Quantifying the dispersion of turbidity currents generated by seafloor mining operations

Abstract

Turbidity currents are common in the waters around the world. They can be caused by earthquakes, collapsing slopes or other geological disturbances. However, turbidity currents can also be caused by deep sea mining activities. Deep sea mining is done with special mining vehicles. There are three main operations within this mining vehicle: collecting nodules, separating the nodules from the mixture and discharging the mixture of sediment and water towards the environment. After impingement of the discharged flow with the seabed, it is expected that the flow will take the form of a turbidity current. Turbidity currents belong to a larger class of flows called gravity currents. Furthermore, turbidity currents are typically defined as dilute flows in which particles are dominantly supported by fluid turbulence. These currents have an interstitial fluid that is a liquid, generally water. The objective of this research is to increase knowledge of the behavior of these currents and to obtain experimental results that can be used for validation of CFD models. In this research the influence of the initial concentration on the dispersion, deposition and entrainment of the current is investigated. In addition, the spread of particles within the current is researched. Furthermore, different types of sediment are used with various particle size ranges and particle properties. The experimental methodology can be divided in two parts. The first part are experiments that involve a full-depth lock release of a fixed volume suspension of a sediment with different particle size ranges and particle properties into water. The initial concentration is varied and the obtained current is recorded with a high-speed camera. The second part consists of multiple calibration procedures. During these procedures, different concentrations of sediment are mixed with water and the resulting solution is recorded. This data is used to create a calibration function that in turn can be used to quantify the concentrations of sediment in the previous recorded currents. The experimental methodology differs from previous work due to this calibration procedure. Afterwards, video processing is used to perform an analysis of the current. The resulting turbidity currents go through three phases as observed in the experiments and previous work. At the start, there is an initial phase where the front is formed and the current accelerates. During this phase a limited amount of particles settle out of the current. Afterwards, the current transitions into a second phase when the velocity of the front starts to decrease due to inertial forces and due to the start of particle settling at the rear of the current. The last phase is when the backflowing bore created by the ambient fluid at the release of the gate reaches the front and decelerates the current until viscous forces start to dominate until the current vanishes. The experiments shows that currents transporting fine particles reach higher velocities and may travel for longer distances in comparison to currents composed of larger particles. Furthermore, the combination of fine and large particles has a substantial effect on the currents dynamics. The currents composed of a mixture of these two sediment types travel longer distances than currents with only large particles. The performed experiments also focus on varying the initial concentration and sediment type. These show that larger particles tent to entrain more with the ambient fluid in comparison to smaller particles and thus they have a larger vertical dispersion. Furthermore, currents composed of sediment with only fine particles will have a more rapid transition into a dense front and a less dense middle and rear pat of the current. Currents composed of fine sediment and a combined sediment with small and large particles have a clear division between a dense basal layer and a less dense top layer within the current. At last, deposition patterns are different per sediment type. Sediment composed of larger particles will have a larger deposition rate than sediment composed of smaller particles. Furthermore combining both sediment types will increase the deposition rate at the beginning of the current.