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NZ Should Learn From Tsunami Disaster

January 6, 2005

New Zealand should learn from Indian Ocean tsunami disaster

A New Zealand engineer and computational hydraulics specialist says New Zealand has for years been living “without seatbelts” in relation to damage caused by disastrous coastal hazards such as a tsunami.

Writing in New Zealand Local Government magazine published just two weeks before the Indian Ocean tsunami disaster, Dr Alastair Barnett said New Zealanders now generally accept that in the rare case of a car crash, their chances are better with the seat belt on, rather than off.

“Coastal hazards are like this, and we have been doing without the seat belts for years now,” he said. “While tsunami warning systems in place in New Zealand would likely avoid significant loss of life or injury, damage to buildings and infrastructure along our coastline could be significant.”
In his article, Dr Barnett wrote that the risk of flooding and damage from tsunamis along New Zealand’s coastline was partly recognised in 2001 when changes were made to this country’s building regulations. “Before then the building regulations gave no room for local councils to withhold building consents in flood-prone areas, except on the grounds of likely flooding "resulting from a storm". “However in 2001 Building Amendment Regulations were issued changing the words ‘a storm’ to ‘an event’.

This change was presumably a response to the Thames floods in particular, but has never been accompanied by any guidance such as an update to an Approved Document to suggest that storm surges and tsunamis must now be considered,” he wrote. “And so, many developers and councils have carried on as before. “Storm surges tend to be localised, and so are within the resources of our insurance industry, but is this true of tsunamis?” Dr Barnett wrote.

He said that on 23 May 1960 a giant earthquake off the coast of South America sent a tsunami across the Pacific. “Luckily the tide was close to low when the main tsunami waves passed through, greatly reducing the damage. Even so, the whole east coast of New Zealand was swept from Port Chalmers to Opua with wave heights of about 1 metre on the open coast and up to 3 metres in harbours.”

He said while the 1960 earthquake is one of the largest ever recorded, tsunamis from much smaller South American earthquakes also generated similar effects along our coast in 1868 and 1877, suggesting their energy was focused in our direction. “Again we were lucky - both happened near low tide in mainland New Zealand. The difference the tide makes can be illustrated by the havoc caused by the 1868 tsunami arriving at mid-tide in the Chatham Islands - waves raced up to 6km inland, sweeping away a village with several casualties resulting.”

In his New Zealand Local Government article, Dr Barnett said the probability on which much national disaster planning is based in this country is a “return period” of about 500 years - such as for an earthquake on the Alpine fault or the Wellington fault.

….2 ….2

“While even a one-in-500 year event sounds like something which can be ignored for another few years, try translating the probability into the chance of something (like a car accident) happening once in a human lifetime. If you go through the maths, you find: * a 1-in-100 year event has a lifetime 55% probability * a 1-in-500 year event has a lifetime 15% probability * a 1-in-3000 year event has a lifetime 3% probability.
We certainly take precautions against car accidents of these orders of probability!,” he wrote.

He said local councils can do something to remove the glaring discrepancy between the treatment of flood hazards and river flood hazards caused by tsunami waves racing up-stream from a coastal river mouth.

“Since the 1960 tsunami the Japanese have shown that ordinary stop-banks provide highly effective tsunami protection.”
He said a methodology is needed to design a suitable height for such stop-banks, but a fortuitous benefit to New Zealand of large hydro-electric power canals has been the work done to study movement of artificial tsunamis at almost full-scale.
“We have been able to follow 1 metre-high waves in 10 metre-deep water for 25 kilometres with laboratory precision, giving us the opportunity to refine our numerical modelling techniques. So, for computer design, New Zealand’s technology at least is well proven,” he wrote. Other methodologies have been developed relating to the energy of New Zealand earthquakes and the height of a resulting tsunami. “Starting from a lifetime 55% probability tsunami being about 1 metre high on the open coast, as indicated by our 160 years of dated tsunami observations, this gives a lifetime 15% probability tsunami being about 2 metres high, and a lifetime 3% probability tsunami being well over three metres high.”

Another factor that must be considered, he said, is that a sea level rise of 0.5 metres is predicted to occur within a lifetime.
“Together, this gives a rule of thumb that coastal land with less than 1.5 metres freeboard above high tide must be considered flood-prone,” he said. “If anything, this underestimates the hazard, as internal reflections can considerably raise wave heights inside harbours and estuaries, and there is no coastal dune system to provide natural protection.

“Local research such as the recent NIWA study in the Bay of Plenty also suggests the real tsunami hazard is somewhat greater, so no-one could complain that this rule of thumb is over-cautious.” Dr Barnett wrote: “We know how to stop tsunamis, (stop-banks) how high the tsunamis will be at the coast, and how they move inside harbours and estuaries.”

He said when councils meet to consider applications for coastal building consents, they should remember that they are required to consider an "event" of similar probability to a car accident. “Isn't it time to buckle up that seatbelt?,” he asked.


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