The Generation of Surfzone Eddies in a Strong Alongshore Current

The surfzone contains energetic two-dimensional horizontal eddies with length scale larger than the water depth. Yet, the dominant eddy generation mechanism is not understood. The wave-resolving model funwaveC is used to simulate surfzone eddies in four case examples, from the SandyDuck field experiment, that had alongshore uniform bathymetry. The funwaveC model is initialized with the observed bathymetry and the incident wave field in 8-m depth and reproduces the observed cross-shore structure of significant wave height and mean alongshore current. Within the surfzone, the wave-resolving funwaveC-modeled E(f, k(y)) spectra and the bulk (frequency and k(y) integrated) rotational velocities are consistent with the observations below the sea-swell band (<0.05 Hz), demonstrating that the model can be used to diagnose surfzone eddy generation mechanisms. In the mean-squared perturbation vorticity budget, the breaking wave vorticity forcing term is orders of magnitude larger than the shear instability generation term. Thus, surfzone eddies (vorticity) generally are not generated through a shear instability, with possible exceptions for very narrow banded in frequency and direction and highly obliquely large incident waves. The alongshore wavenumber spectra of breaking wave vorticity forcing is broad with the majority (>80%) of vorticity forcing occurring at short alongshore scales <20 m. However, the alongshore wavenumber spectra of vorticity is red, which may be due to a 2D turbulence inverse energy cascade bringing energy to longer wavelengths or may result from an amplified vorticity response to direct forcing at smaller k(y).