Short-crested Breaking Waves and Surfzone Vorticity

海滩和邻近的冲浪区受到陆地污染的威胁，这些污染往往排到海岸线上，在冲浪区(海岸线附近的海浪破碎区域)内被输送和扩散。受污染的海水会导致海滩关闭，危害公众健康，但目前还没有关于冲浪区污染的预测模型，也不知道建立这种模型所需的冲浪区分散率。最近的野外工作发现，冲浪区示踪剂(如污染、营养物质、幼虫)的扩散与冲浪区内水平涡流的强度有关。观测和模式结果表明，存在广泛的表面带涡旋大小和时间尺度，但这种丰富的涡度(旋转)场的结构和来源尚不清楚。这项研究将使用独特的圆形海流计阵列在天然海滩上进行首次海浪涌浪(0.05-0.25赫兹)和更低频率的海浪带垂直涡度(例如，从水平涡旋)的测量。2011年进行的一项试点试验表明，圆形阵列测量可用于在较宽的频率范围内估计涡度，这将允许检验平均涡量随平均波角和波高而增加的假设，以及涡度方差随波向扩散、平均波角和波高而增加的假设。有许多潜在的海面带涡度来源。特别是，假设涡度是在破碎的波峰(波峰末端)产生的，最近的一项模拟研究表明，波峰末端可能是海浪带涡旋扩散的主要强迫机制。在这里，表面带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。峰端强迫涡度将与其他可能的涡旋强迫机制进行比较，包括波群和近岸平均流(剪切波)的剪切不稳定，在一系列条件下。表面涡度(平方涡度)谱将与理论谱形状进行比较，并可能指示主要的搅动尺度。更广泛的影响：对涡度的研究将增加对表面带涡旋和混合的了解，并可能导致仅使用视频观测或近海波浪条件来估计涡扩散系数。扩散系数估计对于模拟冲浪区污染，从而改善公共安全和海滩管理决策是必要的。这项工作的结果和团队的实地经验将通过伍兹霍尔科学和技术教育伙伴关系(WHSTEP)与高中海洋学和海岸研究学生分享。该团队将与一名高中教师合作，带领学生进行实地考察和讨论，并将开设一堂关于冲浪地带过程的课程，该课程符合国家科学教育标准，并可适应不同的学习水平。将开发一个网站来托管完成的独立产品(教案、演示文稿、活动和评估)，以便供WHSTEP和全国的教育工作者使用。

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.