The Turbulence Responsible for Horizontal Surfzone Dispersion

提高对冲浪区(海岸线100米范围内)引起物质扩散的水平湍流的了解是本项目的目标。许多海滩的水质不佳对海滩游人的健康构成风险，并对当地经济造成负面影响。最近的研究表明，冲浪带水平旋涡的长度尺度大于水深，它混合和分散了诸如污染等冲浪带示踪剂。虽然冲浪带涡旋的扩散对稀释和物质(包括幼虫)的跨岸交换是至关重要的，但如何在计算机模型中对冲浪带涡旋扩散系数进行参数化是未知的。湍流的欧拉性质(涡流速度、涡流空间和时间尺度)与冲浪区拉格朗日统计量(即色散和涡旋扩散率)之间的确切关系尚不清楚。另外，冲浪区欧拉涡旋特性与入射波场之间的关系也没有完全弄清楚。通过分析现有观测和进行模型实验，将阐明欧拉涡旋特性和色散之间的联系。智力优势：本研究的第一个目标是检查现有观测中的冲浪带水平涡旋特性。由于色散主要取决于速度c、涡长L和涡动时间尺度T，特别是参数α=Ct/L，因此冲浪区值将由空间和时间滞后的速度自相关函数确定。首先，将根据SandyDuck的观测(亨廷顿海滩2006年和帝国海滩2009年也将尽可能地考虑)来计算它们，然后将对观测到的冲浪带涡流场进行表征。较小的Alpha值表示涡旋场在时间上变化非常快，导致的色散与涡流寿命较长(较大的Alpha)时的色散性质不同。将研究观测到的涡旋特性如何依赖于入射波场，以及沿岸流是否改变了所确定的涡场的性质。此外，还将计算冲浪带涡旋与跨岸位置的关系。具体地说，由于冲浪带涡旋只在冲浪带内产生，因此将检查冲浪带涡流场在冲浪带内和冲浪带向海的方向是否不同。第二个目标是对观测到的海浪分辨率和海浪平均的分析进行相同类型的分析，并对观测到的条件(例如桑迪Duck)进行数值模拟。即使波浪平均和波浪分辨速度频率-波数谱可能有显著差异，这些模型是否能再现观测到的涡流速度、长度和时间尺度？这是一个重要的考虑因素，因为色散取决于频率-波数谱的整体特性，而不是其详细形状。模型模拟还将允许计算非常可靠的拉格朗日统计数据，因为虚拟漂移者将被添加到模型中。然后将比较欧拉和拉格朗日涡旋统计，并确定现有理论的相关性，该理论将欧拉和拉格朗日统计联系起来，为各向同性均匀湍流而发展，用于表面带。如果这一理论适用，那么只要知道欧拉涡的统计数据，就可以在没有显式拉格朗日统计的情况下推断出冲浪区的涡扩散系数。广泛的影响：2008年，在美国海滩，7%的海滩水样超过国家卫生标准，关闭和建议关闭的天数超过2万天。城市径流的污染经常直接排到海滩上，对游泳者构成重大的健康风险。管理决策可以受益于对冲浪区湍流和扩散的更好理解。作为教育外展工作的一部分，PI将与非营利性环境组织I Love A Clean San Diego(ILACSD)合作，并协助他们开发教材和教育计划。此外，PI将在选定的ILACSD项目上提供嘉宾演示。与ILACSD的合作将产生巨大的影响，因为他们总共为30,000名高中生(45%为第一职称或服务不足)提供了环境教育，他们与圣地亚哥各地的资源经理密切合作。

Improved understanding of the surfzone (within a few 100 m of the shoreline) horizontal turbulence that causes material dispersion is the goal of this project. Poor water quality at many beaches poses health risks to beach goers and negatively impacts local economies. Recent research has indicated that surfzone horizontal eddies, which have length scales greater than the water depth, mix and disperse surfzone tracers such as pollution. Although dispersion by surfzone eddies is critical to the dilution, as well as the cross-shore exchange of material (including larvae), how to parameterize the surfzone eddy diffusivity in computer models is unknown. Exactly how the Eulerian properties of the turbulence (the eddy velocity, and the eddy space and time scales) are related to surfzone Lagrangian statistics (i.e. the dispersion and eddy diffusivity) is unknown. Also not completely understood is how the surfzone Eulerian eddy properties are related to the incident wave field. By analyzing existing observations and performing model experiments, the link between Eulerian eddy properties and dispersion will be illuminated.Intellectual Merit:The first objective of this study is to examine surfzone horizontal eddy properties in existing observations. As dispersion depends critically on the velocity c, eddy length L, and eddy time scales T, and in particular the parameter alpha = cT/L, surfzone values will be determined from space- and time-lagged velocity autocorrelation functions. First, they will be calculated for SandyDuck observations (Huntington Beach 2006 and Imperial Beach 2009 will also be considered to the extent possible) and observed surfzone eddy fields will then be characterized. Small values of alpha indicate that the eddy field changes very quickly in time and results in qualitatively different dispersion than if the eddies are long lived (large alpha). How the observed eddy properties depend on the incident wave field will be investigated and whether alongshore currents change the nature of the eddy field determined. Also, the dependence of surfzone eddies on cross-shore position will also be calculated. In particular, as surfzone eddies are only generated within the surfzone, whether or not the surfzone eddy field is different within and seaward of the surfzone will be examined.The second objective is to perform the same type of analysis done for the observations on wave resolving and wave averaged numerical simulations of observed (e.g. SandyDuck) conditions. Do the models reproduce the observed eddy velocities, length and time scales even though wave averaged and wave resolving velocity frequency-wavenumber spectra can differ significantly? This is an important consideration as dispersion depends on the bulk properties of the frequency-wavenumber spectra rather than its detailed shape. Model simulations will also allow the calculation of very reliable Lagrangian statistics as virtual drifters will be added to the model. Eulerian and Lagrangian eddy statistics will then be compared and the relevance of an existing theory which relates Eulerian to Lagrangian statistics, developed for isotropic homogeneous turbulence, for the surfzone will be determined. If this theory is applicable, the surfzone eddy diffusivity can then be inferred without explicit Lagrangian statistics as long as the Eulerian eddy statistics are known.Broader Impacts:At United States beaches in 2008, 7% of beach water samples exceed national health standards and there were over 20,000 closure and advisory days. Pollution from urban runoff often drains directly onto beaches and poses significant health risks to swimmers. Management decisions could benefit from better understanding of surfzone turbulence and dispersion. As part of educational outreach efforts, the PI will collaborate with I Love A Clean San Diego (ILACSD), a nonprofit environmental group, and assist them with the development of their teaching materials and education program. Additionally, the PI will provide guest presentations at selected ILACSD programs. Partnering with ILACSD is going to have a large impact as they have provided environmental education to 30,000 high school students (45% Title 1 or underserved) in total and they work closely with resource managers throughout San Diego.