An incoming tsunami would force evacuating residents in the Napier suburb of Westshore to make a tricky choice.

They could try to cross one of two potentially choked bridges to get to the guaranteed safety of Bluff Hill.

Or they could head inland, a potentially long trek with no guarantee of safety for kilometres around.

Westshore residents would face a tough choice when the tsunami warning sirens go off. Photo / Aaron Bryan
Westshore residents would face a tough choice when the tsunami warning sirens go off. Photo / Aaron Bryan

Two workshops - Quicker, Safer Tsunami Evacuations - at the Napier Sailing Club and McLean Park on Tuesday, showed the bleak reality facing those evacuating the suburb in a worst case tsunami scenario.

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Residents living in Westshore as well as Ahuriri, Pandora, Napier South, Maraenui, Te Awa and Marewa were invited to contribute their local knowledge to help GNS improve tsunami evacuation modelling for their area.

GNS scientist William Power told attendees they needed the public's help to create accurate evacuation models during a workshop in Napier on Tuesday. Photo / Paul Taylor
GNS scientist William Power told attendees they needed the public's help to create accurate evacuation models during a workshop in Napier on Tuesday. Photo / Paul Taylor

The packed workshop on Tuesday morning provided crucial interaction between scientists and those who live in the area, to help identify and problem-solve problems like potential congestion on the Pandora Rd and Hawke's Bay Expressway bridges.

Members of the Hawke's Bay Civil Defence also attended to share their knowledge and answer questions from members of the public.

GNS Scientist William Power said one of the key factors they were interested in is what Westshore residents would do should they need to evacuate.

"Do you go to Bluff Hill because you'll have to cross the bridge or do you go in the other direction?

"That's something we really want to have more discussions about in these workshops.

"It's also similar for the suburb of Onekawa, it's on the border line, do you go to Bluff Hill or Taradale? Our models show Bluff Hill, but it's only an assumption."

Hawkes' Bay Civil Defence team leader of hazard reduction Lisa Pearse said the Pandora Rd bridge had long been discussed as a choke point.

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"Back in 2001 there was a report done - a Hawke's Bay Engineering Lifelines report - and that looked at all the different hazards and layered everything up.

The Pandora Rd bridge also has telecommunications, power and water supplies.

"Because of that the NZTA have done a lot of strengthening on that bridge and it should be strong enough to withstand a large earthquake.

"The main concern is the bridge abutments," Pearse said.

"The bridge might be fine, but leading up to it might get a bit busted up and this is why we're telling people not to rely on vehicles, take a bicycle or walk."

Simulations during the workshop showed people walking to the safety point, which was Bluff Hill, and also estimated points where congestion might occur during the evacuation.

"Going by car is obviously problematic when you've got a big town because very quickly you'll end up with a big traffic jam," Power said.

"Obviously by going car would make sense in other parts of town depending on where you are."

A packed workshop on Tuesday morning provided crucial interaction between scientists and those who live in the area. Photo / Paul Taylor
A packed workshop on Tuesday morning provided crucial interaction between scientists and those who live in the area. Photo / Paul Taylor

Pearse said although it may take a tsunami 30-40 minutes to reach the shore, residents should ideally evacuate their homes within 15 minutes.

"People really need to sit down with their families and make a plan because if anything goes wrong or technology fails you've still got that plan in place."

Project Leader for East Coast LAB, Kate Boersen said they had positive feedback from the workshops and it would take some months for them to modernise Napier's evacuation models based on the information given by residents.

Power said the most likely cause of a Hawke's Bay's tsunami would be a major earthquake off the coast.

"If an earthquake occurred on the plate boundary between Australia's plate and the Pacific plate which lies out to the east of Hawke's Bay, we'd it expect a tsunami to reach Napier within 30-40 minutes from that situation.

"We never know where exactly the quake is going to originate, so it's best to have a plan in place and get out as quickly as you can."

"If we have an earthquake it raises or lowers, particularly where it's been raised it lifts the water up above it and then it starts trying to go back to its normal position and that starts the waves spreading out. Kaikoura is a prime example," Power said.

Power said underwater landslides were also an issue and in the geology north of Gisborne, large under-sea landslides had occurred in the past.

Practise your tsunami hīkoi

Scientists are encouraging Napier's communities to practise their tsunami hīkoi and record their routes on a free mobile app.

Residents can download "Open GPX Tracker" for Apple devices and "Open GPS tracker" for Android devices. Once they've completed their hikoi to a safe location they're then encouraged to email their results to quickersaferevac@gns.cri.nz.

The results will be added to the computer-based tsunami evacuation models of Westshore, Ahuriri, Pandora, Napier South, Maraenui, Te Awa and Marewa, which were developed by scientists from GNS Science to simulate the movement of people who have self-evacuated on foot, following a long or strong earthquake that causes a large tsunami.

Power said the additional data would be used to increase the models' realism.

"This information is really valuable as it means we can provide more realistic models that Civil Defence Emergency Management can later use for planning purposes.

Tsunami hīkoi week runs from March 11 to 17.

Boersen said it was good way for families to identify potential obstacles or hazards along the way.

"Walking the route you would take in an actual event is one of the best ways to prepare for a tsunami."