abstract: To understand more fully the fluid and sediment dynamics of upper-stage plane beds, laboratory experiments were conducted using mobile and fixed beds where turbulent motions of fluid and sediment were measured using laser anemometry. Bed-elevation fluctuations on mobile upper-stage plane beds reveal millimeter-high bed waves. Vertical profiles of flow velocity, mixing length and eddy viscosity (diffusivity) are represented well by the law of the wall. For the mobile bed, von Karman's k=0.33 and equivalent sand roughness-to-mean bed-grain size varies from 9 to 17, due to the presence of bed load and low-relief bed waves. For fixed beds with no sediment transport, k=0.41 and equivalent sand roughness is equal to the mean bed-grain size. Mobile-bed turbulence intensities are greater than those for sediment-free fixed beds, due to enhanced wake formation from the lee side of near-bed grains and low-relief bed waves. Sediment diffusivities (Es) calculated in a similar way to fluid diffusivities (E) indicate that Es=E. Sediment diffusivities calculated using the equilibrium balance between upward diffusion and downward settling of sediment are similar to E in near-bed regions, but are larger than E higher in the flow, suggesting that suspended-sediment concentration higher in the flow is not closely related to mean fluid turbulence. Sediment diffusivities calculated for high-magnitude ejection events are comparable to those calculated using the diffusion-settling balance higher in the flow, suggesting that larger, more energetic turbulent eddies are responsible for sediment suspension higher in the flow.