Rotational currents with order 0.1 m/s velocities and periods of about 30 minutes have been observed in many surf zone environments and for a wide range of conditions. These low-frequency surf zone eddies provide a mechanism for cross-shore dispersion and exchange of material between the shore and the inner continental shelf. This study tests the hypothesis that low-frequency surf zone currents may be the result of energy transfer from breaking waves. Existing data from an extensive two-dimensional array of wave and current sensors will be analyzed to investigate the inverse energy cascade to lower frequencies. Due to limitations in the spatial scales resolvable by the existing data, new data will be collected through the deployment of an alongshore array of wave and current sensors in the surf zone. The sensor deployment will occur during a multi-agency-funded field project to take place in 2020, leveraging significant infrastructure and ancillary measurements. In addition to understanding the contribution of these low frequency oscillations to material exchange between the surf zone and the coastal ocean, the study will also address the need for exposure of future scientists and professional engineers in field data collection and analysis. The project will provide students and young coastal professionals field data collection experiences and 1-week long workshops in analyzing field observations. In addition, the research will form part of the dissertation of an MIT-WHOI PhD student advised by the PIs.
There are several hypotheses for the generation of low-frequency surf zone motions, including instabilities of mean currents (shear waves), slow modulations of the incident wave field (wave groups), and nonlinear energy transfers (breaking waves). The wide range of observed wave conditions in existing field data sets, combined with new observations will allow separation and isolation of these mechanisms. Preliminary analyses of surf zone observations suggest that some statistics of the low-frequency current field are consistent with the expectations of a two-dimensional inverse energy cascade. Based on preliminary results, two hypotheses have been formulated and will be investigated as part of this project: (1) vortical energy with spatial scales of a few meters injected into the surf zone at the ends of high frequency (50 to 300 mHz) breaking wind waves is transferred to low-frequency (< 4 mHz) surf zone currents via a two-dimensional inverse cascade, and (2) enstrophy (the integral of vorticity squared) is transferred from breaking-wave-generated vorticity to smaller-scale, higher-frequency motions via a two-dimensional forward cascade. The spatial scale for which the inverse and forward cascades intersect is an indication of the spatial scale of the injected vorticity.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
