— Ruapehu is one of the world's most active volcanoes. The deep crater lies between its peaks and fills with a lake between big eruptions
A dramatic lahar struck on New Zealand's Mount Ruapehu on Sunday, returning the crater lake to its pre-1995 level.
At 1047 local time, a natural dam of volcanic ash and sand began to crumble, sending an estimated 1.3 million cubic metres of water, mud and debris flooding down the side of the volcano. No people are reported to have been injured.
A helicopter inspection on Monday morning showed that the dam has been totally washed away. The lake is now draining into the Whangaehu River.
Scientists knew that a major lahar was imminent (see Volcanic crater lake primed to spill). The 500-metre wide lake had been filling since the eruptions in 1995 and 1996 that deposited the 7-metre-high dam around the crater. It is not clear yet whether heavy rain or volcanic activity was the final trigger for the lahar.
In 1953, a Mt Ruapehu lahar swept away a railway bridge, killing 151 people travelling across it.
Sunday's lahar, which took place in a series of bursts over a period of about 45 minutes, triggered new sensors on the rim of the crater lake. These sensors are linked to an automatic system giving warnings of up to 90 minutes or so. Roads and bridges downstream of the lahar's predicted path were closed, while trains were suspended. Farmers expected to be hit by the lahar were alerted and moved their stock.
The systems worked well, with the lahar behaving as New Zealand Department of Conservation (DoC) scientists had forecast, flooding down the southern side of the mountain, along the Whangaehu River and then out to sea. The surge reached an estimated four metres in height in places.
Damage was slight and seems to have been limited to a DoC footbridge on a track around the mountain, as well as a small farm bridge and road.
The DoC's controversial decision to implement a lahar early warning and management system, rather than to bulldoze a hole in the lake to drain it, was vindicated on Sunday, say DoC officials. This is one of the world's most active lahar regions, and the incident on Sunday showed the systems work well, says conservation minister Chris Carter.
Sunday's lahar will also be the best studied. In anticipation of the event, research groups from the UK, US and New Zealand set up a range of sensors, including vibration sensors, flood height measuring devices and lake level sensors.
"We will get the best scientific data on a lahar, which will have utility around the world," says DoC geologist Harry Keys. "We can model rivers and water flow but predicting lahar size, which is key for hazard management, is very difficult, so this data will be incredible for that."