Abstract
Deep-sea polymetallic nodule mining research activity has substantially increased in recent years, but the expected level of environmental impact is still being established. One environmental concern is the discharge of a sediment plume into the midwater column. We performed a dedicated field study using sediment from the Clarion Clipperton Fracture Zone. The plume was monitored and tracked using both established and novel instrumentation, including acoustic and turbulence measurements. Our field studies reveal that modeling can reliably predict the properties of a midwater plume in the vicinity of the discharge and that sediment aggregation effects are not significant. The plume model is used to drive a numerical simulation of a commercial-scale operation in the Clarion Clipperton Fracture Zone. Key takeaways are that the scale of impact of the plume is notably influenced by the values of environmentally acceptable threshold levels, the quantity of discharged sediment, and the turbulent diffusivity in the Clarion Clipperton Fracture Zone.
Highlights
Deep-sea polymetallic nodule mining research activity has substantially increased in recent years, but the expected level of environmental impact is still being established
The key non-dimensional parameters governing the nature of the dynamic plume are[10] (i) the Reynolds number (Re(z) = 2b(z)w(z)νÀ1), which characterizes turbulence intensity in t(hFerðzÞdy1⁄4nawmðzicÞÀgp0bluðzmÞÁeÀ, 0:5()ii)chathraectedriezninsigmtehtreicrelFatriovuediempnourtmanbceer of momentum to buoyancy forces, and (iii) the buoyancy number (Δ(z) = NðzÞ2wðzÞ2g0À2) characterizing the importance of the background stratification in influencing the plume vertical extent; here, w is the vertical velocity in the centerline of the plume at a given vertical coordinate z, b is the radius of the plume at a given z, ν is the kinematic viscosity of water (10−6 m2 s−1 in this article), g′ is the reduced gravity of the plume (g0 1⁄4 gðρp À ρbÞρÀb 1), g is the gravitational acceleration, ρp is the plume density, ρb is the density
The vessel was stationary and the dynamic plume in the vicinity of the vessel was monitored with a Phased Array Doppler Sonar (PADS) and a vertical profiler
Summary
Deep-sea polymetallic nodule mining research activity has substantially increased in recent years, but the expected level of environmental impact is still being established. The assumed sediment discharge rate was an order of magnitude larger than those estimated for a commercial-scale nodule mining operation[18], and the simulation did not use any dynamic plume model to determine the ambient plume initial conditions.
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