Study models likely destruction from tsunami

A tsunami triggered by a massive South American earthquake would swamp Auckland motorways, coastal roads and low-lying bays, a new study has found.

The Niwa report for the Auckland Regional Council used computer modelling to predict the impact of the wave along vulnerable coastal areas.

A South American tsunami was chosen as the region's most likely risk, expected to reach our shores every 50 to 100 years.

The study looked at the worst-case scenario of a wave striking land at high tide.

Findings showed "significant inundation" to key roads including the Northwestern Motorway between Pt Chevalier and Te Atatu, Tamaki Drive at Hobson Bay, and the harbour bridge approaches. But few people would be forced to flee their homes.

"Our risk is relatively minimal with regards to devastating damage to developed areas," said regional council hazards manager Greg Holland.

"What we've seen is infrastructure, like the approaches to the harbour bridge and the Northwestern Motorway, gets into a little bit of trouble, so that's the areas we'd need to keep away from."

Currents would sweep through the harbour at up to 3m per second. The wave would appear low, but could have enough power to surge 2m to 3m beyond the high tide mark, Holland said.

"You wouldn't want to be down on the beach when the tsunami came in."

On the North Shore, flooding would be mostly confined to the coast, except streams at Long Bay, Winstones Cove and Torbay, and low-lying areas of Browns Bay and Mairangi Bay.

Westhaven Marina and parts of Freemans Bay, Mission Bay and Glendowie would be swamped. Waiheke Island would be inundated at Blackpool and Surfdale.

East of the city, parts of Cockle Bay, Beachlands Marina, Kellys Beach, Te Puru Stream and Maraetai Beach would be affected.

Experts haven't yet determined how many homes would flood, but "but if you live on the waterfront, you should be careful and get to higher ground," Holland said.

Auckland Civil Defence Emergency Management Group controller Harry O'Rourke said the modelling was a valuable tool. "We can now plan and move forward with options for public warning systems."

Work is under way to assess the impact of a tsunami from a closer source, such as the Tonga-Kermadec trench. Such a wave would arrive faster and be more devastating, but is far less likely to occur.

The most recent tsunami warning issued for New Zealand was caused by an earthquake near Vanuatu on October 8.

heather.mccracken@hos.co.nz