Surfzone Mixing: Inhomogeneities and Suppression by Strong Alongshore Currents
Within the surfzone, wave breaking gradients generate rotational horizontal flows, ie. horizontal flows with vertical vorticity. These eddies stir and mix material (such as pollution) within the surfzone. Previous drifter and tracer observations (and modeling studies) have indicated that the surfzone cross-shore eddy diffusivity K is approximately 1 m^2/s and the Lagrangian time-scale is approximately 100 s. Due to limited data (both short and few drifter trajectories), a single diffusivity and a single Lagrangian time-scale were reported for the entire surfzone. However, the surfzone eddy field is highly inhomogeneous with eddy velocities approximately 0.2 m/s (for 1.5 m incident waves) within the surfzone and near zeros 200 m farther offshore. Here, the cross-shore dependence of surfzone mixing is investigated using particle trajectories obtained from a wave-resolving model (funwaveC). Model eddy diffusivities indicate that surfzone mixing depends significantly on the time since release and the drifter cross-shore release location. These dependencies are investigated resulting in a more complete picture of surfzone mixing.
Eddy diffusivities are also found to depend on the mean wave angle. For obliquely incident waves that drive an unstable alongshore current, eddy diffusivities are smaller than diffusivities found for the same wave field but normally incident waves. For strong alongshore currents, eddies propagate at a speed different from the local mean current speed leading to suppressed cross-shore diffusivities. This is the same mechanism that sets the diffusivity for geostrophic eddies [Ferrari & Nikurashin, 2010]. A linear stability analysis indicates that the surfzone eddy phase speed is approximately the phase speed of the most unstable mode of the alongshore current.