Simulating Bathymetric Influences on Sediment Plume Steering for Deep-Sea Mining

Abstract

The global transition to low-carbon technologies is driving increased demand for critical metals, prompting renewed interest in deep-sea mining (DSM) as a complementary alternative to declining terrestrial sources. However, DSM raises environmental concerns, particularly from NGOs and the International Seabed Authority (ISA) (the organization which oversees DSM activities) due to sediment plumes generated by Polymetallic Nodule Mining Tools (PNMT), which may disrupt deep-sea ecosystems. Allseas is developing DSM equipment for the NORI-D area located in the Clarion-Clipperton Zone (CCZ) and aim to minimize these sediment plume impacts. A 2022 pilot study showed lower than expected plume magnitudes, but further mitigation efforts focus on directing plumes toward less sensitive or previously disturbed areas. As bathymetric slopes may influence plume behavior, this study investigates whether a plume, horizontally steered 20° from the diffusers, can maintain its direction across sloped terrain in the NORI-D region.

To explore this, OpenFOAM simulations were performed using the solver by Elerian (2023), which uses multiple sediment fractions for improved plume representation.Flocculation was omitted to reduce computational costs, as it is expected to have minimal impact on the near-field plume dynamics that are the focus of this study. Initial simulations validated the solver against the laboratory experiments of Visser (2024), replicating its setup, sediment/proxy type (glass beads), and slope conditions (–5°, –3°, 0°, 3°, 5°) without steering. Simulated plume behavior aligned well with experiments, with minor discrepancies likely due to the measurement method and/or timing differences. Follow-up simulations used NORI-D sediment properties and a scaled conceptual model of the commercial PNMT diffuser. A horizontally aimed 20° steering angle towards the starboard and the same slope variations were applied. Results showed that plume behavior varied with driving direction. Uphill and flat driving scenarios led to prolonged sediment suspension, enabling greater downstream transport and reducing steering effectiveness, with more plume material drifting to the port side. Flat terrain produced the most extensive plume, likely due to its sustained propagation and enhanced lateral spreading. In contrast, downhill driving promoted earlier, strong deceleration near the impingement zone (where the plume first comes into contact with the seabed), resulting in shorter, more confined plumes. These conditions improved steering performance, with most of the sediment directed to the desired starboard region or remaining behind the PNMT.

These findings demonstrate that downhill driving over relatively steep slopes offers the most favorable conditions for effective plume steering and reduced dispersion. This confirms that bathymetric slopes are a critical factor in sediment plume behavior and should therefore be carefully considered in the planning and execution of deep-sea mining operations.