Observing Surf-Zone Dispersion with Drifters

Abstract

Surf-zone dispersion is studied using drifter observations collected within about 200 m of the shoreline (at depths of less than about 5 m) on a beach with approximately alongshore uniform bathymetry and waves. There were about 70 individual drifter releases, each 10?20 min in duration, on two consecutive days. On the first day, the sea-swell significant wave height Hs was equal to 0.5 m and mean alongshore currents |?| were moderate (<0.1 m s?1). On the second day, the obliquely incident waves were larger, with Hs equal to 1.4 m, and at some surf-zone locations |?| was greater than 0.5 m s?1. The one-particle diffusivity was larger, with larger waves and stronger currents. On both days, the one-particle diffusivity tensor is nonisotropic and time-dependent. The major axis is initially parallel to the cross-shore direction, but after a few wave periods it is aligned with the alongshore direction. In both the along- and cross-shore directions, the asymptotic diffusivity is reached sooner within, rather than seaward of, the surf zone. Two-particle statistics indicate that relative dispersion grows like D2(t) ? t3/2 and that the relative diffusivity is scale-dependent as ? ? l2/3, with l being the particle separation. The observed scalings differ from 2D inertial-subrange scalings [D2(t) ? t3 and ? ? l4/3]. Separations have a non-Gaussian self-similar distribution that is independent of time. The two-particle statistics are consistent with a nonconstant-coefficient diffusion equation for the separation probability density functions. The dispersion is explained by neither irrotational surface gravity waves nor shear dispersion. The observations imply the existence of a 2D eddy field with 5?50-m length scales, the source of which is speculated to be alongshore gradients in breaking-wave height associated with finite crest lengths.