COASTAL SEDIMENT DYNAMICS UNDER ASYMMETRIC WAVES AND CURRENTS: MEASUREMENTS AND SIMULATIONS

iii As waves travel and shoal towards a beach, their surface elevation becomes peaky (sharp crests) and asymmetric relative to the vertical. Within a wave period, the corresponding orbital velocity near the bottom shows a similar (time) variation, presenting both velocity and acceleration skewnesses. These local nonlinearities are inextricably linked to sediment transport, but the processes involved are not well understood. This research intends to increase the understanding of cross-shore sediment transport processes under combined nonlinear waves plus currents, focusing on the importance of the velocity and acceleration skewnesses in sheet flow layer dynamics. For that purpose, new experimental and modelling work are presented in this thesis. Firstly, new experiments under sheet flow conditions conducted in an oscillating water tunnel (OWT) are reported, enabling to study the effects of flow acceleration on instantaneous velocities and sediment concentrations and fluxes. The measured net transport rates show that (i) the acceleration skewness in an oscillatory flow is responsible for a net sediment transport in the direction of the highest (onshore) acceleration; (ii) the net transport in the presence of an opposing current is negative, against the direction of the highest acceleration, and reduces with an increase in flow acceleration; and (iii) the velocity skewness increases th