Wave- and Wind-Driven Flows Near the Beach

沿海岸流动的水流输送沉积物和污染，对海底造成压力，并影响总水位、洪水、海滩侵蚀和随后的恢复。然而，在内浪和激流带，特别是在风暴期间，对这些重要的沿岸流的观测很少，而且在现有观测期间驱动它们的物理因素尚不确定。除了波浪强迫和近岸水位梯度，有证据表明，风力也可能是沿岸极浅水域水流的重要驱动因素。近岸水动力、地形和岸线演变以及波浪漫顶模型需要了解驱动浅水流动的物理机制。该项目将利用2021年秋季在北卡罗来纳州Duck举行的DUNEX研究期间所资助的工作，通过额外的系泊仪器和对非常近岸的遥感(无人机)来评估推动海滩附近沿岸流动的海浪和风过程。该项目将为社区提供一个重要的新数据集，培训一名博士后学者，并通过向遭受风暴强迫侵蚀和淹没的近岸地区的管理人员提供信息，促进社会利益。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中的各项。需要评估的假设是：1)海滩附近沿岸流的强迫是与跨岸(深度)有关的；2)相反的水位梯度和斜波强迫驱动沿岸流在波浪和激流带之间改变方向；3)风暴期间斜浪和风的强迫主导内浪和激流。作为该项目的基础，Dunx资助的野外工作(USCRP、USGS、NSF、USACE和DoD)包括一个近岸冲浪区ADV阵列，一个从沙丘到内海浪的单一跨岸横断面，带有压力计、ADV和激光雷达，以及2021年秋季在美国陆军陆战队野外研究设施(USACE-FRF)进行的频繁水深测量。FRF还维护着8米处的定向波阵、一个潮汐测量仪、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岸的压力计横断面。此外，可以通过跟踪图像序列中破碎波产生的泡沫来估计表面流动。基于无人机的粒子图像测速仪和光学海流计过去曾用于冲浪区条件下。该项目将扩展和磨练这些技术用于冲浪区。将对风暴和中浪条件下的观测进行比较和对比，以检查过程如何随着海浪和风况的变化而不同。这一结果可以显著提高预测海滩附近的水流、相应的风暴引起的海岸地貌变化以及随后的恢复的能力。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.