LAHAR! volcano disaster in New Zealand -- but lots of warning, nobody hurt

TOP: Depiction of an eruption-generated lahar flowing from the summit of Mount Ruapehu in New Zealand. (The Australasian Journal of Disaster and Trauma Studies)

BOTTOM: Recent chart about the expected impending Ruapehu lahar in The Dominion Post newspaper, New Zealand.

A lahar is a huge mudflow or mudslide triggered by a volcanic eruption or by the unstable geology at the top of an active volcano. The damage and deaths are caused not by fire, lava or hot toxic gas, but by huge volumes of fast-descending mud. Lahars are one of the most common ways that volcanos kill people.

In places with an advanced scientific and civil-defense infrastructure, potential lahars can be predicted, and effective and automatic early warning systems can be established. Ruapehu went lahar in 1953, wiped out a railroad bridge, swept a passenger train into a lake, and killed 151 people.

This time the lahar killed nobody.

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Lahar sweeps down
New Zealand mountain

Sunday 18 March 2007 4:59 GMT (1st Lead)

Wellington, New Zealand -- A massive lahar, or volcanic mudflow, swept down New Zealand's 2,797-metre high Mount Ruapehu on Sunday after its steaming crater lake burst its banks releasing thousands of tonnes of rock-filled water.

It had long been expected and police and civil defence officials said alarms and safety system installed after a similar lahar 54 years ago which killed 151 people on a train when a rail bridge was swept away, had worked perfectly.

They said the lahar, confined by a new stopbank, had kept to its expected course down the mountain into the Whangaehu River valley and past the village of Tangiwai, near the rail bridge, without incident.

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.

Hundreds of motorists and train passengers were stranded but officials said the lahar had not reached the road, nobody was hurt and no settlements had been affected.

The lahar kept to its predicted path eventually moving out to the sea.

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

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.

The Department of Conservation had earlier predicted that a lahar would travel at about 21 kilometres an hour down the mountain and a spokesman described Sunday's event as 'moderate.'

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

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© 2007 dpa - Deutsche Presse-Agentur

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The Dominion Post (daily newspaper, New Zealand)
Tuesday 2 January 2007

Dam on the brink of bursting

by Emily Watt

Mt Ruapehu's crater lake is at a record high level, and the dam wall is beginning to erode.

Scientists say if the lake keeps rising as predicted, the dam could blow by March. They say it is a case of "not if, but when" for the lahar, which is expected to burst the dam and flow down the Whangaehu River on the eastern side of the mountain to Tangiwai and out to the coast.

Unlike the 1953 lahar that led to the Tangiwai rail disaster that killed 151 people, this one is unlikely to threaten residential areas but roads, bridges and rail lines could be affected. When a multimillion-dollar early warning system sounds, police will have 20 minutes to cordon off key roads, including State Highway 1. The system also triggers automatic road and rail barriers.

A 300-metre concrete barrier has been built to prevent the flow entering the Waikato Stream and Tongariro River; the road bridge on State Highway 49 has been raised and strengthened; and riverbanks have been cleared of pine trees to lessen the risk of their being wedged against the bridge.

Last Friday, Conservation Department staff measured the lake at a record high of 2.8 metres below the top of the dam, and there is evidence of the dam seeping at up to 10 litres a second. Department earth scientist Harry Keys said that marked the beginning of the dam's eroding. The lower the lake levels when this happens, the less water that escapes. "The sooner it happens the better," Dr Keys said. "This needs to be resolved. It's an issue hanging over the local community."

The lahar warning level remains at two, with a 1 to 2 per cent chance of an immediate lahar. There is also a wave hazard as chunks of the ice cliff fall into the lake, causing water to slosh on to the top of the dam. A large wave could spill over the dam and flow down the mountain. The lahar threat becomes more significant when the lake rises another 0.8 metres, which could be in two weeks, but more likely by February.

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