Field Measurements of the Vertical Structure of Rip Currents

Abstract:

Field measurements of the vertical structure of rip current flows inside and outside the surf zone are examined on a rip-channeled beach at Monterey, California. Wave-group induced very low frequency (VLF) motions were a significant component of the rip current flow, accounting for 37% to 50% of the total energy inside and outside the surf zone. The VLF pulsations in the near-surface rip current flow were aperiodic, with more frequent and stronger pulsations occurring for increased wave energy. The VLF pulses measured inside and outside the surf zone did not occur simultaneously and were not correlated, indicating the VLF rip current pulses outside the surf zone were random and independent of those inside the surf zone. The strength of the VLF pulses added to the mean surfzone background rip current flow, and determined the magnitude of the rip current flow outside the surf zone where there was no background flow. Inside the surf zone, the rip current flow was found to be depth-uniform with minimal rotation throughout the water column. Outside the surf zone the rip current flow was surface-dominated with rotation in the flow with depth. The results suggest that the rip current behaves like a jet as it exits the surf zone and moves over the surfzone break, becoming surface dominated outside the surf zone and remaining between the sea-surface and the depth at the offshore edge of the surf zone. Farther offshore, the rip current flow mixes vertically downward controlled by the depth of the thermocline as it continues to be transported and mixed offshore.