From Particles to Plumes: Role of Flocculation in Turbidity flows

Abstract

The work presented in this thesis investigates the role of flocculation in turbidity flows, with relevance to deep-sea mining but also dredging operations in more shallow waters, as the results are broadly applicable. Deep-sea mining is emerging as an alternative source for critical minerals needed for the transition to clean energy technologies. However, mining activities disturb the seabed, leading to the formation of sediment plumes. These sediment plumes propagate over long distances and impact sensitive marine ecosystems in the area. Similar environmental concerns also arise in the case of dredging projects, where resuspended sediments increase the turbidity in the surrounding waters. These challenges highlight the need for improved understanding of sediment plume dynamics and the processes that control their evolution. Flocculation (the aggregation of small particles into larger ones) could potentially limit turbidity flow propagation. This underpins the work presented in this thesis.


A series of laboratory experiments was designed to study the propagation of turbidity currents under controlled conditions. A lock-exchange flume setup was used to simulate sediment-laden turbidity currents, where detailed investigations of current propagation and floc formation were carried out. The work combines hydrodynamic measurements with advanced particle-sensing techniques to link micro-scale flocculation processes with turbidity current behavior. Experiments were first conducted with clay (illite) in the presence of flocculants (polyacrylamide), and then with a natural clay from the Clarion-Clipperton Fracture Zone (CCZ) that contains organic matter (acting as flocculants). Some experiments were also conducted with non-flocculating quartz as a reference.
The results demonstrate that flocculation can occur rapidly, within tens of seconds, and significantly modifies particle size distributions and settling behavior.

The thesis further explores how seabed characteristics influence turbidity currents. Experiments were performed over beds of different compositions and ages to understand their roles in turbidity flows and floc evolution. The presence of polyacrylamide in the outflow compartment of the lock-exchange flume was found to increase the front velocity when no bed was present, which was attributed to the lubrication effect between the turbidity current and the plexiglass bottom of the flume. It was found that pre-existing beds and their consolidation state affect sediment entrainment and flow propagation. Bed roughness and material type can either enhance or suppress floc formation, thereby altering the mobility of the turbidity current. These findings are directly relevant to operational strategies in dredging and mining, where repeated disturbance of the seabed occurs.

In addition to physical mechanisms, this research also examines the ability of monitoring instruments to detect turbidity current properties, particularly particle size and concentration. Optical Backscatter Sensors (OBS), Acoustic Doppler Velocimeter (ADV), and Laser In-Situ Scattering and Transmissometry (LISST) were used in the lock-exchange setup. Malvern mastersizer and FlocCAM were used to further characterize samples taken at different positions within the lock-exchange flume. The study demonstrates that sensor responses are highly sediment-dependent and that no single instrument can, in situ (lock-exchange) reliably distinguish between primary (unflocculated) particles and aggregates. A combination of lab measurements and in situ sensor techniques is therefore recommended for studying flocculation.

Overall, this thesis provides new experimental insights into the coupling between particle aggregation and turbidity current dynamics. It shows that flocculation, bed interactions, and sensor limitations must all be considered when predicting sediment plume behavior. The outcomes contribute to more reliable assessment and monitoring of environmental impacts associated with offshore engineering activities and offer guidance for future field measurements. The work ultimately strengthens the scientific basis for responsible and sustainable management of deep-sea mining and dredging operations.