Modelling of sediment plumes generated by deep-sea mining in CCZ

Deep-sea mining may bring potential risks such as degradation of biological habitats or loss of biodiversity in deep-sea mining areas. Thus, assessing and predicting the potential environmental impacts of deep-sea mining has become a common concern by the international communities. Among them, predicting the extent of sediment plume is an important element in the environmental impact assessment of deep-sea mining. We developed a high-resolution hydrodynamic numerical model applicable to the deep-sea simulation needs of the COMRA polymetallic nodule contract area in the CCZ, where the source term of the sediment plume under a commercial mining scale scenario was measured and a numerical modeling pilot study of the plume dispersion carried out. The study showed that when the mining tailwater was discharged at the bottom of the COMRA contract area with a discharge depth of 6 m from the bottom and a discharge rate of 63 kg/s in one month of continuous mining in a mining area of 5.35 km² and an average flow velocity about 5 cm/s, the maximum redeposition thickness reached about 4.3 mm and the ranges of redeposition thickness in between 1 mm and 0.1 mm were 9.4 km² and 63 km², respectively. Because of the dilution by seawater, the suspended sediment concentration were very low and all less than 0.01 mg/L. Sensiti vity experiments show that the redeposition thickness increased as the settling velocity increased from 0.08 mm/s to 0.30 mm/s, with a maximum value of approximately 12.9 mm, the area of redeposition thicknesses higher than 1 mm and 0.1 mm are 16.5 km² and 98 km², respectively, and particle in the water column are reduced with 80% of the particle settling to the seabed at the end of the mining operation.