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、水质和有害藻华等其他生态系统挑战的日益严重的威胁。这些紧迫的问题大多直接或间接地与沉积物搬运有关，有些与沙有关，但许多是由于细粒沉积物。细粒沉积物是粘性的，因此它们作为多孔的颗粒聚集体（称为絮凝体）进行运输。通过其复杂的结构，絮凝物是有机碳，营养物质，污染物的载体，有时它们可能含有沙粒。因此，它们的沉降速度很难量化。迄今为止，大多数沿海/河口模型忽略絮凝过程，并采用恒定的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构成了相当大的限制。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，将进行几个不同尺度的综合数值模拟和基于光学的实验室观测，包括与颗粒大小、水流湍流运动和底部边界层有关的模拟和观测。拟议研究的成果将用于更好地装备沿海模型，使其具备沉积物输运能力，以应对沿海社区面临的挑战。研究结果将通过参加会议和与社区表面动力学建模系统（CSDMS）合作，广泛传播到沿海建模社区。待开发的开源代码和实验室实验的数据将通过CSDMS传播，待开发的絮凝制剂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，还计划为CSDMS年会举办絮凝建模诊所。该项目支持2名博士生，他们将接受海岸过程，流体动力学，高性能计算和实验室技术的均衡培训。该项目还为早期职业博士后研究人员提供部分支持。两名本科生将从这个项目中受益，因为他们对粘性沉积物的研究。该项目还加强了与联合王国和德国在新的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/河口底部边界层粘性沉积物输运的关键挑战。该研究利用一种新的颗粒分辨模拟模型，调查非均质沉积物的絮凝和絮凝体结构的物理。这一努力进一步加强了实验室实验，旨在更好地量化粘性和了解砂泥混合物的絮凝。利用波浪-水流底部边界层中细泥沙输运的连续性分辨模拟模型，加上模拟沉降速度的增强絮凝公式，调查人员将研究沿海/河口底部边界层中粘性沉积物的絮凝、再悬浮和沉积之间的相互作用。五个假设的开发，以指导实验和建模工作，这将提供洞察关键的小尺度过程，难以解决的沿海模型。最后，通过整合和综合这些研究成果，该研究将评估一套封闭的沉降速度，由于絮凝，从复杂到简单，通知沿海/河口建模粘性泥沙输运在区域scale.This奖项反映了NSF的法定使命，并已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。

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.