Coherent turbulence structures and sediment resuspension in the coastal benthic boundary layer

This work addresses the complex interactions between bed‐generated coherent turbulence structures and suspended non‐cohesive sediment in wave‐dominated environments and under combined wave‐current flows. Coherent structures are intermittent, connected fluid masses with a defined spatial extent and life‐span and characterised by vorticity; and define entrainment and momentum exchanges in turbulent flows. Prototype‐scale experimental observations of flow, bedforms and suspensions are presented; focusing on the structural, spatial and temporal dynamics of these motions and their role in sediment entrainment. Two scenarios are considered: (a) shoaling (and breaking) erosive and accretive waves in the nearshore of a sandy barrier beach (spilling and plunging breakers); and (b) collinear steady currents aligned with, and opposing the direction of wave propagation over a rippled, sandy bed. Measurements collected in the nearshore of a prototype‐scale sandy barrier beach show intermittent momentum exchanges characterised by large wave induced coherent structures within the benthic boundary layer, corresponding to the mean flow properties. These structures can be described by a 3D bursting sequence which plays a significant role in moving and maintaining sediment in suspension. The temporal variability of these events dictates the net onshore and offshore transport. Periods associated with a succession of powerful turbulent events cause powerful suspension clouds across multiple frequency scales. The bulk of suspension is attributed to wave‐induced fluctuations of low frequencies. These modulate smaller, rapidly decaying high‐frequency turbulence extending outside the boundary layer. As larger structures persist for a considerable amount of time, suspensions near the bed are amplified before decaying as the supply of momentum ceases. Outside the boundary layer, momentum transfer via turbulent fluctuations maintains the suspensions until their energy is depleted. In combined wave‐current flows, the current plays a significant role in dictating the prevalence of specific turbulent motions within a bursting sequence. The current‐aligned structures contribute significantly to the stress, and display characteristics of wall‐attached eddies formed by the pairing of counter‐rotating vortices. In aligned wave‐current flows, the flow is characterised by a local balance between turbulent production and dissipation, and displays the ‘universal’ inertial cascade of energy in the outer flow, while just outside the combined boundary layer a superposition of eddies is observed, often linked to intermittent coherent turbulent structures at an intermediate range between energy production and dissipation. When the current opposes the direction of wave propagation, stress‐bearing coherent structures break rapidly as they are ejected higher in the water column. Suspended clouds through vortex shedding thus cease rapidly in opposing flows, compared to more continuous sediment transport in aligned flow. Both studies indicate that bed‐induced coherent turbulence structures play a significant role in the entrainment and transport of suspended sediment in flows typically encountered in the coastal environment. Suspension clouds induced be vortex shedding are maintained by the continuous supply of momentum through vortex clusters; and transport is described by the advection of these particles through net currents. A complex feedback is then imparted by the suspended particles on these structures. This merits a reevaluation of coastal sediment transport models, and a shift towards a stochastic description of the problem.