Cross‐shore surfzone tracer dispersion in an alongshore current

Abstract

Cross‐shore surfzone tracer dispersion in a wave driven alongshore current is examined over a range of wave and current conditions with 6 continuous dye releases, each roughly 1–2 hours in duration, at Huntington Beach, California. Fluorescent dye tracer released near the shoreline formed shore parallel plumes that were sampled on repeated cross‐shore transects with a jet ski mounted fluorometer. Ensemble averaged cross‐shore tracer concentration profiles are generally shoreline attached (maximum at or near the shoreline), with increasing cross‐shore widths and decreasing peak values with downstream distance. More than a few 100 m from the source, tracer is often well mixed across the surfzone (i.e., saturated) with decreasing tracer concentrations farther seaward. For each release, cross‐shore surfzone absolute diffusivities are estimated using a simple Fickian diffusion solution with a no‐flux boundary at the shoreline, and range from 0.5–2.5 m 2 s −1 . Surfzone diffusivity scalings based on cross‐shore bore dispersion, surfzone eddy mixing length, and undertow driven shear dispersion are examined. The mixing‐length scaling has correlation r 2 = 0.59 and the expected best‐fit slope <1, indicating that horizontal rotational motions are important for cross‐shore tracer dispersion in the surfzone.