Surf zone eddies coupled with rip current morphology

Abstract

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. Copyright 2004 by the American Geophysical Union.