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- Title
Rip Currents Off Rocky‐Shore Surge Channels.
- Authors
MacMahan, Jamie; Thornton, Ed; Patria, Nick; Gon, Casey; Denny, Mark
- 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 a return flow through a network of incised feeder channels that converge to a primary surge channel, directing flow offshore. The rip current extends three 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. Plain Language Summary: A series of field experiments are performed to understand rip currents associated with surge channels, a characteristic of rocky shores. Information from video observations, Global Positioning System‐equipped drifters, and bottom‐mounted current and pressure sensors was obtained for a low‐lying channel (called a surge channel) at the Hopkins Marine Station, Pacific Grove, CA, which defines the rocky shore rip current. The surge channel is a feeder‐channel network that converges into a primary channel, acting like a nozzle. The strength of the rocky shore rip current and its pulsations are governed by the sea swell waves, the modulated wave group, the tidal elevation, and surge channel geometry. Rocky‐shore surge‐channel rip currents differ from the common rip currents on sandy shores. The differences are related to the steep bottom morphology of rocky shores, which does not adjust over time to incoming sea‐swell waves and has a much smaller geometric scale than rip current morphology for sandy shores; the different morphology results in different forcing mechanisms. In addition, the significantly greater bottom roughness of rocky shores enhances the forcing mechanism. These differences modify the wave transformation, bottom friction, and rip‐current response. Our field experiments describe the kinematics and dynamics of a rocky shore rip current. Key Points: Rocky‐shore rip‐current strength varies with sea swell and low‐frequency (LF) wave forcing, channel geometry and hypsometry, and tide elevationLF pulsations are a significant contribution to the rip currents and change from a reflective to dissipative process with tideIn contrast to rip currents on sandy beaches, retention on this rocky shore is low, augmenting cross‐shore transport
- Subjects
RIP currents; OCEAN waves; PRESSURE sensors; IMPULSE (Physics); FIELD research; STORM surges; WAVE forces; SEAS
- Publication
Journal of Geophysical Research. Oceans, 2023, Vol 128, Issue 8, p1
- ISSN
2169-9275
- Publication type
Article
- DOI
10.1029/2022JC019317