Fluid and sediment dynamics of upper stage plane beds

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 Kármán/s κ ≈ 0.33 and equivalent sand roughness to mean bed‐grain size varies from 9 to 17 because of the presence of bed load and low‐relief bed waves. For fixed beds with no sediment transport, κ ≈ 0.41 and equivalent sand roughness is equal to the mean bedgrain size. The decrease in κ for mobile beds is related to the relative motion of grains and fluid. Mobile‐bed turbulence intensities are greater than those for sediment‐free fixed beds because of enhanced wake formation from the lee side of near‐bed grains and low‐relief bed waves. Sediment diffusivities εs calculated in a similar way to fluid diffusivities ε indicate that εs≈ε. Sediment diffusivities calculated using the equilibrium balance between upward diffusion and downward settling of sediment are similar to ε in near‐bed regions (y/d < 0.3) but are larger than ε 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 for y/d > 0.3, suggesting that larger, more energetic turbulent eddies are responsible for sediment suspension higher in the flow.