Collaborative Research: Shoreward Sediment Transport: Combining Highly Resolved Field Observations and Modeling to Examine Fundamental Processes Controlling Shoreline Adjustment

海岸泥沙运输负责在波浪控制的海岸上建造和维护海岸线。由于波浪的振荡运动，使沉积物来回移动，而净输运在许多波浪和条件下平均，是造成海滩淤积或侵蚀的原因。海岸泥沙运输对于理解海滩如何重建以应对海平面上升、风暴和海岸工程结构的建设至关重要。这项研究的重点是提高对海岸运输的理解，这将使更好的模式参数化能够预测这一反应，并预测风暴过后海岸线的潜在恢复。波浪驱动的岸上运动的基本过程是近岸海洋学中最不了解和最难以捉摸的过程之一。许多相互竞争的理论机制可能会有所贡献，但严重缺乏验证数据来确定它们的相对重要性。这项拟议的工作是一项跨学科合作，将海洋学、流体动力学和工程学结合在一起，以解决具有重大社会影响的问题的科学基础。湍流和泥沙的现场测量将在天然海滩上进行。实验数据将与先进的数值模拟技术相结合，从而能够识别天然海滩上最重要的波浪驱动的岸滩机制。除了这项研究的科学和工程价值外，该项目还将为培养博士生做出贡献，并让本科生参与实地研究。将通过开发简短的纪录片形式的视频进行公众宣传。模拟海岸输运需要表现多种输运过程，这些过程在广泛的尺度上发挥作用，从小尺度湍流、泥沙动员和流体泥沙反馈，到波长尺度的非线性波浪过程。过去，对这类过程的直接调查只能在实验室研究中进行，由于尺度效应和简化的动力学，实验室研究对实际近岸环境的适用性不确定。另一方面，全面的实验代表了现实的动态，但往往牺牲了分辨率，例如，只测量了许多波周期的平均传输，留下了关于哪些过程对传输有贡献的不确定性。这项研究的关键组成部分是利用专门建造的宽带脉冲相干声学多普勒剖面仪(MFDOP)和详细的过程模型，这些模型共同解决了假设发生在自然海滩上的一整套传输机制。MFDop系统是一种剖面仪，能够测量高频(50赫兹)的泥沙浓度和速度波动，垂直分辨率为4毫米，能够分辨海床上方几厘米延伸的波浪边界层。MFDop系统和辅助仪器将部署在俄勒冈州的一个天然海滩上，以测量边界层速度和沉积物悬浮的垂直结构，作为自由流速度和压力梯度的函数。收集的数据将指导模拟波长和厘米尺度的水流和泥沙变异性的模拟试验(雷诺平均一维垂直模拟)，以及能够分辨边界层涡和泥沙颗粒的亚厘米到亚毫米尺度的模拟试验(大涡模拟(LES)模拟)。将开发一种嵌套方法，旨在将波长效应添加到LES模型中，同时避免计算限制。该奖项反映了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。

Shoreward sediment transport is responsible for building and maintaining shorelines on wave-dominated coasts. Due to their oscillatory motion waves move sediments back and forth and the net transport, averaging over many waves and conditions, is responsible for accretion or erosion of beaches. Shoreward sediment transport is critical to understanding how beaches rebuild in response to rising sea levels, storms, and the construction of engineered coastal structures. Improved understanding of shoreward transport, the focus of this study, will enable better model parameterizations to predict this response and forecast the potential recovery of the coastline following a storm. The fundamental processes underlying wave-driven shoreward transport are among the least understood and most elusive processes in nearshore oceanography. Numerous competing theoretical mechanisms are likely to contribute, but there is a critical lack of validation data to determine their relative importance. The proposed work is an interdisciplinary collaboration that brings together oceanography, fluid dynamics, and engineering to address the scientific fundamentals of a problem with significant societal implications. Field measurements of turbulence and sediment will be carried out on a natural beach. The experimental data will be combined with advance numerical modelling techniques that will allow identification of the most important wave-driven shoreward mechanisms on a natural beach. Apart for the scientific and engineering values of the study, the project will provide contribute to the training PhD students and it will expose undergraduate students into field research. Public outreach will be conducted through the development of short documentary-style video.Modeling shoreward transport requires representation of multiple transport processes that act across a broad range of scales, from small-scale turbulence, sediment mobilization, and fluid-sediment feedbacks, to wavelength-scale nonlinear wave processes. In the past, direct investigation of such processes has only been possible in laboratory studies, which have uncertain applicability to real nearshore environments due to scaling effects and simplified dynamics. Full-scale experiments, on the other hand, represent realistic dynamics but often sacrifice resolution, e.g. by only measuring average transport over many wave periods, leaving uncertainty as to what processes are contributing to transport. The key components of this study are the utilization of a purpose-built, broadband pulse-coherent acoustic Doppler profiler (MFDop), and detailed process models, which together resolve the full suite of transport mechanisms that have been hypothesized to occur on natural beaches. The MFDop system is a profiler, capable of measuring sediment concentration and velocity fluctuations at high frequency (50 Hz) and with a vertical resolution of 4 mm capable of resolving the wave boundary layer that extends a few cm above the seabed. The MFDop system and supporting instrumentation will be deployed on a natural beach in Oregon to measure the vertical structure of boundary layer velocities and sediment suspension as a function of the free-stream velocity and pressure gradient. The data collected will guide modeling experiments simulating flow and sediment variability at wavelength and cm-scales (Reynolds-averaged 1-D vertical simulations) and at sub-cm to sub-mm scales capable to resolve boundary layer eddies and sediment particles (Large eddy simulation (LES) modelling). A nested approach will be developed aiming at adding wavelength-effects into LES modelling while avoiding computational limitation.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.