Science Centres: Coasts

In the wake of the ferries

 

The sort of wake which can be a concern in Tory Channel. (Photo: Warren Thompson, NIWA)

NIWA-designed ‘Dobie’ data logger.

NIWA has recently completed work to measure the wake waves of the vessels of one Cook Strait ferry operator.

The safety and environmental impacts of wake waves of large commercial vessels are a concern in a number of places around the NZ coast, none more so than Tory Channel, the route of the Cook Strait ferries. Here, vessels operating at a speed of over 15 knots must obtain resource consent, which includes tests to demonstrate that the wakes generated are within the limits prescribed by the Marlborough Sounds Resource Management Plan.

Water depth and wave measurements were made using pressure sensors, incorporated in the NIWA-designed ‘Dobie’ data loggers. Measurements were made at two sites over several days, with the vessels travelling at a range of speeds. In the case of the ferry operator concerned, there was good compliance with the limits of the Resource Management Plan.

Scientist Murray Smith says: "This is a sensitive area, and it is important to have objective measurements that are seen to comply with legal limits. Unfortunately it is difficult for ship operators to predict wake size before bringing the ships here, because the wake wave height depends on a complex interaction with local bathymetry and currents."