Beaches and the adjacent surfzone are threatened by terrestrial pollution that often drains onto the shoreline where it is transported and dispersed within the surfzone (region of breaking waves near the shoreline). Polluted water causes costly beach closures and endangers public health, but currently there are no predictive models for surfzone pollution and the surfzone dispersion rates required for such a model are not known.
Recent fieldwork has found surfzone tracer (e.g., pollution, nutrients, larvae) dispersion is correlated with the strength of horizontal eddies within the surfzone. Observations and model results indicate there is a wide range of surfzone eddy sizes and time scales, but the structure and origin of this rich vorticity (rotation) field is unknown. This study will use a unique circular array of current meters to make the first sea-swell (0.05-0.25 Hz) and lower frequency measurements of surfzone vertical vorticity (e.g., from horizontal eddies) over a range of incident wave conditions on a natural beach. A pilot test done in 2011 suggests the circular array measurements can be used to estimate vorticity over a wide frequency range, which would allow the testing of the hypotheses that mean vorticity increases with mean wave angle and wave height, and that vorticity variance increases with wave directional spread, mean wave angle, and wave height. There are many potential sources of surfzone vorticity. In particular, it is hypothesized that vorticity is generated at the ends of a breaking wave crest (crest ends), and a recent modeling study suggests that crest ends may be a primary forcing mechanism for surfzone eddy diffusion. Here surfzone vorticity measurements will be combined with video observations to quantify changes in vorticity during crest-end events, and to estimate crest-end vorticity generation. Crest-end forced vorticity will be compared with other possible eddy forcing mechanisms, including wave groups and shear instabilities of the mean alongshore current (shear waves), over a range of conditions. The surfzone enstrophy (squared vorticity) spectrum will be compared with theoretical spectral shapes and may indicate a primary stirring scale.
Broader Impacts: This investigation of vorticity will increase knowledge of surfzone eddies and mixing, and may lead to eddy diffusivity estimates using only video observations or offshore wave conditions. Diffusivity estimates are necessary to model surfzone pollution, and thus to improving public safety and beach management decisions. The results of this work and the team's field experiences will be shared with high school oceanography and coastal studies students through the Woods Hole Science and Technology Education Partnership (WHSTEP). In collaboration with a high school teacher the team will lead fieldtrips and discussions with students, and will create a lesson on surfzone processes that fits within National Science Education Standards and can be adapted to different learning levels. A website will be developed to host the finished self-contained product (lesson plans, presentations, activities, and assessments) so it can be used by educators in the WHSTEP and across the nation.
