The suppression of surfzone cross‐shore mixing by alongshore currents

Mixing by horizontal surfzone eddies is important to the cross‐shore exchange of material through the surfzone. Surfzone cross‐shore mixing for normally and obliquely incident waves, with very similar incident significant wave heights and directional spreads, is quantified using Boussinesq modeled drifter trajectories. For t < 100 s, surfzone cross‐shore diffusivities K ( t ) are independent of incident wave angleθ ̄ owing to similar eddy velocities U ≈ 0 . 1  m s −1 . For 600 < t < 10,000 s, K ( t ) is maximum for normally incident waves and decreases with incident wave angle. Thus, the mixing parameterization K ∝ U L Eis not completely applicable to the surfzone, because U and L E do not change for these experiments. Reduced mixing for obliquely incident waves results from large eddies (wavelengths >200 m) propagating at a velocity C different than the mean current velocity V ( x )—the same mechanism that reduces mixing for mesoscale eddies.