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

大多数从大陆运往海洋的沉积物都是由非常细粒度的颗粒组成的泥浆。泥质沉积物的命运在很多方面都很重要。泥浆运移决定着河流污染物、重金属和有机碳的命运；泥浆也是一种关键资源，可以恢复沿海地区，使平坦、低洼的景观适合居住。越来越多的证据表明，河流中的泥土和有机物经常通过一种叫做絮凝的过程结合在一起，形成大的集合体。该项目将开发一个絮凝泥在河流中运移的预测数学模型，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。研究结果将与海湾地区来自地方、州和联邦机构、学术界和环境非政府组织的科学家分享，以影响密西西比三角洲沿海湿地的管理决策。该项目将邀请新奥尔良大学的学生参加加州理工学院的一个夏季研究项目，这些学生直接受到密西西比三角洲土地流失的影响。与沙子相比，人们对河流系统中泥浆运输和沉积的基本机制知之甚少。这是一个重大的知识空白，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。泥浆也是陆地和海岸景观的主要组成部分：它形成了洪泛平原、海岸湿地和三角洲，影响了河岸侵蚀速率和堤防稳定性，并决定了河流地层的沉积结构。在河流中，泥浆通常被认为是“冲刷物”，沉降速度非常小的颗粒，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质经常絮凝在一起形成聚集体，其沉降速度相当于沙子。本项目将利用新水槽实验的理论和半经验关系建立淡水絮凝的机理模型。将通过浓度-深度剖面的反演来编制河流絮体沉降速度数据库，以使用现场数据测试淡水絮体模型。泥沙输运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工程改道重建湿地所需的湿地增加速率的影响。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。

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.