The general objective of my thesis is to develop a better understanding of non-hydrostatic 3D processes and their impact on nearshore hydrodynamics. The nearshore zone includes the surf zone and a subtidal zone, the inner shelf, extending offshore to a depth of about 20 m. The nearshore zone is particularly complex, as processes of very different temporal and spatial scales coexist and interact. It is therefore essential to improve our knowledge to correctly represent the transport of biogeochemical tracers and sediments. Since most simulations of the nearshore zone are performed using depth-averaged (2D) Boussinesq models, the impact of 3D non-hydrostatic dynamics on eddies in the surf zone and offshore transport currently remains unknown. Yet, Boussinesq models struggle to describe field observations and may miss important processes.
Recently, RANS-type, 3D wave-resolving models with a free surface have been made available to researchers to study the nearshore zone. The model used here is CROCO (Coastal and Regional Ocean Community model), enriched with a compressible, non-hydrostatic solver allowing us to explicitly resolve surface waves and the transfer of motion through surfing to the three-dimensional coastal circulation (Marchesiello et al., 2021). It allows for the simulation and understanding of coastal processes with a reduced number of unknown parameters. The overall goal of the thesis is therefore to analyze the complete 3D dynamics, including eddy structure, instability mechanisms, turbulent cascades, and transport processes. As CROCO can be progressively degraded to a depth-averaged or even wave-averaged model, it is possible to evaluate the realism of simplified equations commonly used in scientific and engineering studies.
