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Port Otago's plans to deepen its shipping channel by 2m to 15m is essentially a future-proofing project so the port remains viable for major shipping lines.
However, myriad environmental, recreational and commercial concerns may be raised because of the removal and disposal of up to 7.2 million cubic metres of a mix of sand and silt.
There will be effects on avian and aquatic wildlife to consider, the integrity of Aramoana's salt marsh, the dispersal of sediment in the harbour and its dumping 6.5km out to sea off Taiaroa Head.
The release this week of Port Otago's assessment of environmental effects of the "next generation" project, compiled over two years and running to more than 100 pages, plus maps and technical data, is yet to be considered by affected parties.
At present, Port Otago intends to apply for resource consent from the Otago Regional Council in April but will not progress with dredging until shipping lines, such as its largest customer Maersk, confirm they will bring larger ships on a regular basis.
Shipping lines have collectively lost billions of dollars during the global recession.
They have scaled back calls by investing in larger ships to call less often but carry more containers, prompting numerous ports to dredge their channels to at least 15m to accommodate ships' draughts of 14.5m.
Port Otago chief executive Geoff Plunket said in an interview that large-volume exporters believe "it is only a matter of time" before the bigger ships arrive on the horizon.
"Because of the lead-in time, we need to be prepared.
"That's why we need resource consent now," Mr Plunket said.
The 13km channel to Port Chalmers now accommodates Albatross class ships of 281m length and 12.5m draught carrying 4100 Teu (twenty foot container equivalents) on a weekly basis; with one 5000 Teu vessel having been handled recently.
The new generation of ships already plying the world's waterways have a draught of 14.5m, are between 300m and 347m long, and carry respectively 6000 or 8000 Teu.
Larger ships are planned, but their sheer size means they cannot navigate Otago harbour; regardless of dredging beyond 15m.
"We can't be sure of the timing.
"Shipping lines will have to come to us and say they will be bringing 6000 Teu to us on a regular basis before we start - which would be about a year out [before their arrival]," Mr Plunket said.
Mr Plunket is confident of sourcing funding from existing banking facilities, which would include purchasing a fourth Shanghai-made crane and larger tug, to cope with bigger ships.
A proposed 135m wharf extension, on the right hand side of Boiler Point, was necessary to cope with the mix of cruise ships and container ships requiring space, as the wharf had a bend in it which compromised space availability.
Port Otago has been saying for several years it does not intend reclaiming any more land and the wharf extension would be pile-driven, taking up to 12 months to build.
Combined with its locality within the port's heart and with plans to include a 30m fishermen's platform, it would appear unlikely to attract much opposition.
In the channel, however, the proposal is for Port Otago to employ a contractor to shift most of spoil, most of which would be deposited 6.5km out to sea northeast of Taiaroa Head.
Mr Plunket said the sediment drift to the north was expected to cover 12km to 15km of an existing sand bank in 27m of water, and not interfere with Blueskin Bay or fishing grounds closer to Karitane.
Aside from the 2m deepening, the 13km channel would be widened in two places to assist the handling of longer ships, one area being opposite Harington Point which would be widened toward the salt marsh from 205m wide to 250m.
Port Otago concludes this will have little effect on the salt marsh.
Over at Harington Point, Port Otago propose to build an 80m rip-rap (loose boulder) groyne to the north of mainly cribs at erosion-prone Harington Point then pump 45,0000cu m of spoil to create a 200m beach.
When asked, Mr Plunket agreed there were "issues with vessel wash" at Harington, where erosion has eaten into the banks of crib-owners sections, but said there was a "balance" between "nature taking its course" and the possible contribution of ship-wash eroding the banks.
"This is a relatively simple solution to see some progress [on the erosion] issue for the community," he said.
He said the Harington project, which was separate from the channel-widening project, was designed to stabilise the shoreline, but he wanted all its residents to agree on the design before separate resource consent was applied for - the implication being it does not want it contested but rather just get on with the job of restoration unimpeded.
Mr Plunket acknowledged anyone, not only residents or crib-owners, had the right to challenge any application, but he hoped putting all the information into the public realm would clarify Port Otago's intentions.
Mr Plunket has concluded there will be only minor effects associated with channel dredging, and referred to the worst effect of the last major dredging operation in 1976, which identified sediment suspended in the saltwater, but otherwise naturally dispersing over time.
He said it was too early to speculate on whether the issue could go to the Environement Court, noting public submissions would be held and that any outcome could be appealed by Port Otago or opposers in the court.
However, the size, scope and number of potentially affected parties, from recreational to communities and commercial users, in the larger channel deepening project. would appear likely to attract close scrutiny and some opposition.
• The assessment report is at www.portotago.co.nz