Impact of 3D non-hydrostatic dynamics on tracer transport in the nearshore region

The nearshore zone, including the surf zone and the inner shelf (up to about 20 m depth), is a particularly chaotic zone where processes with very different scales coexist and interact permanently. This very active region, which marks the transition from the continent to the sea, is of crucial importance in many aspects (beach erosion/accretion, dispersion of pollutants or larvae, users’ safety, etc.). Several studies have already looked at the transport of passive tracers in the nearshore zone, but they generally use depth-averaged models. As a result, little is known about the non-hydrostatic 3D processes governing the nearshore zone, while depth-averaged models struggle to accurately represent measured data. For example, they tend to underestimate transport in the surf zone and overestimate it on the inner shelf. Recently, 3D wave-resolving models with a free surface have been made available to researchers. One of these models, CROCO (Coastal and Regional Ocean Community model) is used here and allows the simulation of nearshore processes with a reduced number of unknown parameters. The model is applied to a large-scale dye release experiment in California (Imperial Beach, 2009), for which simulations with the depth-averaged funwaveC model have already been performed. The IB09 survey is ideal to study processes as the beach is almost alongshore uniform. Several diagnostics are performed, such as tracer mass balance or quantification of the exchange between the surf zone and the inner shelf. The diagnostics highlight the vertical shear and 3D instabilities at work in the nearshore zone and infer the ability of 3D wave-resolving models to adequately reproduce the available observations, particularly with respect to surf-shelf exchange. This work may in the future suggest feedback for better parameterization in coarser, simpler models for a more accurate depiction of coastal pollution fate and sediment transport.