Wave-induced horizontal diffusivity from optically sensed dye tracer fields in impermeable beach laboratory experiments

Contaminants in nearshore regions can have negative consequences for aquatic life, public health, and the economic value of beaches. The associated risk in these regions depends on the relative concentrations of the contaminant at different distances from the shore. To address this concern, we performed passive tracer studies during a series of experiments in a laboratory wave basin, releasing dye near-instantaneously into the swash zone and outside of the breaking zone under monochromatic waves of varying heights and incident angles. By tracking dye patch evolution with cameras, we approximated horizontal diffusivity of the dye from the time rate of change of its variance in the cross-shore and alongshore directions. We performed approximately 50 dye release experiments with a combination of three wave heights and three wave angles. From these experiments, we approximate cross-shore and alongshore diffusivities ( κ x , κ y ) and explore parameterizations of these diffusivities on the basis of cross-shore location and wave conditions. The results indicate an order of magnitude increase in both κ x and κ y from the region of wave shoaling to the surf and swash zones. The nearshore diffusivity estimates show good agreement with previous empirical models and values reported in the literature, and for the first time provide insight on the detailed cross-shore distribution of horizontal diffusivity inside and outside of the wave breaking region. • Dye is released across the surf and swash zones in a wave basin. • Horizontal diffusivity is quantified from spreading of optically sensed dye patches. • Diffusivity increases by an order of magnitude from shoaling region to the swash zone.