Wave- and Wind-Driven Flows Near the Beach

沿着海岸流动的水流携带沉积物和污染物，对底部施加压力，影响总水位、洪水、海滩侵蚀和随后的恢复。然而，对这些重要的沿岸流动的观测很少，特别是在风暴期间，在现有的观测中，驱动它们的物理因素是不确定的。有证据表明，除了波浪作用和沿岸水位梯度外，风的作用也可能是非常浅的沿岸水流的重要驱动因素。近岸流体动力学、形态和海岸线演化以及波浪过顶模型都需要了解浅水流动的物理驱动力。该项目将利用2021年秋季在北卡罗来纳州达克市进行的DUNEX研究期间的资助成果，使用额外的系泊仪器和远程（无人机）感知非常近的海岸，以评估推动海滩附近沿岸流动的波浪和风力过程。该项目将为社区提供重要的新数据集，培养博士后学者，并通过为受风暴侵蚀和淹没影响的近岸地区的管理提供信息，为社会利益做出贡献。该项目将获得一系列波浪条件的现场和遥感现场观测，以估计控制近岸区的动量平衡项。待评估的假设是：1)靠近海滩的沿岸流的强迫与海岸（深度）有关；(2)反对水位梯度和斜波强迫驱动沿岸水流，使其在冲浪带和冲刷带之间改变方向；3)在风暴期间，斜波和风强迫主导了内冲流和冲流。该项目建立在dunex资助的实地工作（USCRP、USGS、NSF、USACE和DoD）的基础上，包括海岸带ADVs阵列，从沙丘到内部冲浪的单个跨海岸样带，带有压力计、ADVs和激光雷达，以及2021年秋季在美国陆军部队野外研究设施（USACE- frf）进行频繁的测深测量。FRF还维护一个8米高的定向波阵列、一个潮汐计、几个风速计和一个每小时收集海滩和沙丘地形的激光雷达。这里提出的扩展观测包括两个跨海岸的压力计横断面。此外，表面流动可以通过跟踪破碎波产生的泡沫序列图像来估计。过去，基于无人机的粒子图像测速和光学电流计已经在地表条件下使用。本项目将扩展和完善这些技术的冲击区。将对风暴和中波条件下的观测结果进行比较和对比，以检验随着波浪和风条件的变化过程有何不同。该结果可以显著提高预测海滩附近流量、相应的风暴引起的海岸形态变化以及随后恢复的能力。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Currents flowing along the shore transport sediments and pollution, place stress on the bottom and affect total water levels, flooding, beach erosion, and subsequent recovery. Yet few observations of these important alongshore flows exist in the inner-surf and swash zones, especially during storms, and the physics driving them during existing observations is undetermined. Along with wave-forcing and alongshore water-level gradients, evidence suggests wind-forcing also may be a significant driver of very shallow-water alongshore currents. Understanding of the physics driving shallow-water flows is needed for nearshore hydrodynamical, morphological and shoreline evolution, and wave overtopping models. This project will leverage funded efforts during the DUNEX study in Duck, NC in fall 2021 with additional moored instrumentation and remote (drone) sensing of the very-near shore to enable evaluation of the wave and wind processes driving alongshore flows near the beach. The project will provide an important new dataset to the community, train a post-doctoral scholar, and contribute to societal interests through informing management of nearshore zones subject to storm-forced erosion and inundation. This project will obtain in situ and remotely sensed field observations for a range of wave conditions to estimate the terms in the momentum balance governing the nearshore zone. Hypotheses to be evaluated are, 1) the forcing of alongshore flows near the beach is cross-shore (depth) dependent; 2) opposing water-level-gradient and oblique-wave forcing drive alongshore flows that change direction between the surf and swash zones, and; 3) oblique-wave and wind forcing dominate inner-surf and swash flows during storms. The DUNEX-funded fieldwork (USCRP, USGS, NSF, USACE, and DoD), which this project builds upon, includes an alongshore array of surfzone ADVs, a single cross-shore transect from the dune to the inner surf with pressure gages, ADVs, and lidar, and frequent bathymetry surveys at the U.S. Army Corps Field Research Facility (USACE-FRF) in fall 2021. FRF also maintains a directional wave array at 8-m, a tide gauge, several anemometers, and a lidar that collects beach and dune topography hourly. The expanded observations proposed here include two cross-shore transects of pressure gages. Additionally, surface flows can be estimated by tracking breaking-wave-generated foam in sequences of images. Drone-based particle image velocimetry and the Optical Current Meter have been used in the past in surfzone conditions. This project will extend and hone these techniques for the swash zone. Observations during storms and moderate wave conditions will be compared and contrasted to examine how processes differ as wave and wind conditions change. The results could significantly improve the ability to predict flows near the beach, the corresponding storm-induced changes to coastal morphology, and the subsequent recovery.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.