Rip currents transport contaminants, nutrients, larvae, and sediment between the surf zone and deeper waters. Transient rip currents can eject particles offshore even on alongshore-uniform beaches. During directionally spread wave conditions, waves break along finite-length regions, generating rotational motions (vorticity) near crest ends. Energy associated with the injected vorticity may be transferred to larger scale horizontal rotational currents (eddies) that can interact and episodically eject offshore as transient rip currents. However, a scarcity of observations has restricted our understanding of the processes connecting the wave field to the formation of transient rip currents.

This talk explores and characterizes surf-zone eddy dynamics that can lead to rip currents with large-scale wave basin experiments. We find that increasing the wave field directional spread results in more crest ends that inject vorticity into the water column. Also, the along-crest gradient in wave energy dissipation varies with the wave field properties, altering small-scale eddy generation, as assessed using remotely sensed water surface elevation maps. Lastly, by employing optically derived surface velocities, our results suggest that low-frequency currents are consistent with an inverse energy cascade from small to larger scale eddies during wave conditions with large directional spreads. However, we do not find evidence of an inverse energy cascade for wave fields with moderate and low directional spreads. These findings improve our understanding of the processes that enhance rip current activity, aiding predictions of swimmer hazards and pollutant transport.