Kiwi safety breakthrough
Navigation of the notoriously dangerous entrance to Tory Channel/Kura Te Au at the top of New Zealand’s South Island is set to be significantly improved.
The Marlborough District Council alongside technical services providers Oceanum, Calypso Science and iXblue have devised a system to measure water level gradients within the channel that can determine tidal and non-tidal flows at its entrance.
The project follows a council-commissioned navigational safety assessment of what is described as the main trunk line for ferries travelling between the North and South Islands that identified 67 risks including grounding, collision, limited tug availability and the need for better aids to navigation.
Clues within the channel
Acknowledging the difficulties to accurately measure currents at the channel entrance, the project team noted that currents were entirely governed by water level gradients within the channel itself. Hypothesising that those gradients could be used to infer tidal and non-tidal currents, it successfully completed a desktop test of the theory on six months of 2017 data and then evolved to deploy a live trial system.
“The strong currents and turbulent flows that characterise the entrance to Tory Channel are a well-known challenge to shipping,” says Captain Luke Grogan, manager of the council’s nautical and coastal team and former Marlborough harbourmaster.
Tory Channel/Kura Te Au is the main trunk line for ferries travelling between the North and South Islands and other large vessels calling at Picton (Credit: Marlborough District Council)
“We know that published current and tide predictions do not always align with what is experienced. This means that mariners cannot be certain of the conditions when they arrive. It’s a complex and challenging waterway.”
As the most direct sea route from Queen Charlotte Sound/Tōtaranui to Cook Strait, the waterway is in continuous daily use by ferries and a number of other large vessels, such as cruise liners and log ships. The entrance to the narrow and winding channel – in which prevalent tidal and current conditions have been measured to generate water speeds of up to seven knots – has long posed a serious navigational threat.
A history of groundings
Over the past 20 years alone the entrance – which is also affected by high winds and within which vessels are required to make an almost 90-degree turn – has been the site of at least five serious groundings or near-grounding incidents. A Transport Accident Investigation Commission report into the most recent of which, the grounding of the cruise ship Azamara Quest in January 2016, pointed to navigators having a low level of understanding of the waterway’s tidal conditions.
“Without exception, in all cases, currents or more specifically the complexity of currents, were a major factor in these incidents,” says Captain Grogan.
“If we can create a system of real-time monitoring of currents at the entrance this would be an important risk mitigation measure. It will give us an evidence-based approach to navigational safety.”
Using real-time tidal and gradient observations via new tide stations at two sites in Tory Channel - Te Weka Bay and Okukari Bay - and one site at Motuara Island in Queen Charlotte Sound, this data is integrated with a hydrodynamic model simulation of the Sounds to “nowcast” and “forecast” currents.
One of the new measuring stations in Tory Channel delivering real-time tidal and gradient data (Credit: Marlborough District Council)
Having sought guidance from Land Information New Zealand, the project team has launched a validation process for the developmental solution in co-operation with the two local ferry operators, Interislander and StraitNZ, that entails technology being installed onboard the ferries to record the actual conditions experienced by the vessels, which is then being compared to the system’s predictions.
“This collaboration will ensure we can confirm sufficient confidence in the data across our key stakeholders and help us chart a course for the data to be made more widely available to inform safe navigation.”
A timely advancement
Describing the work as a significant advancement towards understanding tides and currents in the ‘Sounds, Captain Grogan also notes that the solution provides an “innovative approach to unravelling these influences for any coastal region”.
Captain Grogan adds that the work has only been possible in the ‘Sounds due to the council’s investment in a 2016 multi-beam survey of Queen Charlotte Sound.
“We were able to draw on the information about tides gathered over a three-month period during this research.”
Captain Grogan says the solution is particularly timely as the Interislander, one of two current passenger and cargo ferry operators on Cook Strait/Rau Kakakawa, is planning to introduce vessels about 40 metres longer and five metres wider than any of its current stock by 2024.
Iain MacIntyre is a freelance journalist