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

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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.