Flocculation and Settling of Clay from a Radially Spreading Intrusion in a Two-Layer Fluid

In the process of deep-sea mining, poly-metallic nodules on the ocean abyssal floor are carried to the surface along with bottom sediments. After the nodules are extracted, the sediments are re-injected into the ocean at depth. This motivates the present study of how a mixture of clay and water injected vertically into the stratified fluid spreads laterally at its neutral buoyancy level and how the clay eventually descends from this intrusion. Laboratory experiments are performed in which a mixture of clay and fresh water is injected downwards into a two-layer salt-stratified fluid. From side and top view movies, we measure the radial spread rate of the intrusion and settling speed of the particles. The intrusion spreads radially with faster speed if the source density is larger until the settling of particles below the intrusion begins, at which point the advance slows. The time at which settling below the intrusion begins decreases with increasing source density, with the settling speed being much larger than that predicted for individual clay particles as a result of flocculation.