Temporal evolution, morphology, and settling of the sediment plume in a model estuary

Abstract

We examine the transport and settling of particles in a laboratory-scale model estuary using direct numerical simulation. The configuration is a shallow saltwater-filled basin with large horizontal dimensions. The particle-laden freshwater enters the basin over a relatively small inlet. Turbulence is generated by the collapse of Kelvin–Helmholtz vortices triggered in the freshwater/saltwater stratified mixing layer. The flow is computed until a statistically stationary solution is attained. The results demonstrate a significant increase of the particle settling speed compared to Stokes particle settling. For the onset of the simulation, this increase is attributed to sheet and finger convection. At later stages, we find that the increased settling speed is a result of the turbulent mixing of the particle suspension with clear ambient fluid, the density differences between the different phases, and the Stokes settling. The increase of the settling velocity is most pronounced for large Reynolds numbers, Richardson numbers of the particle suspension, and Stokes settling speeds. Apart from rather large Richardson numbers of the particle suspensions, variations of these parameters do not alter the flow qualitatively. For sufficiently small Richardson numbers, the shape of the particle plume is characterized by a distinct near-surface plume, a zone of fast particle settling beneath this plume, and a nepheloid layer at the bottom of the basin. These features are a result of the buoyant freshwater-particle mixture, the entrainment of ambient saltwater and the settling enhancement of the particulate phase.