Foveaux's rogues demand vigilance

22:10, Mar 19 2012

Known as one of the most unpredictable stretches of water in the world, a rogue wave in Foveaux Strait claimed more lives this week.

In the past 15 years rogue waves have been cited as the cause of several boat capsizes in the south, including the Easy Rider on Wednesday night.

Ryal Bush man Barry Bethune knows better than most the power of rogue waves in the strait.

Mr Bethune survived four hours in the strait after his boat was capsized by a rogue wave in January. The freezing waters claimed the life of his son, Shaun Bethune, and best friend Lindsay Cullen. Two others were rescued from the water by Easy Rider skipper Rewai Karetai.

Mr Bethune said he had been looking the other way when the wave hit.

"All I felt was my boat just getting picked up and tipped over," he said. Another passenger on the boat had been looking out the window when the wave came, and told Mr Bethune all she had seen was a wall of water.

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The wave had been two or three times bigger than any other waves they had seen that day, he said.

"I didn't have any time to react and move away from the wave, to surf in front of it or anything like that."

He had been steering through the waves to make the trip more comfortable, because they did not come from just one direction, but had not expected the rogue wave, he said.

National Institute of Water and Atmospheric research scientist Richard Gorman said rogue waves were at least twice the size of the significant wave height at the time.

For example, if the significant wave height at the time were 4 metres, there could still be a mixture of wave heights of 6m to 7m, Mr Gorman said.

"If it is starting to get above 8m that is very unusual for the normal sea conditions, if it is over twice as high we define those as rogue waves," he said.

"The main problem seems to be they are particularly steep waves, just being a high wave doesn't necessarily cause a problem. If it is a long swell and not particularly steep (the boat) will ride over it, if it is steep it causes capsize."

Foveaux Strait generally has reasonably energetic wave conditions, and Niwa data showed westerly wave conditions were coming into the strait at the time of capsize, Mr Gorman said.

Mr Bethune said rogue waves were a lot more common than people realised.

"Talk to any fisherman out there, every one of them has got a story to tell about a rogue wave," he said.

There were some "bloody horror stories" among recreational fisherman about rogue waves, he said, that never made the press and people needed to be more aware of how common they were.

"People underestimate Foveaux Strait."

The Southland Times