Rip Currents: Coupling and Feedback between Waves, Flows, and Morphology

激流将生物群、污染物和沉积物从海岸线输送到大陆架，对游泳者来说是一个危险，每年有100多人死亡。这些狭窄的、瞬变的、近海定向的射流很难观察到，因此很少有关于它们的产生、演化、三维结构和与不断演变的海底形态的反馈的假设的现场测试。激流可以由波浪破碎的近岸变化驱动，通常与沙洲的缝隙或海浪带海底的凹陷有关。将利用现有的在冲浪区疏浚的渠道附近的现场观测和经过现场观测检验的数值模式来研究撕裂渠道内和附近的波浪、水流和地形之间的耦合和反馈。虽然该项目的主要重点是研究冲浪带海底大扰动附近的流体动力学和地貌动力学的基本物理，但项目调查人员将与联邦机构的同事合作，这些机构对近岸过程有更多的应用兴趣。特别是，这位博士后研究员将与开发该模型的USGS同事以及其他联邦机构科学家密切合作，进行模型模拟，以提供与他们的任务一致的结果。该模型将成为这些机构预测海浪、水流和海滩变化的有用工具，包括对风暴和工程项目的响应。此外，项目调查人员将与国家气象局和国家海洋局的科学家合作，改进对激流和其他危险海滩条件的预报和警告。2012年，在冲浪区疏浚了大型岸上垂直航道，其基本假设是，在航道上破碎的海浪的近岸变化将驱动强大的激流环流单元(岸上平行支流汇聚，并在离岸定向的冲刷射流上转向大海)。测量了一系列波浪条件和航道大小的三维环流和演变形态，包括从流经航道的离岸撕裂流到流经航道的近岸流的转变，以及迁移和充填的航道。在疏浚岸边垂直航道的情况下，对波浪、海流和水深的现场观测将被用来证实事实并校准数值模型。该数值模式将被用来研究一系列波浪强迫和测深扰动的水动力和地形响应。尽管这项研究将集中在我们挖掘的裂谷水道上，但其结果将适用于广泛的近岸系统，无论是否具有复杂的水深特征。具体地说，将讨论以下问题：(I)三维近岸环流如何对不同的波浪条件和水深做出反应，以及是什么力量控制着近岸流动和激流之间的过渡？(Ii)波浪、水流、泥沙输送和地形演变之间的耦合和反馈是什么，导致河道迁移、生长或衰退？

Rip currents transport biota, pollutants, and sediment from the shoreline to the continental shelf, and are a hazard to swimmers with over 100 fatalities per year. These narrow, transient, offshore-directed jets are difficult to observe, and thus there are few field tests of hypotheses for their generation, evolution, three-dimensional structure, and feedback with evolving seafloor morphology. Rip currents can be driven by alongshore variations in wave breaking, often associated with gaps in sandbars or depressions in the surf zone seafloor. Existing field observations near channels dredged across the surf zone and a numerical model tested with the field observations will be used to investigate the coupling and feedback between waves, currents, and morphology in and near rip channels. Although the main focus of the project is on the fundamental physics of hydrodynamics and morphodynamics near large perturbations to the surfzone seafloor, the project investigators will be collaborating with colleagues from federal agencies with more applied interests in nearshore processes. In particular, the post-doctoral fellow will work closely with the USGS colleagues who developed the model and other federal agency scientists to perform model simulations designed to provide results in line with their missions. The model will be a useful tool for these agencies to predict waves, flows, and beach changes, including responses to storms and engineering projects. In addition, the project investigators will collaborate with National Weather Service and National Ocean Service scientists to improve forecasts and warnings of rip currents and other hazardous beach conditions.In 2012 large shore-perpendicular channels were dredged in the surf zone with the underlying hypothesis that alongshore changes in wave breaking over the channels would drive strong rip current circulation cells (shore-parallel feeder currents converging and turning seaward at an offshore-directed rip jet). The three-dimensional circulation and evolving morphology was measured for a range of wave conditions and channel sizes, and included transitions from rip currents flowing offshore through the channels to alongshore currents flowing across the channels, and channels that migrated and filled. The field observations of waves, currents, and bathymetry in the presence of dredged shore-perpendicular channels will be used to ground truth and calibrate the numerical model. The numerical model will be used to investigate the hydrodynamic and morphologic response for a range of wave forcing and bathymetric perturbations. Although this study will focus on the rip channels we excavated, the results will be applicable to a wide range of nearshore systems, with or without complex bathymetric features. Specifically, the following questions will be addressed: (i) How does the three-dimensional nearshore circulation respond to different wave conditions and bathymetries, and what forces control transitions between alongshore flows and rip currents? (ii) What are the couplings and feedbacks between waves, currents, sediment transport, and morphological evolution that lead to channel migration, growth, or decay?