Surfzone eddies and undertow vertical shear effects on tracer dispersion: a 3D wave-resolving model approach

– Simon Treillou, LEGOS –

Résumé :

The nearshore zone, encompassing the surf zone and the inner shelf, is a highly dynamic region where surfzone eddies and rip currents of varying scales coexist and interact. This critical interface between land and sea determines the transport of various elements, including sediments, contaminants (such as heavy metals, microplastics, and pathogens), as well as biological tracers like plankton and larvae. These tracers are central to addressing key coastal challenges, such as beach erosion, coastal pollution, and ecosystemic services. On longshore-uniform sandy beaches, one of the primary transport mechanisms is transient rip currents, driven by wave directional spread. While numerous studies have investigated passive tracer transport under these conditions, most have relied on depth-averaged wave-resolving models (Boussinesq models). Although these models offer valuable insights, they fail to capture the effect of vertical shear resulting from shoreward surface flow induced by breaking waves and seaward undertow. They typically underestimate mixing within the surf zone and overestimate offshore dispersion. Recently, 3D wave-resolving models such as CROCO have provided researchers with the tools to conduct more comprehensive studies. The objective of this thesis is to contribute to the ongoing improvement of these models and to assess the impact of undertow vertical shear on tracer dispersion. The first step was to correct a coherent interference problem in the CROCO wavemaker, then to validate its ability to resolve transient nearshore dynamics using a recent wave basin experiment. After confirming the robustness of the model, the influence of vertical shear was examined through two dye release experiments, one in a wave basin and the other during a large-scale field experiment at Imperial Beach, California. Comparisons of simulations with and without undertow vertical shear revealed two key findings: a reduction in offshore dispersion due to a weakening in the 2D inverse kinetic energy cascade, and enhanced mixing within the surf zone through a newly identified 3D process associated with « mini-rips », a type of intermediate-scale transient current recently discovered. This research, which provides a more accurate representation of transport mechanisms in the nearshore zone, offers valuable feedback for improving parameterizations in coarser models.

Jury :

• Volker ROEBER, Rapporteur, Université de Pau et des Pays de l’Adour
• Damien SOUS, Rapporteur, Université de Pau et des Pays de l’Adour
• France FLOC’H, Examinatrice, Université de Bretagne Occidentale
• Laurent LACAZE, Examinateur, CNRS (IMFT)
• Rafael ALMAR, Examinateur, IRD (LEGOS)
• Patrick MARCHESIELLO, Directeur de thèse, IRD (LEGOS)
• Pascal NOBLE, Membre invité, INSA
• Franck DUMAS, Membre invité, SHOM