Tsunami defences needed in North

By Peter de Graaf
A coastal property boom and lax development rules are putting Northlanders' lives at risk from tsunamis, a visiting expert says.
Hamilton-based hydraulic engineer Alastair Barnett, who was due to give a free public lecture in Whangarei tonight, says people ignored the danger posed by tsunamis, despite the Boxing Day event that claimed almost 250,000 lives around the Indian Ocean.
A major tsunami hitting Northland was a one-in-500-years event, which meant people on the coast had a roughly 15 percent chance of encountering a giant wave in their lifetime. That was about the same chance as being involved in a serious car accident, Dr Barnett said.
And yet, tsunami protection was all but neglected in New Zealand. "It's still seen as a problem only where the last big one happened. Councils ought to be looking at their coastlines and taking sensible precautions," he said.
Councils enforced protection against river flooding, requiring stopbanks to withstand one-in-500-years floods, and buildings were designed to stay standing after a one-in-500-years earthquake - but tsunamis were treated differently. "It seems a bit inconsistent to ignore them. Why are tsunamis so different?"
Dr Barnett said coastal land should face the same kind of planning restrictions as flood-prone river plains and development needed tighter controls. Houses built along the coast - some of which were "practically on the high tide mark" - should be set back, or raised on timber piles or structural concrete walls.
Such houses would still be damaged in a tsunami, but the people inside would be protected.
Whangarei was "absolutely" at risk, as harbours could amplify a tsunami. Extra protection for the city and oil refinery was needed.
Dr Barnett said he had been involved in the development of Northport, which was designed with tsunamis in mind.
Northland's most recent tsunami was a three-metre wave that hit Tutukaka in 1960, generated by a magnitude 9.6 earthquake off the coast of Chile. That tsunami hit at low tide, and few people lived around Tutukaka then. "Our luck held that time, but we've allowed a lot of coastal development since then, and a lot of clever things like turning campgrounds into housing. You can evacuate campsites quite easily, and at most you lose a few tents. We're much more exposed to huge national damage now than in 1960 when the last big one came through," he said.
Tsunamis on the scale of the 1960 wave, or one in 1868, were caused by distant underwater quakes or eruptions. A nearby earthquake could trigger a tsunami 30m high - but such events were very rare, about once every 4000 years, and all but impossible to protect against.
James Goff, also speaking tonight, believes a 32-metre tsunami struck the Far North in the 15th century. Geological evidence of the giant wave at Henderson Bay was backed up by Maori folklore.
Graeme MacDonald, hazard management team leader for the Northland Regional Council, said an independent assessment had found Northland was "clearly at risk" from tsunamis.
The Civil Defence Ministry launched a national study into tsunami risk after the Boxing Day event, and the council was waiting for the findings before continuing its own research. In the meantime a warning plan was in place, which would be triggered by an alert from the Pacific tsunami monitoring centre in Hawaii.
A system of warning sirens, like one operating in parts of the Bay of Plenty, was on its own not the answer - public education was essential. "Otherwise it's a case of alarms going off and people wondering what they mean," Mr MacDonald said.
Dr Barnett will be joined by William Powers, from Wellington's Institute of Geological and Nuclear Sciences, and James Goff, of the National Institute of Water and Atmospheric Research, for tonight's lecture at 7.30pm in the Cafler Suite, Forum North. Topics will include the Boxing Day tsunami and the effects of a giant wave on Northland.