The project goal is an improved understanding of two dimensional eddies with length-scale greater than the water depth. Beaches throughout the United States are chronically impacted by poor water quality, making swimmers sick and affecting coastal economies. Run-off pollution often drains directly into the surfzone and the mechanisms dispersing and diluting pollution or other tracers are not clear. Recent observations implicate surfzone 2D turbulent eddies in tracer dispersion and dilution. Surfzone 2D turbulence is generated either from a shear instability of the alongshore current, from finite-crest length breaking of individual waves, or from alongshore gradients in wave-group forcing. These vorticity sources all have distinct properties and the dependence of the surfzone eddy field upon these vorticity sources is not understood. In addition, the effects of cross-shore variable (both monotonic and barred) surfzone bathymetry on the 2D turbulence eddy field is unclear.
Intellectual Merit: The first set of proposal objectives is to examine surfzone turbulent eddies in both models and existing field observations. The magnitude, time- and spatial scales of surfzone turbulent eddies in two different wave-averaged (with and without wave-group forcing) and one wave-resolving models (finite crest length wave-breaking) will be compared to Eulerian measures such as frequency-wavenumber spectra and Lagrangian observations like relative dispersion. The 3 models with distinct vorticity sources will also be inter-compared. For each vorticity model, the length-scales of forcing and of the resulting eddies will be examined, as will the length-scales where inverse-energy or enstrophy cascade regions are present. Particle separation statistics (e.g., relative diffusivities) will be estimated for each model to identify turbulent eddy length-scales. Analysis of the potential-vorticity and potential-enstrophy budgets within each model will illuminate the dynamical processes underlying the evolution of surfzone eddies. The factors that affect the vorticity forcing, such as wave directional-spread and the alongshore current magnitude, will be varied to reveal how the surfzone eddy field depends upon the various forcing mechanisms. The second set of objectives includes developing an understanding of how monotonic or barred topography affects classical 2D turbulence. This will improve the understanding of the evolution and dynamics of surfzone 2D turbulent eddies. Two scenarios will be examined: 1) freely decaying turbulence and 2) forced/dissipated turbulence. The effects of variable bathymetry upon the spatial distribution and isotropy of the eddies will be examined. In freely decaying turbulence, the preferential migration to deep or shallow water of decaying eddies will be examined. Furthermore, the effect of cross-shore variable bathymetry upon other properties of 2D turbulence, such as the time-dependent vortex number and size, and the inverse-energy and enstrophy cascades, will be determined. For forced/dissipated turbulence, how the potential vorticity and enstrophy budgets depend upon cross-shore position will be examined.
Broader Impacts: At United States beaches, there were over 20,000 closure and advisory days in 2008, and 7% of beach water samples exceed national health standards. Significant health risks are associated with bacterial pollution typically from run-off sources that drains directly onto the beach. Quantitative knowledge of the mixing of surfzone pollutants, needed for informed management decisions, requires an improved understanding of the surfzone turbulent eddy field. As part of educational outreach efforts, the PIs will co-host with the Ocean Institute two Teen SurfScience Conferences for about 100 high-school students. This will be a continuation of previous successful Teen SurfScience conferences co-hosted by the PI. In addition, PIs will present talks on surfzone/beach science to high school students of low socio-economic status through the Preuss School and curriculum materials will be developed.
