Collaborative Research: Understanding the physics of flocculation processes and cohesive sediment transport in bottom boundary layers through multi-scale modeling

由于气候变化、海平面上升和人为开发，沿海社区面临着洪水、土地流失、水质和其他生态系统挑战（如有害的藻华）日益严重的威胁。这些紧迫的问题大多直接或间接地与沉积物的运输有关，有些与沙子有关，但许多是由于细颗粒的沉积物。细粒度的沉积物具有凝聚力，因此它们以多孔颗粒聚集体的形式运输，称为絮凝体。通过其复杂的结构，絮凝体是有机碳、营养物质、污染物的载体，有时它们还含有沙粒。因此，它们的沉降速度很难量化。迄今为止，大多数海岸/河口模型忽略了絮凝过程，采用恒定沉降速度来估计细粒沉积物的沉积，这对上述各种挑战的预测能力造成了相当大的限制。为了了解絮凝的基本动力学及其对细粒沉积物再悬浮和沉积的影响，将进行不同尺度的综合数值模拟和基于光学的实验室观测，包括与粒径、水湍流运动和底边界层相关的观测。拟议研究的成果将用于更好地为沿海模型提供泥沙输送能力，以应对沿海社区面临的挑战。研究结果将通过参与会议和与社区表面动力学建模系统（CSDMS）的合作，广泛传播给沿海建模界。待开发的开源代码和实验室实验数据将通过CSDMS传播，待开发的絮凝配方代码将通过最近发布的Python建模工具包（PyMT）集成到CSDMS建模框架中。此外，还计划在CSDMS年会上举办一个絮凝建模的临床讲座。该项目支持2名博士生，他们将在海岸过程、流体动力学、高性能计算和实验室技术方面接受平衡的培训。该项目还为早期的博士后研究人员提供部分支持。两名本科生将受益于本项目对粘性沉积物的研究。该项目还加强了与英国和德国在新型观测和计算工具方面的合作。这项合作研究的主要目标是解决沿海/河口底部边界层中粘性沉积物运输的关键挑战。该研究利用一种新的颗粒分解模拟模型来研究非均质沉积物的絮凝和絮体结构的物理特性。为了更好地量化粘性和了解砂泥混合物的絮凝作用，实验室实验进一步加强了这一努力。利用湍流解析波流底边界层细粒沉积物输运模拟模型，结合增强絮凝公式模拟沉降速度，研究者将研究海岸/河口底边界层粘性沉积物絮凝、再悬浮和沉积之间的相互作用。提出了五个假设来指导实验和建模工作，这将为在沿海模式中难以解决的关键小尺度过程提供见解。最后，通过整合和综合这些研究成果，本研究将评估一套由复杂到简单的絮凝沉降速度，为区域尺度上的粘性泥沙迁移的海岸/河口模型提供信息。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。

Due to climate change, sea level rise and anthropogenic development, coastal communities have been facing increasing threats from flooding, land loss, water quality, and other ecosystem challenges such as harmful algal blooms. Most of these pressing problems are directly or indirectly associated with sediment transport, some related to sands, but many are due to fine-grained sediments. Fine-grained sediments are cohesive and hence they transport as porous aggregates of particles, called flocs. Through their complex structures, flocs are vehicles of organic carbon, nutrients, contaminants and sometimes they can contain sand grains. Consequently, their settling velocities are very difficult to quantify. To date, most coastal/estuarine models neglect the flocculation process and adopt a constant settling velocity to estimate deposition of fine-grained sediments, which poses a considerable limitation of their predictive capability for the various challenges addressed above. In order to understand the fundamental dynamics of flocculation and their impact on fine-grained sediment resuspension and deposition, several integrated numerical simulations and optical-based laboratory observations across different scales will be carried out, including those associated with the particle size, water turbulence motions, and bottom boundary layer. Outcomes from the proposed research will be used to better equip coastal models with sediment transport capability to tackle challenges facing the coastal communities. The research findings will be widely disseminated to the coastal modeling community through participation in conferences and collaboration with the Community Surface Dynamics Modeling System (CSDMS). The open-source code to be developed and the data from the laboratory experiments will be disseminated through CSDMS and the flocculation formulation codes to be developed will be integrated into the CSDMS modeling framework via the recently released Python Modeling Toolkit (PyMT). In addition, a clinic on flocculation modeling is planned for the CSDMS annual meeting. This project supports 2 PhD students who will receive balanced training in coastal processes, fluid dynamics, high performance computing and laboratory techniques. The project also provides partial support for an early career postdoc researcher. Two undergraduate students will benefit from this project for their research on cohesive sediments. The project also strengthens collaboration with the United Kingdom and Germany on novel observational and computational tools.The primary goal of this collaborative study is to address key challenges of cohesive sediment transport in coastal/estuarine bottom boundary layers. The study utilizes a novel particle-resolved simulation model to investigate the physics of flocculation and floc structures for heterogeneous sediments. This effort is further augmented by laboratory experiments designed to better quantify stickiness and understand flocculation of sand-mud mixtures. Using a turbulence-resolving simulation model for fine sediment transport in a wave-current bottom boundary layer, coupled with enhanced flocculation formulations to model settling velocity, the investigators will study the interplay between flocculation, resuspension and deposition of cohesive sediments in coastal/estuarine bottom boundary layers. Five hypotheses are developed to guide the experimental and modeling work which will provide insight into key small-scale processes that are difficult to resolved in coastal models. Finally, by integrating and synthesizing these research outcomes, the study will evaluate a suite of closures for the settling velocity due to flocculation, from complex to simple, to inform coastal/estuarine modeling of cohesive sediment transport at regional scale.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.