Aerial imagery of the rocky shoreline (RS) at Hopkins Marine Station in Pacific Grove, CA, was captured with an unmanned aerial vehicle. Imagery was georectified and post-processed to observe the surface flow of 2 persistent, stationary rip currents (rips) on the RS. Waves propagating toward the shoreline break as they interact with the irregular bathymetry. The collision of reflected waves in combination with a network of small feeder channels converge into a larger surge channel, directing a jet of water offshore. Breaking waves also create bubbles that inundate the shoreline with a dense bubble zone. Farther-extending bubbles within the rips were used to determine the cross-shore (CS) extent. Rip extents in channels 1 and 2 ranged between 14.3–49.2m and 8.8–33.1m. The respective mean extents were 33.1m and 18.1m. A dependence on wave height and tidal elevation is observed; as wave heights increase, the extent of the rips increases and, inversely, as tidal elevations decrease, extents increase. Slopes of ejected foam trajectories were measured for CS velocity. Calculated slopes show a velocity decay as the flow moves farther offshore. RS rips exhibit a direct offshore transport circulation. Drifters released from the shoreline immediately exited the surf zone and traveled along foam streaks, indicative of Lagrangian coherent structures. Maximum foam extents were observed at 154m, indicating surface material moving substantially offshore relative to the small surf zone.

Approved for public release; distribution is unlimited.

http://archive.org/details/opticalkinematic1094561242

Lieutenant Commander, United States Navy