Surf zone eddies coupled with rip current morphology

Energetic very low frequency (VLF; frequencies <0.004 Hz) surf zone eddies (SZEs) were observed on a beach composed of shore‐connected shoals with quasi‐periodic (∼125 m) incised rip channels at Sand City, Monterey Bay, California. Incident waves consisted of predominantly shore‐normal narrow‐banded swell waves. SZEs were located outside the gravity region in alongshore wave number, ky, spaced within the VLF band, and did not appear to exist in higher‐frequency bands. The SZEs were significant (Urms,VLF ∼ 0.25 m/s) and constant in intensity within the surf zone (shore‐connected shoals and rip channels) and rapidly decreased offshore. The alongshore and cross‐shore SZE velocity variances were similar in magnitude. VLF SZE velocities were not forced by VLF surface elevations and were not well correlated with rip current flows (r2 = 0.18). There is an indication that the SZEs were related to wave forcing, with the SZEs statistically correlated with incoming sea‐swell wave height (r2 = 0.49). F‐ky spectral estimates illustrate a strong relationship between rip channel spacing and SZE cross‐shore velocities (ky = ±0.008 m−1) and minimal SZE alongshore velocity variation (ky = 0 m−1). Data analysis suggests that the SZEs are not simply instabilities of an unstable rip current jet. A simple conceptual model suggests that SZE f‐ky spectra can be explained by the entire rip current circulation cells oscillating predominantly in the cross shore and slightly in the alongshore.