Rip Currents Off Rocky‐Shore Surge Channels

Abstract

More than half the world’s coastlines are rocky, but rip‐current dynamics on these topographically complex shores have not been studied. Field experiments on a typical rocky shore using video, drifters, and in‐situ current meters and pressure sensors reveal that incoming narrow‐banded swells result in incident wave groups that force breakpoint low‐frequency (LF) waves, which act in phase inside the surf zone to generate set‐up, run‐up, overtopping, and mass flux, pooling water atop the shore. Set‐up is enhanced by the change in momentum of the LF waves owing to dissipation by bottom friction over the rough seafloor, and by depth‐limited breaking, evidenced by low reflection. A 1D momentum balance results in a mean bottom‐drag coefficient of 1 owing to the rough bottom and vegetation. During wave‐group minima, the hydraulic head of the pooled water forces return flow through a network of incised feeder channels that converge to a primary surge channel, directing flow offshore. The rip current extends 3 surf‐zone widths offshore with a maximum velocity of 1 m/s, and (in contrast to sandy shores) flow exits the surf zone, augmenting cross‐shore transport. Rip current strength is a function of channel hypsometry, overtopping, and exit constriction, factors that vary with the tide. As the tide rises, the mean flow decreases as the hydraulic head decreases and the constriction diminishes. Similar to rip currents on sandy beaches, rip currents on rocky shores are modulated by tides and sea‐swell, but they differ in geometric scale and forcing mechanism.