Collaborative Research: Flocculation Dynamics in the Fluvial to Marine Transition

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.