Lahar sweeps down New Zealand mountain (Roundup)

Mar 18, 2007, 8:07 GMT

Wellington - A massive lahar, or volcanic mudflow, swept thousands of tonnes of rock-filled water harmlessly down a New Zealand mountainside on Sunday, reviving memories of a similar event 54 years ago which killed 151 people when a bridge was swept away shortly before a train passed over it.

Alarms and safety systems installed on 2,797-metre high Mount Ruapehu following the disaster on Christmas Eve 1953 worked perfectly, allowing roads and the railway track to be closed immediately the volcano's steaming crater lake burst its banks.

'The lahar travelled down the path as predicted, and the early warning response system worked exactly as planned,' said Conservation Minister Chris Carter.

Civil Defence Minister Rick Barker said bad weather over the weekend had fortuitously kept hikers and climbers off the mountain, which is the North Island's highest peak.

Nobody was hurt and damage was limited, ironically, to a memorial to those who died in the 1953 tragedy, and a hikers' footbridge as the lahar, which uprooted trees and boulders in 3 to 4-metre high waves moving at more than 20 kilometres an hour, made its way down the mountain and out to the Tasman Sea.

The lahar had long been expected and officials were delighted that, confined by a new stopbank, it kept to its expected course down the mountain into the Whangaehu River valley and past the village of Tangiwai, near the rail bridge, without harming anybody.

Police, alerted by a series of automatic alarms monitoring the crater lake's temperature and level, closed all roads in the area, including the highway between the capital Wellington and the country's biggest city Auckland, and stopped trains on the main trunk line, stranding hundreds of motorists and rail passengers for several hours.

Scientists had been closely monitoring the 17-hectare crater lake, which sits about 250 metres below the summit of Mount Ruapehu, since January when seeping water threatened to sweep away the rim.

Weathermen said extremely heavy rain had fallen on the mountain for more than three hours which probably accounted for the rising lake level which breached the dam walls.

A Department of Conservation spokesman described the lahar as 'moderate.'

The National Crisis Centre in Wellington was activated and officials said the lahar emergency response plan had worked as expected.

It was the first real test of the safety measures and Conservation Minister Carter said, 'I am pleased that we now have a robust system in place to manage this kind of natural event - and to do so with a great degree of advance warning, with the least amount of damage to people and property is the ideal situation.'