Wave- and Wind-Driven Flows Near the Beach

沿海岸沿着流动的水流输送沉积物和污染物，对海底施加压力，并影响总水位、洪水、海滩侵蚀和随后的恢复。然而，这些重要的沿岸流的观测存在于内冲浪和冲击区，特别是在风暴期间，和物理驱动他们在现有的观测是不确定的。沿着波浪强迫和沿岸水位梯度，有证据表明风强迫也可能是非常浅水沿岸流的重要驱动因素。了解物理驱动浅水流动需要近岸水动力学，形态和海岸线的演变，和波浪越浪模型。该项目将利用2021年秋季在Duck，NC进行的DUNEX研究期间的资助工作，并对非常靠近海岸的额外系泊仪器和远程（无人机）传感，以评估沿海滩附近的海岸流动的波浪和风过程。该项目将为社区提供一个重要的新数据集，培养一名博士后学者，并通过对受风暴侵蚀和洪水影响的近岸地区的信息管理为社会利益做出贡献。该项目将对一系列波浪状况进行现场和遥感实地观测，以估计控制近岸区的动量平衡条件。待评估的假设是：1）海滩附近沿岸流的强迫是跨岸（深度）依赖的; 2）相反的水位梯度和波浪强迫驱动沿岸流，改变冲浪和冲浪区之间的方向; 3）波浪和风强迫在风暴期间主导内冲浪和冲浪流。DUNEX资助的实地工作（USCRP，USGS，NSF，USACE和DoD）是该项目的基础，包括一个沿岸的冲浪区ADV阵列，一个从沙丘到内冲浪的单一跨海岸断面，带有压力计，ADV和激光雷达，以及2021年秋季在美国陆军部队实地研究设施（USACE-FRF）进行的频繁测深调查。联邦共和国部队还维持一个8米的定向波浪阵列、一个验潮仪、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断面的压力计。此外，可以通过跟踪破碎波生成的泡沫序列的图像来估计表面流。基于无人机的粒子图像测速仪和光学海流计过去曾在冲浪带条件下使用。这个项目将扩展和磨练这些技术的冲洗区。在风暴和中等波浪条件下的观测将进行比较和对比，以研究波浪和风条件变化时过程的不同。研究结果可以显著提高预测海滩附近水流的能力，相应的风暴引起的海岸形态变化，以及随后的恢复。该奖项反映了NSF的法定使命，并被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.