Collaborative Research: Freshwater flocculation and its impact on sustaining floodplains and deltaic wetlands

从大陆到海洋的大多数沉积物是由非常细的颗粒组成的泥浆。淤泥沉积物的命运在许多方面都很重要。 泥浆的运输控制着河流污染物、重金属和有机碳的命运;泥浆也是恢复沿海地区并使平坦、低洼的景观适合居住的关键资源。越来越多的证据表明，河流中的淤泥和有机物经常通过一种称为絮凝的过程结合在一起形成大的聚集体。这个项目将开发一个预测性的数学模型，用于河流中絮凝泥浆的运输，并将该模型应用于密西西比三角洲湿地的实际海岸恢复问题。该项目汇集了来自加州理工学院（Caltech），新奥尔良大学（UNO）和海湾水研究所的团队，以解决墨西哥湾地区的环境和社会挑战。研究结果将与来自地方、州和联邦机构、学术界和环境非政府组织的海湾地区科学家分享，以影响密西西比三角洲沿海湿地管理决策。该项目将吸引新奥尔良大学的学生，他们直接受到密西西比三角洲土地流失的影响，参加加州理工学院的夏季研究项目。与沙子相比，人们对河流系统中泥浆输送和沉积的基本机制知之甚少。这是一个主要的知识差距，因为泥浆吸附污染物，营养物质和有机碳，这种材料的命运直接影响环境质量和全球碳循环。泥浆也是陆地和沿海景观的主要组成部分：它形成洪泛平原、沿海湿地和三角洲，影响河岸侵蚀速率和堤坝稳定性，并形成河流地层的沉积结构。在河流中，泥浆通常被认为是“冲刷物”，沉降速度很小的颗粒，它们可以作为被动的水示踪剂。然而，越来越多的证据表明，河流系统中的泥浆和有机物往往絮凝在一起形成聚集体，其沉降速度与沙子相当。本计画将利用新水槽实验的理论与半经验关系，发展淡水絮凝的机制模式。通过浓度-深度剖面反演，将汇编河流絮凝物沉降速度数据库，以使用现场数据检验淡水絮凝物模型。泥浆输送理论将被整合到用于密西西比三角洲湿地恢复的数值模型中。一系列的数值实验将评估絮凝对湿地增长率的影响，以用于重建下沉湿地所需的工程改道。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。

Most sediment transported from continents to the oceans is mud that consists of very fine-grained particles. The fate of muddy sediment is important in many ways. Mud transport governs the fate of riverine pollutants, heavy metals and organic carbon; mud also is a key resource that can restore coastal areas and make flat, low-lying landscapes habitable. Growing evidence indicates that mud and organic matter in rivers are often bound together into large aggregates through a process called flocculation. This project will develop a predictive mathematical model for the transport of flocculated mud in rivers and apply the model to real-world coastal restoration problems in the Mississippi Delta wetlands. The project brings together a team from the California Institute of Technology (Caltech), the University of New Orleans (UNO), and the Water Institute of the Gulf, to solve environmental and societal challenges in the Gulf of Mexico region. The results will be shared with scientists in the Gulf Region from local, state, and federal agencies, academia, and environmental NGOs to influence coastal wetlands management decisions in the Mississippi Delta. The project will engage students from the University of New Orleans, who are directly affected by Mississippi Delta land loss, in a summer research program at Caltech.Compared to sand, far less is known about the basic mechanics of mud transport and deposition in river systems. This is a major knowledge gap because mud adsorbs pollutants, nutrients and organic carbon, and the fate of this material directly impacts environmental quality and the global carbon cycle. Mud also is the primary building block of terrestrial and coastal landscapes: it builds floodplains, coastal wetlands and deltas, affects bank erosion rates and levee stability, and sets the depositional architecture of fluvial strata. In rivers, mud is typically considered as “washload,” particles with settling velocities so small that they act as passive water tracers. However, growing evidence indicates that mud and organic matter in fluvial systems are often flocculated together into aggregates with settling velocities equivalent to sand. This project will develop a mechanistic model for freshwater flocculation using theory and semi-empirical relations from new flume experiments. A database of floc settling velocities from rivers will be compiled by inversion from concentration-depth profiles, to test the freshwater floc model using field data. The mud transport theory will be integrated into numerical models used for wetland restoration in the Mississippi Delta. A series of numerical experiments will evaluate the impact of flocculation on wetland accretion rates for engineered diversions that are needed to rebuild sinking wetlands.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.