Mean Lagrangian flow behavior on an open coast rip-channeled beach: A new perspective

The accepted view of rip currents is that they are an efficient mechanism for transporting material out of the surf zone. Previous rip current campaigns on natural beaches have focused on Eulerian measurements with sparse in situ pressure and current meter arrays. Here, for the first time, spatially synoptic estimates of rip current flow patterns, vorticity, and Lagrangian transport behavior are measured in the field using a fleet of 30 position-tracking surfzone drifters during multiple rip current occurrences on an open coast beach in Monterey, CA. Contrary to the classic view (Shepard et al., 1941), the rip current flow field consisted of semi-enclosed, large-scale vortices that retained the drifters and resulted in a high number of Lagrangian observations that are temporally and spatially repeated. Approximately 19% of the drifters deployed in the rip currents exited the surf zone per hour, on average during the experiments. The observed surf zone retention of drifters is consistent with measurements from different open coast beach rip current systems (14% at meso-macrotidal Truc Vert, France and 16% at macrotidal Perranporth, United Kingdom). The three-hour-average cross-shore rip current velocity at Monterey was 30 cm/s with peak time-averaged velocities of 40–60 cm/s depending on wave and tidal conditions. Drifters that episodically exited the surf zone were transported approximately 2 surf zone widths offshore at ∼ 20 cm/s.