Observing surf-zone dispersion with drifters

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 H-s was equal to 0.5 m and mean alongshore currents vertical bar(v) over bar vertical bar 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 vertical bar<(v)over bar>vertical bar 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 D-2(t) similar to t(3/2) and that the relative diffusivity is scale-dependent as mu similar to l(2/3), with l being the particle separation. The observed scalings differ from 2D inertial-subrange scalings [D-2(t) similar to t(3) and mu similar to l(4/3)]. 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.