A model of suspended sediment transport by internal tides

Abstract

The ability of internal tides to resuspend and advect sediment over continental shelves and slope regions is investigated theoretically using an internal wave and sediment transport model. Numerical experiments are carried out, firstly, with a linearly sloping continental slope and the ratio of bathymetry and internal waves characteristics creating critical, subcritical, and supercritical conditions, and secondly, for an observed section of the Australian North West Shelf. In the former cases, the model is forced with an internal tide propagating through the model domain. The latter application involves forcing by a barotropic tide which in turn generates internal waves at the shelf slope. Internal wave-generated bottom layer shear stresses are large enough to resuspend sediment. The application of a turbulence closure scheme results in the creation and maintenance of a thin nepheloid layer. The thickness of the suspended sediment layer is controlled by vertical diffusion which is large within the bottom boundary layer, but very small outside. The residual velocity and the asymmetry associated with the velocity field, result in both down- and upslope net suspended sediment fluxes, and deposition of resuspended material onto the shelf. These suspended sediment fluxes are largest for critical bottom slopes. The parting point between down- and upslope net sediment flux is found to be sensitive to the formulation of vertical mixing with the parting point moving downslope for increased mixing. At the Australian North West Shelf, near the shelf break and upper slope, the net flux of resuspended material is influenced by both the barotropic and internal tide. The phase relationship in bottom layer shear stresses generated from those two tides causes regions of enhancement and regions of reduction in the resuspension rates and net suspended sediment fluxes.