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

向岸沉积物输运是在波浪控制的海岸上建立和维持海岸线的原因。由于波浪的振荡运动，使沉积物来回移动，在许多波浪和条件下平均的净迁移是造成海滩增生或侵蚀的原因。向岸沉积物输运对于理解海滩如何重建以应对海平面上升、风暴和工程海岸结构的建设至关重要。本研究的重点是加深对向岸运输的理解，这将使模型参数化能够更好地预测这种响应，并预测风暴后海岸线的潜在恢复。波浪驱动的向岸输运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。 拟议的工作是一个跨学科的合作，汇集了海洋学，流体动力学和工程，以解决具有重大社会影响的问题的科学基础。湍流和沉积物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，这将使最重要的波驱动的向岸机制的天然海滩的识别。除了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，并将使本科生参与实地研究。将通过制作纪录片式的短片向公众宣传。模拟向岸输运需要表现多种输运过程，这些过程跨越各种尺度，从小尺度湍流、沉积物移动、流体-沉积物反馈到波长尺度的非线性波浪过程。过去，对此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，这些研究对真实的近岸环境的适用性不确定。另一方面，全尺寸实验代表了真实的动态，但往往牺牲分辨率，例如，仅测量许多波周期的平均输运，从而无法确定哪些过程对输运有贡献。这项研究的关键组成部分是利用一个专门建造的，宽带脉冲相干声学多普勒剖面仪（MFDop），和详细的过程模型，它们一起解决了全套的传输机制，已假设发生在自然海滩。MFDop系统是一个剖面仪，能够以高频（50赫兹）测量沉积物浓度和速度波动，垂直分辨率为4毫米，能够分辨出海床上方几厘米处的波浪边界层。MFDop系统和配套仪器将部署在俄勒冈州的天然海滩上，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。收集到的数据将指导模拟波长和厘米尺度（哈罗德平均一维垂直模拟）和厘米以下至毫米以下尺度的水流和沉积物变化的建模实验，这些实验能够分辨边界层涡流和沉积物颗粒（大涡模拟建模）。将开发一种嵌套方法，旨在将波长效应添加到LES建模中，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。

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.