This document summarizes research on the environmental impacts of particle sedimentation from deep sea mining. It finds that mining release plumes become arrested by ocean stratification, causing particles to spread radially and settle onto the sea floor. Particle settling speed and plume coherence determine whether sedimentation occurs as a convecting plume or separated particles. Typical fall speeds and mining volumes mean most particles settle within 1-100 km of the mining site.

1. Enviromental Impact of Particle Sedimentation
from Deep Sea Mining
Andy Woods / Nicola Mingotti
University of Cambridge
(Inistitute for Energy and Environmental Flows)
aww1@cam.ac.uk

2. Effect of Stratification : Release plume of particles and fluid into a tank of stratified fluid
Plume arrested by stratification – particles spread radially, then settle out to floor of tank

3. Particles fall at settling speed – no convection below the intrusion
Waves of settling particles
andy@bpi.cam.ac.uk

4. In cross flow – current swept
downwind and moves with the flow
Plume behaves as a line thermal

6. Slow settling speed
coherent plume
Fast settling speed
separation

7. Large
Small
Large
Small
Convecting Plume Model Sedimentation model
Dashed line Dashed line
Data Coloured lines Data coloured line

9. Ocean stratified – source water typically less dense than ambient
Dense mixture of fresh
water and particles
Buoyant mixture of fresh
water and particles

10. Particles separate from fluid – fluid reverses direction
and moves upwards

11. Interesting convective
Sedimentation
Particle clouds settle below
the plume with higher speed
than fall speed.
Coherent motion – particle
convection

12. Dispersal of the Particles
Typical fall speeds and volume fluxes : -
* Sedimentation speed 0.01-0.0001 m/s
* Current speed 0.01 m/s.
Near Source Plume flow : Far field sedimentation
à Max Lateral transport 1-100 km from ship
prior to settling on sea floor
Zone of impact may be 1-100 km around perimeter of mining site