Sedimentation from buoyant fine-grained suspensions

Abstract

Laboratory experiments were performed to observe the sedimentation of natural sediment in buoyant turbulent suspensions. A series of experiments were conducted in a flume modified to investigate sedimentation from a steady, stratified shear layer. Ambient salinity stratification in the lower fluid allowed the upper layer to remain buoyant, while turbulence induced by free shear in the water column produced instabilities between the two fluids. These experiments showed that convective plumes dominated sedimentation. These convective plumes had vortex tips and vertical velocities of 1–2 cm/s. The velocities observed are two orders of magnitude larger than those predicted by Stokes settling of the constituent particles. Surface plume concentrations as low as 380 mg/l were documented to support robust mixing-induced convective sedimentation. At high concentrations (>6000 mg/l), the experiments produced a divergent plume with a considerable amount of the sediment-laden fluid being diverted along the bottom of the flume. Our observed vertical sediment fluxes demonstrated a 50% increase as compared to previous studies of double-diffusive sedimentation. A simple scaling analysis using the dissipation rate of turbulent energy and the surface plume sediment concentration was able to collapse the experimental data and indicated a positive correlation between turbulent energy and sedimentation. The empirical scaling relationship accurately predicted effective settling velocities measured on the Eel River margin in 1997–1998. The quality of the agreement between the laboratory and field measurements provides strong evidence that sedimentation by mixing-induced convection may be the dominant mode of sediment removal from energetic, highly concentrated, buoyant river plumes.