Warning of tsunami risk from rockslide

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A senior engineering geologist with more than 40 years’ experience has warned that a rockslide on the shoreline of Lake Wanaka could generate a tsunami.

At a 2016 international landslip conference in Italy, Geosolve senior engineering geologist Graeme Halliday presented a paper which found that ongoing slow deformation on steep rock slopes around the lake makes them susceptible to rapid rockslides.

These can be triggered by earthquakes or extreme rainfalls.

There was a high probability of a major M8 earthquake on the “strained-up”Alpine Fault, with a 30% chance of a significant earthquake occurring within the next 50 years, which was likely to trigger many landslips in the Southern Alps, including Lake Wanaka, he said.

The National Institute of Water and Atmospheric Research was conducting studies into Lake Wakatipu, but Mr Halliday said it was important to look into Lake Wanaka as well.

“We really need more detail – both underwater work and detailed geological work along the shoreline of the lake to predict what might occur.

“It’s pretty much up in the air at the moment.”

A British student was conducting a PhD study into the tsunami risk, he said, which would involve surveying the lake bed to better understand past landslips that had impacted the lake.

Knowing the size and location of past rockslides would help plan for future events, he said.

“Once the study is done by the university in England then we should have a much better feel for the level of risk.”

But a lack of data in the meantime made it impossible to predict the size of the wave, he said.

“As the wave travels away from the landslide impact point the height will decrease . . . but a problem with Wanaka is the semi-circular shape of the bay and the gradual shallowing of the water.”

These factors could amplify arriving tsunami waves, he said.

“The configuration of the lakefront at Wanaka’s not good in that respect.”

During peak season, Wanaka’s population can swell to 40,000 people and the lakefront is well-known for attracting large crowds.

It was important for the public to be aware of the risk and know how to stay safe in an emergency, he said.

In the case of an earthquake, timing was key.

“A problem with Lake Wanaka is the arrival time of a tsunami wave after an earthquake would likely be very short – only a matter of minutes.

“If you are near the shoreline and feel a strong or prolonged earthquake, you need to get away very quickly and move uphill or inland.

“It’s exactly the same tsunami advisory Civil Defence give for the sea coast of New Zealand.

“You’ve got to get away quickly – you can’t hang around.”