Abstract
The interest in polymetallic nodule mining has considerably increased in the last few decades. This has been largely driven by population growth and the need to move towards a green future, which requires strategic raw materials. Deep-Sea Mining (DSM) is a potential source of such key materials. While harvesting the ore from the deep sea by a Polymetallic Nodule Mining Tool (PNMT), some bed sediment is unavoidably collected. Within the PNMT, the ore is separated from the sediment, and the remaining sediment–water mixture is discharged behind the PNMT, forming an environmental concern. This paper begins with surveying the state-of-the-art knowledge of the evolution of the discharge from a PNMT, in which the discharge characteristics and generation of turbidity currents are discussed. Moreover, the existing water entrainment theories and coefficients are analyzed. It is shown how plumes and jets can be classified using the flux balance approach. Following that, the models of Lee et al. (2013) and Parker et al. (1986) are combined and utilized to study the evolution of both the generated sediment plume and the subsequent turbidity current. The results showed that a smaller sediment flux at the impingement point, where the plume is transformed into a turbidity current, results in a shorter run-out distance of the turbidity current, consequently being more favorable from an environmental point of view.
Highlights
The industrial revolution and the rising living standards around the world require an unconventional search for resources for many key materials (e.g., Cobalt (Co), Nickel (Ni), Tellurium (Te), Titanium (Ti), Platinum (Pt), and rare Earth metals)
The continuous decrease of the grade of these minerals on land makes it worthwhile for mining companies to go into the deep ocean, which has the advantage of having better ore grades [3], for instance researchers estimated that the largest reserves of cobalt, nickel, and manganese are found on the sea bed [4]
Turbidity currents can be described as particle-laden underflows, which are driven by the excess hydrostatic pressure resulting from the density difference between the ambient water and the sediment–water mixture [52,53]
Summary
The industrial revolution and the rising living standards around the world require an unconventional search for resources for many key materials (e.g., Cobalt (Co), Nickel (Ni), Tellurium (Te), Titanium (Ti), Platinum (Pt), and rare Earth metals). The released suspended sediment, whether it is from the VTS or from the PNMT, forms a sediment cloud termed a “sediment plume” This plume increases the turbidity level of deep-sea water [9,10], which negatively impacts the marine ecosystem [11]. The objective of this paper is to present an analysis of the characterizations of plumes generated by a PNMT in terms of the transport, spreading, and deposition of sediments This will help determine the optimal discharge conditions to minimize the environmental impact caused by sediment plumes. In this regard, the emphasis will be on the nearfield effects (see Section 2.1 for the nearfield/farfield definitions), where engineering and the design of equipment can significantly influence the spread and deposition of the generated plume.
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