Transient Rip Current Dynamics: Laboratory Measurements and Modeling of Surfzone Vorticity

近岸地区作为重要的潮间带栖息地和休闲场所，经常受到病原体和陆地径流过量营养供应的损害。当一个地区受到污染时，可能会导致海滩关闭、生态破坏和相关的经济损失。了解整个近岸地区的物质运输对生态系统和人类健康具有重要意义。以往对跨大陆架交换的分析表明，瞬态离岸流是驱动冲浪带与内大陆架交换的主要机制。证据还表明，瞬态离岸流是由冲浪带漩涡合并产生的，短波峰波是旋转的来源。本项目将利用实验室和数值模拟来研究形成瞬态离岸流的冲浪带涡流的产生和演变。通过本研究将大大提高对近岸地区混合和交换的认识，这将对营养物、幼虫和污染物的运输具有重要意义。与美国国家海洋和大气管理局的科学家合作，将开发瞬时离岸流的简单预测器，以改进对各种环境的危害预测。这是对人类安全至关重要的信息，因为离岸流是海滩上死亡和救援的主要原因。研究生将通过参与这个项目获得宝贵的训练和教育。突出离岸流的外展活动将作为O.H. Hinsdale波浪研究实验室外展计划的一部分，旨在扩大科学，技术，工程和数学（STEM）的参与。该项目旨在建立一个从摇篮到坟墓的图像，通过短峰波破裂，合并形成更大的涡流，以及形成瞬态离岸流，将涡流注入冲浪区。总体目标是确定冲浪区是否可以被视为二维湍流箱。本研究将通过提出四个具体假设，提高对导致瞬态离岸流和相关跨大陆架交换的冲浪带涡的产生和演化动力学的理解：1)短峰波是短时空尺度上重要的涡度源，2)小涡合并形成大涡，3)冲浪带涡衰减受底部摩擦控制，4)冲浪带外喷射到陆架上的涡流偏离了二维范式。实验室实验将包括现场和远程（立体可见光和红外）测量海浪高度、周期和方向传播范围的海面高度和速度。最先进的数值模拟将用于指导实验室取样，分析一系列条件下的动力学，并评估摩擦耗散和三维循环特性的重要性。如果将冲浪区视为一个受强迫的二维湍流箱，则可以建立具有适当强迫的简化模型，以促进对冲浪区在陆海运输中的作用的理解。

The nearshore region, valued as essential intertidal habitat and recreational grounds, is often compromised by pathogens and excess nutrient supply from terrestrial runoff. When an area becomes contaminated, beach closures, ecological disruption, and associated economic losses can result. Understanding the transport of materials throughout the nearshore region can have important implications for ecosystem and human health. Previous studies analyzing cross-shelf exchange suggest that transient rip currents are the dominant mechanism driving exchange between the surf zone and the inner shelf. Evidence also suggests that transient rip currents are generated from the coalescence of surf zone eddies, with short-crested waves serving as the source of rotation. This project will utilize a laboratory and numerical modeling to study the generation and evolution of surf zone eddies to form transient rip currents. The knowledge of mixing and exchange in the nearshore region will be significantly advanced through this study, which will have important implications for the transport of nutrients, larvae, and pollutants. In collaboration with National Oceanic and Atmospheric Administration scientists, simple predictors of transient rip currents will be developed to improve hazard forecasts for a broad range of environments. This is essential information for human safety as rip currents are the leading cause of fatalities and rescues on beaches. A graduate student will gain valuable training and education through involvement in this project. Outreach activities highlighting rip currents will be developed as a part of the O.H. Hinsdale Wave Research Laboratory Outreach Program seeking to broaden participation in Science, Technology, Engineering and Mathematics (STEM).This project seeks to establish a cradle-to-grave picture of the injection of eddies into the surf zone by short-crested wave breaking, coalescence to form larger eddies, and formation of transient rip currents. The overall objective is to determine if the surf zone can be treated as a two-dimensional turbulence box. This study will improve understanding of the dynamics of surf zone eddy generation and evolution leading to transient rip currents and associated cross-shelf exchange by addressing four specific hypotheses: 1) short-crested waves are an important vorticity source at short spatial and temporal scales, 2) smaller eddies coalesce to create bigger eddies, 3) surf zone eddy decay is controlled by bottom friction, and 4) eddies ejected outside of the surf zone onto the shelf deviate from the two-dimensional paradigm. Laboratory experiments will include in situ and remote (stereo visible and infrared) measurements of sea-surface-elevations and velocities for a range of wave heights, periods, and directional spreads. State-of-the-art numerical modeling will be used to guide laboratory sampling, to analyze dynamics for a range of conditions, and to assess the importance of frictional dissipation and three-dimensional circulation properties. If the surf zone can be treated as a forced two-dimensional turbulence box, simplified models with appropriate forcing can be established to advance the understanding of the role of the surf zone in land-to-sea transport.