In recent years, the demand for minerals and rare-earth elements are escalating due to rapid technological advancements and developments. This condition raises the importance of Deep-Sea Mining (DSM) as an option to fulfill the global demands, for the sake of future ambitious projects. On the other hand, DSM still faces some drawbacks and obstacles in its operations, e.g. environmental impact of its tailings discharge. Thus, the presence of tools for predicting the behavior and environmental impact of DSM tailings becomes crucial for the sake of conducting sustainable DSM operations. Researches, both numerical and laboratory experiments, are then done to achieve this goal. In the study of DSM tailings behavior through numerical simulation, the challenge lies in the ability to implement the complex physics phenomena around DSM plumes to a numerical model. This research is thus aimed to observe one of the parts of the so-called physical phenomena, and numerical constraints on the simulation of DSM plumes: the effects of implementing arbitrary non-orthogonal mesh. Arbitrary non-orthogonal mesh would give users the freedom to refine the mesh based on the required resolution on a certain area in the simulation domain. In this research, an arbitrary non-orthogonal mesh is constructed forming a domain geometry of 3D tank with round pipe as a source of tailings discharge, adapting the laboratory setup of Byishimo (2018). Mesh refinement is done around the pipe discharge area, where the jet mainstream expected to occur, and near the bottom, where settling of solids fraction and high velocity-gradients are expected to occur. Top-hat approximation theory is then used for defining the inlet boundary condition of the simulation domain, and smooth solid wall is used for the bottom boundary. In observing the effect of arbitrary mesh, two parameters are varied: solids settling rate, and differentiation scheme. Following this, six simulation cases are prepared, containing three solids-settling conditions (minimum, realistic, and extreme solids fraction settling) and two differentiation schemes (Gauss Gamma and Gauss Linear). These cases are then simulated using the CFD drift-flux model in OpenFOAM for two incompressible fluids, with ambient fluid and tailings mixture as the two incompressible fluids. Turbulence is modeled using LES method, with Wall-Adapting Local Eddy-viscosity (WALE) LES model for modeling the subgrid-scale of the turbulence. Two simulated field variables are picked to be observed and compared with the laboratory measurement data: flow velocities and local Suspended Solids Concentration (SSC). Simulations show that the constructed domain able to generate stable results using not only Gauss Gamma differencing scheme but also Gauss Linear, which originally expected to give unbounded results. From simulations with various solids settling conditions, it is analyzed that the implementation of solids settling using the mentioned function leads to constant settling rate with inability to re-suspend the settled solids. Thus, simulation of cases with extreme solids settling leads to hyper-concentration of solids fraction in the cells on the bottom. The top-hat profile of the inlet boundary condition is constantly sustained throughout the simulation, resulting in uniform jet vertical velocity profile. The simulations also show that momentum-driven jet-like flow can be observed around the impingement point, while gravity current generated further from the impingement point. The simulation results show that the mesh refinement is enabling simulating and validating the flow with the required resolutions. Moreover, the constructed domain also shows that Gauss Linear can be used for simulating DSM plumes, furthermore using full tank domain. The simulated SSC also turns out not only affected by the simulation domain and mesh, but also the differencing scheme, and the presence of settling and pick-up functions.

A lot of uncertainties exist about the impact of deep-sea mining on the Benthic environment. One of these uncertainties concerns the sediment plumes created by the mining operation. The return slurry released above the sea floor creates a large plume of sediments. With numerical models the behavior of such plumes is modelled in order to minimize these uncertainties. Particle size distributions in these plume models are currently based on individual particle sizes. In situ, at the bottom of the ocean in an salt water environment, flocculation can occur. Flocculation can have effects on the particle size distribution, settling rates and floc structure. The objective of this research is to investigate the significance of flocculation on the settling behavior and if it should be taken into account in these plume models. Shear rate and particle concentration, as function of time, are the most governing processes that have effect on the flocculation behavior. The influence of these parameters on the flocculation process are investigated. Different methods are used in the search for a suitable technique to measure the in situ particle size distribution. Next to the particle size distribution the floc shapes, structure and settling rates are measured under the influence of the factors mentioned above. In the shear range of 6 s-1 and particle concentration of 0.7 to 40 g/l flocculation does occur and has a large impact on the settling behavior of the sediment. Flocculation at other shear rates and particle concentrations remain unknown. Figure \ref{fig:vol_dem_salt_floccu1} shows the time until 100 $\%$ of the mass is settled when the suspension is released at a height of 10 m. This graph shows the difference in settling time between a suspension in demi and salt water. A settling time difference of 85 days (25 days for salt water and 110 days for demi water) shows the importance of including flocculation behavior in plume models.

Growing markets for sustainable technologies such as solar panels, wind turbines, and electric cars require an enormous amount of raw materials that might not be available from sources on land. Exploitation of seafloor mineral resources could secure the supply of raw materials for the future. However, proposed deep-sea mining (DSM) systems will return wastewater containing fine sediments and other effluents back to the sea via a pipe discharging plumes close to the seabed. Concerns are that the DSM plumes will have significant environmental impacts on the mostly uncharted deep-sea ecosystem. Until today, the environmental licensing procedure for DSM operations is incomplete. Accurate numerical models to predict the behavior of returned DSM plumes are still lacking.

This research aims to support environmental impact assessments for DSM operations. Laboratory tests were carried out to provide validation data for numerical models that can be used to predict the spreading of DSM plumes in the near-field domain. Additionally, Computational Fluid Dynamics (CFD) technique was used to predict the same laboratory simulations. Wide ranges of initial parameters such are the plume release elevations relative to the bed, source initial Suspended Sediment Concentrations (SSC) and different bed slopes were considered.

Experiments and CFD results agreed well. Negatively buoyant plumes fall vertically towards the bed and impinge generating turbidity currents. Excess buoyancy gained by increasing the initial SSC at the source leads to a considerable increase in the plume dispersion rate. Plume release height is found to be an important parameter. Discharging close to the bed leads to the formation of a circular levee around the impingement point at a distance 10 times the orifice diameter. The sedimentation ring acts as a barrier against the turbidity current leading to the deposition of sediments right outside the ring. As a consequence, the plume dispersion rate reduces and DSM wastes stay closer to the disposal location. In contrast, when plumes are released at a greater height from the bed, due to long falling distance and settling of particles, generated turbidity currents accelerate and the sediments particles are transported in suspension to large distances from the disposal location.

The main goal of this research, to provide a unique set of validation data for numerical models was achieved and a Large Eddy Simulation CFD model was successfully used, tested and approved for its capability to accurately predict the near-field spreading of DSM plumes. Based on obtained results, in order to minimise the environmental impacts, the flow velocity at the bed should be minimised to prevent re-suspension of already deposited sediments. It is recommended that sediment wastes and other effluents should be released close to the seabed. CFD model setup considered only sedimentation of particles and ignored erosion. Based on observed seabed morphological changes during the laboratory experiments, it is recommended that erosion should be considered within numerical models. To fully predict the environmental impacts of DSM operations, the near-field model used in this research should be coupled to a far-field model but also considering multiple environmental pressures caused by DSM tailings.