Observations of drifter dispersion in the surfzone: The effect of sheared alongshore currents

Abstract

Surfzone dispersion is characterized with single‐particle Lagrangian statistics of GPS‐tracked drifters deployed on 5 days at Huntington Beach, California. Incident wave heights varied weakly between days, and stationary rip currents did not occur. Generally, the time‐dependent bulk surfzone cross‐shore diffusivity xx was similar on all days, reaching a local maxima of approximately 1.5 m2 s−1 between 160 and 310 s, before decreasing to about 1 m2 s−1 at 1000 s. The alongshore diffusivity increased monotonically to 1000 s and was variable between the 5 days. For times greater than 30 s, the alongshore diffusivity is greater than the cross‐shore diffusivity, consistent with previous observations. The observed diffusivities are fit to analytic functional forms, from which asymptotic diffusivities and Lagrangian timescales are determined. The asymptotic alongshore diffusivity varies between 4 and 19 m2 s−1, and this variation is related to the variation in the maximum of the mean alongshore current m, broadly consistent with a shear dispersion scaling ∼ . Cross‐shore variation in dispersion processes, lumped together in the bulk κ, is apparent in the non‐Gaussian probability distribution function of drifter displacements at intermediate times (30 s). Both biased and unbiased diffusivity sampling errors depend on the number and length of drifter trajectories and limit aspects of the analysis.