Role of lee wave turbulence in the dispersion of sediment plumes

Sediment plumes threatening benthic ecosystems are one of the potential major environmental hazards associated with seafloor interventions such as bottom trawling, cabling and marine mining operations. This study focuses on sediment plume release from deep-sea mining activities, emphasizing its interaction with turbulent ocean currents in regions characterized by complex seafloor topography. In such environments, turbulent lee waves may significantly enhance the scattering of released sediments, pointing to the clear need for appropriate impact assessment frameworks. Using a seamount in the Lofoten Basin of the Norwegian Sea as a natural bathymetry case study, we present a numerical analysis that explores the interplay between lee wave turbulence and sediment plume dispersion. We show that the turbulence occurs within semi-horizontal channels that emerge beyond the topographic highs and extend into sheet-like tails close to the seafloor. In scenarios simulating sediment release from various sites on the seamount, our model predicts distinct behavior patterns for different particle sizes. Particles with larger settling velocities tend to deposit onto the seafloor within 50-200 m of release sites. Conversely, particles with lower settling velocities are more susceptible to turbulent transport, potentially traveling greater distances while experiencing faster dilution. Based on our scenarios, we estimate that the plume concentration may dilute below 1 ppm at about 2 km distance from the release site. Although our analysis shows that mixing with ambient seawater results in rapid dilution to low concentrations, it appears crucial to account for the effects of topographic lee waves turbulence in impact assessments related to man-made sediment plumes. Our high-resolution numerical simulations enable the identification of sediment particle size groups most likely affected by turbulence, providing valuable insights for developing targeted mitigation strategies.