Observations of sediment resuspension by non-linear internal waves at two sites on the Northwest Shelf of Australia

William Edge1, Nicole L Jones2, Matt Rayson2, Gregory N Ivey3 and Andrew Zulberti4, (1)United States, (2)University of Western Australia, Oceans Graduate School and Oceans Institute, Crawley, WA, Australia, (3)University Western Australia, Oceans Graduate School and Oceans Institute, Crawley, WA, Australia, (4)University of Western Australia, Perth, WA, Australia
Abstract:
On many continental shelves around the world non-linear internal waves (NLIWs) play a significant role in ocean processes, driving currents, ocean mixing and bottom boundary layer (BBL) dynamics. Despite this, the exact mechanisms of NLIW-induced sediment resuspension and transport remains uncertain due to a paucity of suitable field observations.

Here we present observations of NLIW-forced sediment resuspension from two field experiments (Browse Basin, 2017, Canning Basin, 2019). Total suspended sediment (TSS) data is inferred from calibrated turbidity, laser scattering, and acoustic measurements within the bottom boundary layer. We use TSS coupled with through-water-column density, current velocity and near-bed turbulence observations to infer the mechanisms driving vertical resuspension.

Forcing varies between the sites resulting in different modes of resuspension and advection. Shoaling waves of depression propagating in trains dominate the near-bed current and resuspension events at the Browse Basin site. Shoaling bores and associated drawdown appear to advect turbid water up and down a pelagic ridge at the Canning Basin site. At both sites, TSS measurements associated with NLIW forcing exceed peak spring tide resuspension measurements by an order of magnitude or more.

Different mechanisms of resuspension are determined through examination of bed shear stress, Reynolds stress, boundary layer vorticity, and mean vertical advection. The near-bed current-resuspension hysteresis is investigated and we present a preliminary vertical balance of sediment resuspension. The straining of the near-bed stratification by the NLIWs plays an important role in the vertical transport of the sediment.