Collaborative Research: Flocculation Dynamics in the Fluvial to Marine Transition

河流将上游景观侵蚀的细泥质沉积物输送到沿海地区。到达后，泥浆可能会存款在该地区的河流、海湾、河口、沼泽和三角洲中，或通过这些区域移动。预测泥浆到达时的去向对社会很重要。例如，准确预测这些地区泥浆的运动和沉积，对于预测和评估旨在利用沉积物沉积来构建三角洲景观、促进健康水质和/或评估泥浆沉积如何使航道变浅从而阻碍船舶航行的河流改道项目的有效性非常重要。然而，通过沿海沃茨的泥浆运动的预测是困难的。困难在很大程度上是由于测量和模拟泥浆颗粒悬浮在水中时如何变化的能力有限。小的泥粒往往聚集在一起，形成被称为絮凝物的聚集体，絮凝物的生长或收缩取决于水流中的湍流水平、盐度、生物含量以及水中沉积物的数量和类型。泥沙的沉降速度与絮体的大小和密度直接相关.这项研究将通过收集详细的测量结果来评估河流和沿海水道中泥粒运动的性质，这些测量结果将为絮凝物动力学的物理模型提供信息和验证。该研究将实验室实验和世界上最大的沿海三角洲之一路易斯安那州的密西西比河下游的全面实地调查结合起来。该团队将通过弗吉尼亚理工大学的夏令营项目吸引对STEM职业感兴趣的高中女性，并培训来自代表性不足的少数群体的研究生和本科生了解沉积物运输的物理过程。通过结合工程和地球科学的研究，学生将接触到跨学科的目标，因此有能力处理基础和工业科学，和/或政策相关的field.The研究团队将量化泥絮大小作为不断变化的湍流剪切，盐度和水柱沉积物浓度的条件下，存在于整个河流到海洋过渡（FtMT）的密西西比河的函数。这一过渡的三个主要区域是淡水河流段，咸分层和横向限制的河口河段，和横向无限制的近岸河流羽。将检验四个主要假设：（1）在淡水河流河段，絮凝体产生泥浆的垂直分层;（2）在河口河段近底盐楔界面处的混合产生严格的絮凝体生长;（3）通过横向无约束的河流羽流对湍流剪切的修改是絮凝体生长的主要驱动力，从而增加泥浆沉降速度;絮凝体大小与当地湍流和盐度之间的相关性极小，部分原因是絮凝体大小与当地条件之间的不平衡。这些假设将通过整合现场和实验室研究进行测试，这些研究测量了FtMT上絮凝体大小、泥沙浓度、盐度和湍流剪切的垂直、纵向和时间分布。该团队将利用这些知识为絮凝模型提供信息和验证，这些模型可以集成到沿海科学家和工程师使用的更大的流体动力学模型中，以加强对细沉积物运输和命运的预测。作为研究的一部分开发的测量也将启动一个全球社区数据库，该数据库包含开发和校准预测泥浆运动和沉积的计算机模型所需的关键信息。该奖项反映了NSF的法定使命，并被认为值得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来支持。

Rivers deliver fine muddy sediment eroded from upstream landscapes to coastal regions. Upon arrival, mud can deposit in or move through the rivers, bays, estuaries, marshes, and deltas of the region. Forecasting where the mud goes when it arrives is important for society. For example, accurately forecasting the movement and deposition of mud in these regions is important for predicting and evaluating the effectiveness of river diversion projects aimed at using sediment deposits to build deltaic landscape, fostering healthy water quality, and/or evaluating how mud deposits shoal waterways and therefore impede vessel navigation. Yet, the prediction of mud movement through coastal waters is difficult. The difficulty is due in large part to the limited ability to measure and model how mud particles change when suspended in water. Small mud particles tend to clump together and form aggregates known as flocs, which grow or shrink depending on turbulence levels in the flow and the salinity, biological content, and amount and type of sediment in the water. The settling speed of muddy sediment is directly related to the size and density these flocs. This research will evaluate the nature of mud particle movement in river and coastal waterways by collecting detailed measurements that will inform and validate physically-based models of floc dynamics. The research pairs laboratory experiments and comprehensive field surveys from one of the largest coastal deltas in the world, the lower Mississippi River in Louisiana. The team will engage high school women interested in STEM careers through summer camp programs at Virginia Tech, and train graduate and undergraduate students from underrepresented minority groups about physical processes of sediment transport. By combining engineering and earth science studies, students will be exposed to cross-disciplinary objectives and therefore equipped to handle basic and industry science, and/or policy-related fields.The research team will quantify mud floc sizes as a function of the changing turbulent shear, salinity, and water-column sediment concentration conditions that exists across the fluvial to marine transition (FtMT) of the Mississippi River. The three primary zones of this transition are the freshwater fluvial section, the saline-stratified and laterally confined estuarine reach, and the laterally unconfined near-shore river plume. Four central hypotheses will be tested: (1) in the freshwater riverine reach, flocs produce vertical stratification of mud; (2) mixing in the estuarine reach at the interface of the near-bottom salt wedge produces rigorous floc growth; (3) modifications to turbulent shear through the laterally unconfined river plume is a central driver of floc growth and thus increased mud settling velocity; and (4) minimal correlation between floc size and local turbulence and salinity is due in part to disequilibrium between floc size and local conditions. These hypotheses will be tested by integrating field and laboratory studies that measure the vertical, longitudinal, and temporal distributions of floc size, sediment concentration, salinity, and turbulent shear over the FtMT. The team will use this knowledge to inform and validate flocculation models that can be integrated into larger hydrodynamic models used by coastal scientists and engineers to enhance predictions of the transport and fate of fine sediment. Measurements developed as part of the study will also initiate a global community database housing key information needed to develop and calibrate computer models predicting the movement and deposition of mud.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.