Mount sports fields safe haven from killer tsunami

One of the Tauranga City Council tsunami evacuation zone maps, showing the risk areas in the region. Images/supplied

Blake Park has emerged as a surprise safe haven when Mount Maunganui people are fleeing a killer tsunami.

Most people living in the Mount would instinctively head for Mauao, but new tsunami evacuation maps showed that Blake Park and an adjoining swathe of central Mount Maunganui would be just as good.

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The maps published by the Tauranga City Council would be delivered to householders along the coastal strip next week. Emergency manager Paul Baunton said the maps would let people know the nearest safe location or zone and the best route to get there.

He said the maps were based on a 14 metre tsunami which was considered the maximum credible tsunami based on current understandings of earthquakes and plate tectonics. It would be generated by an earthquake somewhere along the Kermadec Trench north of East Cape.

"To produce a tsunami of this height, the earthquake would need to be greater than magnitude 9, would last a long time and be felt very strongly in Tauranga."

The seemingly incongruous selection of Blake Part as a destination was because the dune system was an excellent first line of defence against all tsunami. He said people assumed that the Mount was flat, but some of the roads sat quite a bit higher than the streets on either side.

"The tsunami will lose a lot of power as it encounters these dunes, even as it overtops them or travels around them."

Mr Baunton said a tsunami entering from the harbour side would be significantly lower than the ocean side because much of the energy and power was lost as it squeezed through the harbour entrance.

Only a rare tsunami had the potential to overtop the dunes along Tauranga's coastline, with most not coming anywhere near the worst-case scenario, he said.

"A tsunami from White Island is unlikely to overtop the dune systems because it simply won't be large enough."

Mr Baunton said the best evacuation method in Tauranga would be to walk or cycle rather than drive.

"There will be crippling traffic jams during an evacuation. Think about what happens to traffic after a concert or car accident, then magnify that in the aftermath of a big earthquake."

The tsunami survive checklist included advice that a tsunami could arrive 50 minutes after a major earthquake. "Don't wait for an official warning to evacuate."

In a formal Civil Defence evacuation, people could be asked to move from the orange zone into the yellow zone. Every tsunami affected the red zone along the beachfront.

A worst-case tsunami similar to the one that hit Japan in 2011 was predicted to flood the yellow zone. Mr Baunton said there was a very low chance that this would happen in a lifetime, but if it did there would be no time for official warnings.

It would take about 50 minutes for a major tsunami from the Kermadec Trench to reach the coast plus maybe another 30 minutes to flood the yellow zone.

"The further into the yellow zone you get, the shallower and slower moving the water will be," he said.

Coastal Tauranga tsunami evacuation maps
- Based on four in-depth studies from 2012-15
- Studies peer reviewed by technical experts for accuracy
- Update whenever new information comes to hand
- Most recent update 2016