Sediment Resuspension on the Inner Shelf under Combined Internal Tides and Waves and Surface Waves: Field Observations and Modeling

Abstract This study investigates sediment resuspension by internal wave processes on the shallow California shelf, using high-resolution field observations collected by the Naval Research Laboratory during the 2017 Inner-Shelf Dynamics Experiment. High-resolution observations of pressure, flow velocity profiles, temperature stratification, and acoustic backscatter intensity collected near the 35-m isobath show that California inner shelf is a dynamic environment of coupled multiscale perturbations that include barotropic surface tides, internal tides and waves, and surface gravity waves. Here, we analyze sediment resuspension by an internal wave group event observed on 15 September 2017, using several theoretical formulations of wave–current boundary layer flows. Our analysis indicates that the oscillatory bottom stress induced by surface waves exceeded the mobilization threshold throughout the entire event allowing internal wave/tide currents to easily transport or resuspend the mobilized sediments. The analysis of the acoustic backscatter recorded by a velocity profiler suggests that sediment resuspension by the combined internal tide/waves flows occurred primarily under internal wave crests rather than troughs. We show that resuspension is caused by increased near-bed turbulence due to the phase coupling of internal tide and waves, which skews near-bed flows to the direction of the internal wave propagation. This is consistent with previous results showing increased turbulence during the onshore internal wave phase (Becherer et al. 2020). Because the dominant mechanism of internal wave generation on a shallow shelf is the nonlinear disintegration of the internal tide, the phase relation between the internal tide and internal waves is not random but governed by the history of their nonlinear shoaling evolution. This suggests the possibility that the sediment transport direction is modulated by the phase relation between internal tides and internal waves. Significance Statement This study investigates sediment resuspension under the multiscale turbulence due to surface waves and internal waves and tides on a shallow shelf. Results show that all turbulence scales contribute to sediment resuspension. Surface waves stresses consistently exceed sediment mobilization threshold, while the stresses due to the combined internal tide/waves flow exceed the resuspension threshold when the two oscillations are in phase. The phase relation determines the direction of sediment transport. This effect is significant for shallow shelves, where internal waves are generated by the disintegration of the internal tide, and therefore, their phase relationship is a product of their coupled evolution.