Tracer Dispersion by Surfzone Eddies: Assessing the Impact of Undertow Vertical Shear

Abstract The nearshore region, which includes both the surf zone and the inner shelf, is a highly dynamic environment where eddies and rip currents of different scales coexist and interact. Understanding transport and mixing in this interface between land and sea is essential for many reasons, not least to better predict the fate of pollutants, fish larvae, and sediments. Previous modeling studies on the transport of passive tracers have primarily used depth-averaged Boussinesq models. Although these models have provided valuable information, they seem to generally underestimate mixing in the surf zone, while they overestimate it on the inner shelf. Coastal and Regional Ocean Community (CROCO), a 3D free-surface, wave-resolving model, is used here to study the effect of vertical dimension on surfzone eddies and nearshore tracer dispersion. The model is applied to both a directional wave basin experiment and a large-scale dye release experiment conducted in 2009 at Imperial Beach, California. By comparing simulations with and without vertical shear in surfzone currents, we isolate various mixing processes, such as shear dispersion, flash rips, and mini-rips, recently identified as resulting from undertow vertical shear instability. Quantifying each dispersion process in terms of bulk diffusivity reveals the important role of vertical shear in weakening flash rips (through a reduction in inverse kinetic energy cascade) and hence tracer dispersion offshore, while increasing mixing in the surf zone due to mini-rips. This study suggests potential improvements for the parameterization of coarser or simpler models to obtain a more accurate representation of nearshore transport and dispersion mechanisms. Significance Statement The purpose of this study is to investigate the dispersion of tracers (e.g., contaminants, larvae, sediments) in the nearshore. We combined laboratory and field experiments with a 3D wave-resolving model, resulting in a novel view of the physical processes involved, to explore the effect of undertow-driven vertical shear. Two main results emerge. First, vertical shear reduces stirring by transient rip currents between the surf zone and the inner shelf waters. Second, mini-rips (a process resulting from vertical shear instability) appear to be the dominant mixing process in the surf zone itself. These new results on turbulence and mixing in the nearshore zone call for a reexamination of dispersion rates, critical environmental factors, and parameterizations in coarser models.