Collaborative Research: Flocculation Dynamics in the Fluvial to Marine Transition

河流将上游地区被侵蚀的细泥质沉积物输送到沿海地区。一旦到达，泥浆就会沉积在该地区的河流、海湾、河口、沼泽和三角洲中或穿过它们。预测泥浆到达时的去向对社会很重要。例如，准确预测这些地区的泥沙运动和沉积对于预测和评估河流改道项目的有效性非常重要，这些项目旨在利用泥沙沉积物建立三角洲景观，促进健康的水质，和/或评估泥沙沉积物如何使水道变浅，从而阻碍船舶航行。然而，预测通过沿海水域的泥浆运动是困难的。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变化的能力有限。小的泥浆颗粒倾向于聚集在一起，形成称为絮凝体的聚集体，絮凝体的生长或缩小取决于水流中的湍流程度、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凝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数据来评估河流和沿海水道中泥浆颗粒运动的性质，这些测量数据将为基于物理的絮体动力学模型提供信息和验证。这项研究对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进行了实验室实验和全面的实地调查。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，并对来自代表性不足的少数群体的研究生和本科生进行有关沉积物运输物理过程的培训。通过结合工程和地球科学的学习，学生将接触到跨学科的目标，从而具备处理基础科学和工业科学，以及/或政策相关领域的能力。研究小组将量化泥块的大小，作为密西西比河河流向海洋过渡（FtMT）过程中不断变化的湍流剪切、盐度和水柱沉积物浓度条件的函数。这一过渡的三个主要带是淡水河段、盐层状和侧向受限的河口河段和侧向无受限的近岸河羽。本文将检验四个中心假设：(1)在淡水河段，絮凝体产生垂直的泥浆分层；(2)近底盐楔界面河口河段的混合作用使絮体生长剧烈；(3)通过横向无约束河羽的湍流剪切变化是絮体生长的主要驱动力，从而增加了泥浆沉降速度；(4)絮体大小与当地湍流和盐度之间的相关性很小，部分原因是絮体大小与当地条件之间的不平衡。这些假设将通过整合实地和实验室研究来验证，这些研究将测量FtMT上的浮冰大小、沉积物浓度、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来告知和验证絮凝模型，这些模型可以集成到沿海科学家和工程师使用的更大的水动力学模型中，以增强对细沉积物的运输和命运的预测。作为该研究的一部分而开展的测量还将启动一个全球社区数据库，其中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。

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.