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 |υ| 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 |υ| 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 μ ∼ l2/3, with l being the particle separation. The observed scalings differ from 2D inertial-subrange scalings [D2(t) ∼ t3 and μ ∼ 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.

Temporal observations of rip current circulation on a macro-tidal beach

“…MacMahan et al (2009) also compared drifter and dye releases indicating that drifter observations are valid Lagrangian observations as well as comparing well with in situ observations. The positional data were quality controlled to remove erroneous points (greater than three velocity standard deviations from the mean) and gaps in the time series were linearly interpolated when 410 s and spline interpolated when o 10 s, consistent with drifter observations by Spydell et al (2007). Drifters were released into the surf zone in clusters of 4 and allowed to circulate with the currents until they either grounded at the shoreline or escaped from pre-defined longshore and offshore boundaries.…”

“…If a drifter re-entered the same bin it was considered a new independent observation if t 4 l g =U had elapsed, where t is time, l g is the bin length (10 m) and U is the mean speed for all drifter observations within that bin (MacMahan et al, 2010a). Bins containing greater than 5 independent observations were considered statistically significant following Spydell et al (2007).…”

Surfzone to inner‐shelf exchange estimated from dye tracer balances

“…Yet, despite the detriment of contaminated coastal water to our health and economy, understanding of nearshore tracer transport and mixing remains relatively poor. Several field experiments have tracked Lagrangian surface drifters on alongshore-uniform beaches [e.g., Spydell et al, 2007Spydell et al, , 2014 and rip-channeled beaches [e.g., Brown et al, 2009;MacMahan et al, 2010;Brown et al, 2015] to investigate dispersion in the nearshore. Similarly, fluorescent dye [e.g., Harris et al, 1963;Inman et al, 1971;Grant et al, 2005;Clark et al, 2010] has also been used to explore nearshore mixing.…”

Vortical VLF Motions Under Shore-Normal Incident Waves

“…This can lead to strengthening of existing rip currents or the generation of transient rip currents (MacMahan et al 2006, Johnson andPattiaratchi 2004). Moreover, the eddy like behavior of vortical VLF motions is found to significantly affect the mixing of material (larvae, nutrients, pollutants) inside the surfzone (Spydell et al 2007(Spydell et al ,2009) as well as the exchange of material with the shelf (Reniers et al 2009.…”