Observations of nearbed turbulence intermittency over mobile bedforms in combined, collinear wave-current flows

Collinear wave-current shear interactions are often assumed to be the same for currents following or opposing the direction of regular wave propagation; with momentum and mass exchanges restricted to the thin oscillating boundary layer (zero-flux condition) and enhanced but equal time-averaged bed shear stresses. To examine these assumptions, a prototype-scale experiment investigated the nature of turbulent exchanges outside of the combined boundary layer in flows with currents aligned to, and opposing, wave propagation over a mobile sandy bed. Estimated mean and maximum stresses (from measurements outside the theoretical oscillatory boundary thickness) exceeded predictions by models of bed shear stress subscribing to the assumptions above. The core flow experiences upward turbulent fluxes in aligned flows, coupled with sediment entrainment by vortex shedding at flow reversal, whilst downward fluxes of eddies generated by the core flow, and strong adverse can enhance near-bed mass transport, in opposing currents. Current-aligned coherent structures contribute significantly to the stress, and display characteristics of wall-attached eddies formed by the pairing of counter-rotating vortices. These preliminary findings suggest a notable difference in wave-following and wave-opposing wave-current interactions, and highlight the needed to account for intermittent momentum-exchanges in predicting stress, boundary layer thickness and sediment transport.