Sediment Resuspension Due to Near-Bed Turbulent Effects: A Deep Sea Case Study on the Northwest Continental Slope of Western Australia

Abstract

Sediment transport equations often consider a mean velocity threshold for the initiation of sediment motion and resuspension, ignoring event-based turbulent bursting processes. However, laboratory experiments have suggested that near-bed sediment resuspension is influenced by intermittent turbulent coherent structures. In the field, accessibility constraints for deployment of easily operated equipment has largely prevented further identification and understanding of such processes, which may contribute to resuspension in the marine environment. Field experiments were conducted on the Northwest Slope, Australia, under conditions where the mean current velocities were below the estimated and measured time-averaged critical velocity to investigate the relationship between near-bed turbulent coherent structures and sediment resuspension. Results indicate that sediment resuspension occur even when velocities are below the estimated and measured mean critical values. The majority of turbulent sediment flux is due to ejection and sweep events, with lesser contributions from up-acceleration and down-deceleration (vertical flow) events. Spectral and quadrant analysis indicated the anisotropic and intermittent nature of Reynolds stresses, and wavelet transform revealed a group of turbulent bursting sequences associated with sediment resuspension. These observations, in flow conditions where resuspension was not expected to occur based on mean threshold concepts, reveal that intermittent turbulent events control sediment resuspension rather a single time-averaged critical velocity. This highlights the need of considering turbulence as a significant factor in sediment resuspension and should be further investigated for inclusion into future sediment transport modeling.