Expert witnesses quizzed over seabed mining effects
A sediment plume from a seabed mining operation proposed for South Taranaki would likely be visible in the water from the air, a decision-making committee heard yesterday.
How far the plume would spread and how it was likely to behave was the main topic discussed on the second day of the Environment Protection Authority’s independent decision-making committee (DMC) hearing at the TSB Hub in Hāwera.
The hearing, which is expected to last a total of eight days over three months, has already had to defer some scheduled speakers, because the DMC members have needed more time for questioning experts who were giving evidence on behalf of the applicant, Trans-tasman Resources (TTL).
They also asked the experts to provide further information.
The computer modelling used to determine how the sediment plume would behave has come under scrutiny from the panel, because it is relied upon by other expert witnesses in making predictions for how the proposed operation would affect marine mammals and sea birds.
Dr Helen Macdonald presented research that showed the sediment plume from the mining activity would not be very noticeable closer to the shore, where there was already generally more sediment in the water.
Closer to the mining site, the plume would be more noticeable because there was less sediment already in the water, but she described the effect as moderate.
Dr Michael Dearnaley, director of science at HR Wallingford in the United Kingdom, spoke via audiovisual link.
One of the five-member decision-making panel, Dr Andrea Byrom, asked him what a passenger on a flight to New Plymouth would see if the seabed mining project got the go-ahead.
Dearnaley said the effect of the plume would be more visible in water which normally had low levels of background sediment, as the proposed mining site would have.
“You might not see this as a streak that’s emanating from a point of obvious activity, you might see it appearing as a broad discolouration of the water column 500m away from the activity that’s on the seabed.
“The human eye is very good at detecting small amounts of particulate change in water, it is something easily detected, particularly when the water is clear.”
Dearnaley said the plume modelling used was robust and followed international best practice.
“I am of the view that the results of the analysis and the modelling can be relied upon for the impact assessment.”
The mining activity would be undertaken by a seabed crawler, launched from the ship, attached by pipes to the vessel, Dearnaley told the panel.
“It will advance across seabed, swinging a cutter across the seabed, removing the resource, and pumps it back into the ship where it is processed, then the material that is not ‘the product’ is returned down a tail pipe and discharged about 4m above the seabed.”
The crawler would work in a mining lane of about 900m, he said.
“The action of the crawler, it’s like sucking at the bottom of a wall of the seabed up to 11m deep, about 5m in depth and on one pass across the width of the mining lane the crawler would extract material about a 24m width.”
As it started operating, it would excavate the beginning of the pit, and return the dredged material to the seabed from a pipe at the other end of the vessel, 300m away.
For the first 300m of the mining lane, the tailings would be deposited directly in to the seabed, creating a mound 4m or 5m high under the vessel.
“After the crawler has excavated the first part of the mining lane, the deposition from the vessel is going into the pit the crawler has created,” he said.
“When the crawler gets to the end there will be a 300m-long pit remaining at the end of the lane that isn't refilled.”
About 90% of material excavated would be returned to the seabed, he said.
The hearing continues today.