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, k y , spaced within the VLF band, and did not appear to exist in higher‐frequency bands. The SZEs were significant ( U rms,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 ( r 2 = 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 ( r 2 = 0.49). F ‐ k y spectral estimates illustrate a strong relationship between rip channel spacing and SZE cross‐shore velocities ( k y = ±0.008 m −1 ) and minimal SZE alongshore velocity variation ( k y = 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 ‐ k y spectra can be explained by the entire rip current circulation cells oscillating predominantly in the cross shore and slightly in the alongshore.
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.
Measurement data of a rip-current system are used in a comparison with model results to verify a coastal area model operating on the time scale of wave groups. The measurements were performed as part of the RDEX field experiment at Palm Beach (Australia). Continuous coverage of rip current velocities over a large number of tidal cycles was obtained, with wave conditions varying from calm to storm conditions. Video time exposures of the surf zone are used to calibrate the model dissipation coefficients by comparing the measured intensity and the computed roller energy. Selecting a subset of the measurements covering 11 days at the beginning of the experiment, the agreement with computed results is generally good.
Observations of velocity fluctuations with periods between about 4 and 30 min, thus longer than infragravity waves and referred to as very low frequency (VLF) surf zone motions, are described and compared with numerical simulations. The VLF motions discussed here exclude instabilities (generated by the wave‐driven alongshore current velocity shear) that occur in the same frequency range by selecting cases with weak alongshore currents only. Numerical simulations are based on the linear shallow water equations including friction and forced by nonlinear difference‐frequency interactions between incident sea and swell waves. The model is initialized with sea and swell frequency directional spectra observed seaward of the surf zone. Modeled and observed VLF velocity fluctuation magnitudes agree within a factor of 2; both increase approximately linearly with increasing incident wave height and rapidly decay seaward of the surf zone. Observed frequency‐wave‐number, f ‐ k y , spectra of VLF velocity fluctuations, estimated with instrumented alongshore arrays, are energetic in a broad range of k y in the vortical band. Observed and modeled VLF pressure fluctuations are relatively weak. Still, the model momentum balance suggests that VLF pressure gradients are as important as the nonlinear wave group forcing by sea and swell in accelerating/decelerating the VLF velocities. Model calculations demonstrate that the VLF‐ f ‐ k y response is a function of the modulations of short‐wave forcing associated with the frequency directional distribution of the incident sea and swell spectra. This results in VLF motions which span the surf zone and have O(50–1000 m) alongshore scales with O(200–2000 s) time scales. Given the fact that modulations of short waves resulting from directionally spread incident waves are common during field conditions we expect VLFs to be ubiquitous.
Abstract : NICOP funding for this project ends at the end of September 2001, but the goals of the consortium of researchers continue. Our overall goal is to achieve a better understanding and better predictions of coastal behaviour at intermediate (event/season/year/decade) scales. We aim to bring together researchers from Europe and North America to gain the best possible benefit from developments in field observation, theory and numerical modelling.
Low-cost, handheld, L1 (1575.42 MHz) global positioning systems (GPSs) provide scientists with the opportunity to acquire position and velocity estimates at reduced expense (order of [O]$100), size (∼cell phone), weight (O[70 g]), and engineering time. Two different low-cost, handheld GPS units and four different position-correcting configurations are evaluated here to determine their practicality in measuring surf-zone currents. Three of the simpler configurations result in relative position and velocity errors of O(2 m) and O(0.5 m s−1) for stationary tests. Surf-zone position and velocity signal-to-noise spectral ratios for the three configurations suggest that only motions <0.01 Hz can be confidently estimated for these surf-zone systems. For the fourth configuration, a GPS handheld unit that internally records GPS carrier phase is postprocessed using more sophisticated software for position corrections to obtain absolute position and velocity estimates. Simple modifications are required to improve the position accuracy by reducing patch antenna signal multipathing. For this configuration, the absolute position error for dynamic surveys was ∼0.40 m, and the velocity error on land relative to a survey-grade GPS system was 0.01 m s−1. The handheld GPS was attached to a surf-zone drifter and evaluated in the field. The flow field of a rip-current system was obtained with 24 surf-zone drifters. The drifters tracked simultaneous dye releases well, verifying that the observations are valid Lagrangian estimates. Owing to the low cost and small size of the handheld GPS, a large number of drifter systems can be deployed for absolute position tracking and velocity estimates of surf-zone currents.
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