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S.A. Wahab
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1
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.
...
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.
...
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.
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.
In this study, the influence of a bed on turbidity current propagation and flocculation dynamics has been investigated using a lock-exchange setup. Experiments were performed in saltwater using sediments sampled from a deep-sea mining location in the Clarion Clipperton Zone (CCZ). Results showed that the presence of a bed influenced the propagation velocity of turbidity currents. Flocs were denser and larger than those observed when no bed was present. The floc settling velocities also increased in the presence of a bed. Additionally, in the case of a (freshly) formed bed, sediment resuspension occurred due to the disturbance of organic matter, which contributed to flocculation. This study also sheds light on the role of the age of the bed on turbidity current propagation, with (freshly) formed beds being efficient in reducing sediment spread. These findings are important for predicting the spread of a turbidity current during deep-sea mining activities.
...
In this study, the influence of a bed on turbidity current propagation and flocculation dynamics has been investigated using a lock-exchange setup. Experiments were performed in saltwater using sediments sampled from a deep-sea mining location in the Clarion Clipperton Zone (CCZ). Results showed that the presence of a bed influenced the propagation velocity of turbidity currents. Flocs were denser and larger than those observed when no bed was present. The floc settling velocities also increased in the presence of a bed. Additionally, in the case of a (freshly) formed bed, sediment resuspension occurred due to the disturbance of organic matter, which contributed to flocculation. This study also sheds light on the role of the age of the bed on turbidity current propagation, with (freshly) formed beds being efficient in reducing sediment spread. These findings are important for predicting the spread of a turbidity current during deep-sea mining activities.
This study investigated the impact of various types of bed composition on turbidity current propagation in relation to flocculation. A lock exchange setup was used, comprising a mixing section and an outflow compartment. The bed types investigated were a quartz bed, a quartz bed topped with (unflocculated) illite clay, and a quartz bed with flocculated illite. The findings confirmed that the presence of a bed influenced the turbidity current propagation. In particular, it was found that the front velocity was strongly reduced when the bed was composed of freshly made flocs compared to the case where the bed was made of quartz alone, which does not form flocs. While propagating, either illite clay or flocs were picked up and aggregated into larger flocs. These larger flocs were then deposited further downstream during propagation. Moreover, the front velocity was higher over a quartz bed when no flocculant was added to the outflow compartment water than when flocculant was present. This confirms that flocculation occurs in the water column during propagation.
...
This study investigated the impact of various types of bed composition on turbidity current propagation in relation to flocculation. A lock exchange setup was used, comprising a mixing section and an outflow compartment. The bed types investigated were a quartz bed, a quartz bed topped with (unflocculated) illite clay, and a quartz bed with flocculated illite. The findings confirmed that the presence of a bed influenced the turbidity current propagation. In particular, it was found that the front velocity was strongly reduced when the bed was composed of freshly made flocs compared to the case where the bed was made of quartz alone, which does not form flocs. While propagating, either illite clay or flocs were picked up and aggregated into larger flocs. These larger flocs were then deposited further downstream during propagation. Moreover, the front velocity was higher over a quartz bed when no flocculant was added to the outflow compartment water than when flocculant was present. This confirms that flocculation occurs in the water column during propagation.
Turbidity currents are a subclass of gravity currents where a particle-laden fluid flows through a relatively lighter fluid under the effect of gravity. The particles in this case are mostly suspended by the turbulence created due to the forward motion of the current along the boundary of the domain [1]. Turbidity currents are an inevitable part of any dredging or deep-sea mining activity. They have a potential impact on the local ecosystem [2]. They have the tendency to propagate further from the area of operation before settling down.
This study examines the behavior of turbidity currents which are quite dilute in nature, as they flow over different bed types both pre-existing and freshly deposited ones. The pre-existing bed here refers to the ocean, river or channel bed while the freshly deposited bed consists of a layer of materials deposited from previous run, which has loose materials on its surface. ...
This study examines the behavior of turbidity currents which are quite dilute in nature, as they flow over different bed types both pre-existing and freshly deposited ones. The pre-existing bed here refers to the ocean, river or channel bed while the freshly deposited bed consists of a layer of materials deposited from previous run, which has loose materials on its surface. ...
Turbidity currents are a subclass of gravity currents where a particle-laden fluid flows through a relatively lighter fluid under the effect of gravity. The particles in this case are mostly suspended by the turbulence created due to the forward motion of the current along the boundary of the domain [1]. Turbidity currents are an inevitable part of any dredging or deep-sea mining activity. They have a potential impact on the local ecosystem [2]. They have the tendency to propagate further from the area of operation before settling down.
This study examines the behavior of turbidity currents which are quite dilute in nature, as they flow over different bed types both pre-existing and freshly deposited ones. The pre-existing bed here refers to the ocean, river or channel bed while the freshly deposited bed consists of a layer of materials deposited from previous run, which has loose materials on its surface.
This study examines the behavior of turbidity currents which are quite dilute in nature, as they flow over different bed types both pre-existing and freshly deposited ones. The pre-existing bed here refers to the ocean, river or channel bed while the freshly deposited bed consists of a layer of materials deposited from previous run, which has loose materials on its surface.
Turbidity flows are known to be affected by the density difference between sediment plumes and the surrounding water. However, besides density, other factors could lead to changes in flow propagation. Such a factor is the presence of suspended organic matter. Recently, it was found that flocculation does occur within plumes upon release of a sediment/organic matter mixture in a lock exchange flume. In the present study, mineral sediment (illite clay) was released into the outflow compartment containing water and synthetic organic matter (polyacrylamide flocculant). Even though the density of water was barely affected by the presence of flocculant, flow head velocity was observed to be larger in the presence of flocculant than without. Samples taken at different positions in the flume indicated that flocs were created during the small current propagation time (about 30–60 s) and that their sizes were larger with higher flocculant dosage. The size of flocs depended on their positions in the flow: flocs sampled in the body part of the flow were larger than the ones sampled at the bottom. All these properties are discussed as a function of sediment–flocculant interactions.
...
Turbidity flows are known to be affected by the density difference between sediment plumes and the surrounding water. However, besides density, other factors could lead to changes in flow propagation. Such a factor is the presence of suspended organic matter. Recently, it was found that flocculation does occur within plumes upon release of a sediment/organic matter mixture in a lock exchange flume. In the present study, mineral sediment (illite clay) was released into the outflow compartment containing water and synthetic organic matter (polyacrylamide flocculant). Even though the density of water was barely affected by the presence of flocculant, flow head velocity was observed to be larger in the presence of flocculant than without. Samples taken at different positions in the flume indicated that flocs were created during the small current propagation time (about 30–60 s) and that their sizes were larger with higher flocculant dosage. The size of flocs depended on their positions in the flow: flocs sampled in the body part of the flow were larger than the ones sampled at the bottom. All these properties are discussed as a function of sediment–flocculant interactions.