standardization of environmental data and information - International ...

Modelling and testing
Wiedicke described calculations of mining impacts that might result
from the use of a self-propelled mining and collector system about three
metres high and six metres wide, moving at the speed of one metre per
second and employing water jets to dislodge manganese nodules from the
sediment surface. Using data gathered in the Peru Basin concerning the
thickness of the top layer and information from other groups that not all of
the fine sediment would be redistributed and resuspended, one group in
Berlin 22 had calculated that a maximum of only 20 percent of the sediment
would be resuspended; most would quickly be remoulded into chunks of
sediment deposited behind the miner. This was regarded as a first
approach that demonstrated the great need for additional parameters
before the numbers could be taken seriously.
Another group had calculated near-bottom sediment transport
assuming sediment discharge of 10 kilograms/second, between 1 and 6
days duration, and bottom-current velocity less than 10 cm/s. They had
come up with a plume-residence time in the order of 1.5 to 6 days, a
maximum distance of 15 km from the track before background
concentration was reached and sediment coverage of about 0.5 mm in the
1-2 km zone alongside the track. For continuous discharge, they had
modelled sediment coverage as thick as 30 mm close to the track of the
miner. They had not calculated flocculation effects. According to this
model, the plume would have a predominantly local impact. These
calculations concerned the bottom plume produced by the miner, not the
tailings from above.
Still another group had looked at the long-term propagation of
tailings discharged from a processing ship at the surface. They had used a
well-established geostrophic ocean model that was fairly evolved for this
purpose, although it did not take account of tidal movement. A bottomcurrent
velocity of between three and five centimetres per second was
assumed. The model had utilised two different grain-size distribution
patterns: one as defined in laboratory tests and the other resulting from
practical experiments in which the material was not completely dispersed in
the water column. The differences in result were striking. Under the worst
case scenario, resettlement of 90% percent of the tailings would take three
to ten years, depending on release depth: three years at 3000 m below the
surface and ten years at 500 m below. Resettlement of 95% would take 5
to 14 years. Transport under El Niño conditions would be predominantly
eastward, with deposition on the coast; under La Niña conditions, the
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