Sediment resuspension mechanisms associated with internal waves in coastal waters

Large-amplitude, vertically trapped internal waves can induce sizable velocities and trigger hydrodynamic instabilities in the bottom boundary layer, thereby contributing to the resuspension of sediments and the maintenance of sediment concentration in the water column. We discuss numerical simulations of several different situations in which the boundary layer in the wave footprint undergoes hydrodynamic instability, with a resultant increase in the incidence of spatiotemporal structures that could facilitate sediment resuspension. For the case of internal solitary waves we provide bounds in parameter space separating regions in which internal waves can be expected to efficiently resuspend sediment from those in which the boundary layer in the wave footprint is both laminar and stable. A notable finding is that the onset of instability is a strong function of the background current. The Lagrangian transport of passive particles due to the instability is explored, and some quantitative measures of the efficiency of the particle transport process are provided. We subsequently discuss the evolution of the power spectra of the bottom shear stress with time and find that while the general characteristics of the instability are robust, lowering either the Reynolds number or the strength of the background current leads to an increase in the typical length scales associated with the mature instability. Finally, we discuss instabilities during the internal wave generation process and alternative instability mechanisms when the bottom is not flat.