FLUID AND SEDIMENT DYNAMICS OF UPPER STAGE PLANE BEDS

To understand more fully the fluid and sediment dynamics of upper stag e plane beds, laboratory experiments were conducted using mobile and f ixed beds where turbulent motions of fluid and sediment were measured using laser anemometry. Bed-elevation fluctuations on mobile upper sta ge plane beds reveal millimeter-high bed waves. Vertical profiles of f low velocity, mixing length, and eddy viscosity (diffusivity) are repr esented well by the law of the wall. For the mobile bed, von Karman's kappa approximate to 0.33 and equivalent sand roughness to mean bed-gr ain size varies from 9 to 17 because of the presence of bed load and l ow-relief bed waves. For fixed beds with no sediment transport, kappa approximate to 0.41 and equivalent sand roughness is equal to the mean bed-grain size. The decrease in kappa for mobile beds is related to t he relative motion of grains and fluid. Mobile-bed turbulence intensit ies are greater than those for sediment-free fixed beds because of enh anced wake formation from the 1ee side of near-bed grains and low-reli ef bed waves. Sediment diffusivities epsilon(s) calculated in a simila r way to fluid diffusivities epsilon indicate that epsilon(s) approxim ate to epsilon. Sediment diffusivities calculated using the equilibriu m balance between upward diffusion and downward settling of sediment a re similar to epsilon in near-bed regions (y/d < 0.3) but are larger t han epsilon higher in the flow, suggesting that suspended-sediment con centration higher in the flow is not closely related to mean fluid tur bulence. Sediment diffusivities calculated for high-magnitude ejection events are comparable to those calculated using the diffusion-settlin g balance for y/d > 0.3, suggesting that larger, more energetic turbul ent eddies are responsible for sediment suspension higher in the flow.