The historically unpredictable sea currents in Tory Channel, the gateway for ferries travelling between the North and South Island, may have just become easier to understand.
Marlborough District Council nautical and coastal manager and former harbourmaster Luke Grogan. Photo: Supplied/Ricky Wilson/Stuff
A new project can forecast the currents at any location in the channel up to three hours before it happens, by measuring the gradient of the water in the Kura te Au/Tory Channel using three separate "tide stations".
The technology is being validated - by comparing data collected from ferries and the currents they experience when navigating Cook Strait, with data collected by the three tide stations.
Marlborough District Council nautical and coastal manager and former harbourmaster Luke Grogan, talked the council through the new technology at an environment committee meeting last month.
Grogan said the motivation for the project was to enable improved guidance of Tory Channel - a narrow and winding channel known for "grounding" ships.
"It really all comes back to the grounding of the Azamara Quest, the 181m cruise ship on its maiden voyage in January 2016," he said.
"That was the showstopper, in terms of making us really look and reflect on Tory Channel."
A report prepared by Grogan for the meeting, said the incident record of the channel for the last 20 years showed there had been at least five serious groundings or near grounding incidents at the entrance of Tory Channel.
For the Azamara Quest specifically, the subsequent Transport Accident Investigation Commission report pointed to navigators having a low level of understanding of tidal conditions in the channel, the report said.
And, while risk assessments of Tory Channel had been identified, the published current and tide predictions did not always align with what was actually happening.
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"This means that mariners cannot be certain of the conditions that will be experienced until they have entered the channel," the report said.
Grogan said the flows of the channel were "dominated" by the ingoing and outgoing tide, but there was also a strong non-tidal component too.
"The way the tides work in New Zealand, you can see a tidal bulge travelling around the coast of New Zealand every 12 hours, in an anticlockwise direction," he said.
But the Marlborough Sounds and Cook Strait were more complex than this, he said.
The tide stations had been placed at Te Weka Bay and Okukari Bay - and one site at Motuara Island in Queen Charlotte Sound.
One of the new tide stations in Tory Channel delivering real-time tidal and gradient observations. Photo: Supplied/Marlborough District Council
Speaking after the meeting, Grogan said they were able to use the tidal station method, because the council had invested in a 2016 multibeam survey of the Queen Charlotte Sound.
"So we have this highly detailed understanding of the sea bed," Grogan said.
"These tide stations are giving us the gradient, and then using the wonders of computers and scientific modelling, we were suddenly able to reveal exactly what the currents are doing at any point within the Queen Charlotte Sound and Tory Channel."
Councillor Jamie Arbuckle, a self-proclaimed "non-boatie", asked who would make the ultimate decision about coming into the channel, if this data was going to be made publicly available.
Grogan said ultimately this would be up to the master of the ship, but now they would have a whole new level of understanding, which could help inform people going forward.
The programme received $250,000 from KiwiRail, as part of an "enforceable undertaking" imposed on the ferry service, following an accident on the Kaiarahi Interislander ferry in April 2019.
Local Democracy Reporting is Public Interest Journalism funded through NZ On Air