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Water turbidity model shows encouraging results

12 November 2012

Chatham Rock Phosphate water turbidity model shows encouraging results

Highly sophisticated computer models of the turbidity from material disturbed during extraction of rock phosphate nodules by Chatham Rock Phosphate have shown encouraging results.

The modelling work is being undertaken by Dutch applied research organisation Deltares using complex modelling techniques developed at their Delft headquarters. Deltares was asked to look at the dispersion behaviour of sediments released during the proposed extraction process.

The modelling results will now be independently evaluated.

CRP is planning to take rock phosphate nodules from the seabed on the Chatham Rise using a trailing suction dredge. This will involve jetting to loosen the sediment, sucking the material up a pipe to the ship, separating the nodules and re-depositing the remaining material onto the seafloor.

The Deltares models are designed to predict the turbidity effects and sedimentation patterns of the material returned to the seafloor, taking into account seasons, currents, temperatures, salinity, velocity, location, method of deposition, etc. The models are based on the proposed mining plan that involves extraction in identified areas for four days out of 10 to 12, with the rest of the time dedicated to transit to and from port and for unloading.

The model parameters are derived from analyses of sand and silt samples recovered during research voyages earlier this year. The modelling is being carried out to identify potential impacts on water column or seabed marine life and to help design a mining system that minimises these effects.

The initial findings predict that most of the sediment (10 – 20 cm per cycle) is returned to the seabed parallel to the mining track, near its point of origin. The thickness of sediments deposited outside the mining area is in the order of a few mm per cycle, and small amounts are transported through the water.

The modelling predicts “high sediment concentrations are only observed during the mining activities. During the transitional period (in between mining cycles) sediment concentrations reduce rapidly.

“Sediment concentrations outside the mining area are generally low (less than 1 milligram per litre). Due to direct placement on the seabed, the average vertical dispersion is limited; sediment concentrations decay rapidly in the lower 10m of the water column.

“Once mining activities are interrupted, maximum sediment concentration values reduce below 0.02 milligrams per litre for the clay fraction (within 72 hours) and below 0.01 milligrams per litre for the silt fraction (within 24 hours).”

CRP chief executive Chris Castle said these initial results, which are now to be subjected to further independent review, were encouraging.

“The environmental effects have been a key focus in the design of the mining system because we want to minimise the amount of material dispersed in the water column and across the seafloor.

“With the current mining plan we expect that about 30 square kilometres will be disturbed each year. Over the entire project life of 15 years that’s just 0.1 per cent of the Chatham Rise.

“We’re encouraged by the potentially low environmental impacts predicted by the models. When added to the low cadmium of the product, its low carbon footprint (compared to imported alternatives) and low run-off features when directly applied to pastures, this potentially provides another significant environmental plus.”

Mr Castle said more modelling is being done and will be evaluated, and the results will form part of the information being compiled for the marine consent application to the Environmental Protection Authority next year.


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