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.

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.