Short-crested Breaking Waves and Surfzone Vorticity

海滩和邻近的表层受到陆地污染的威胁，这些污染经常排入海岸线，并在海岸线附近的破碎波区域内运输和分散。受污染的水造成代价高昂的海滩关闭，并危及公众健康，但目前还没有海面污染的预测模型，而且这种模型所需的海面扩散率尚不清楚。最近的实地调查发现，表层示踪剂（如污染、营养物、幼虫）的扩散与表层内水平涡流的强度有关。观测和模式结果表明，存在广泛的表面涡大小和时间尺度，但这种丰富的涡度（旋转）场的结构和起源是未知的。这项研究将使用一种独特的圆形流计阵列，在天然海滩的一系列入射波条件下，进行第一次海浪（0.05-0.25 Hz）和较低频率的表层垂直涡度测量（例如，来自水平涡流）。2011年进行的一项试点测试表明，圆形阵列测量可以用于估计较宽频率范围内的涡度，这将允许测试平均涡度随平均波角和波高而增加的假设，以及涡度方差随波的方向传播、平均波角和波高而增加的假设。有许多潜在的地表涡度来源。特别是，假设涡度是在破碎波峰的末端产生的（波峰末端），最近的一项模拟研究表明，波峰末端可能是表层涡旋扩散的主要强迫机制。在这里，表面涡度测量将与视频观测相结合，以量化峰端事件期间涡度的变化，并估计峰端涡度的产生。在一系列条件下，将峰端强迫涡度与其他可能的涡强迫机制进行比较，包括波群和平均岸流（剪切波）的剪切不稳定性。表面区域熵（平方涡量）谱将与理论谱形状进行比较，并可能表明一个初级搅拌尺度。更广泛的影响：对涡旋度的研究将增加对表层涡旋和混合的认识，并可能导致仅使用视频观测或近海波浪条件来估计涡旋扩散率。扩散系数估算对于模拟表层污染是必要的，从而可以改善公共安全和海滩管理决策。这项工作的结果和团队的实地经验将通过伍兹霍尔科技教育伙伴关系（WHSTEP）与高中海洋学和海岸研究学生分享。在与一名高中教师的合作下，该团队将带领学生进行实地考察和讨论，并将创建一个符合国家科学教育标准的关于表层过程的课程，可以适应不同的学习水平。将开发一个网站来存放完成的独立产品（课程计划、演示、活动和评估），以便WHSTEP和全国各地的教育工作者使用。

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.