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An inspection of Crater Lake this morning has confirmed that one part of the seven metre high soft rock and ash dam has collapsed right down to the original hard rock rim, virtually eliminating the chances of a further lahar.
Department of Conservation scientists flew up Mt Ruapehu
early this morning to inspect the flashpoint for yesterday's torrent of water, rocks and earth which surged down the mountain in a standing wave up to 4m high.
The temporary dam, built up from volcanic eruptions on Ruapehu in 1995 and 1996, had been weakening under the pressure of pent-up water.
Erosion had been detected in one part of the dam as early as December, and 40mm of rain in a span of four hours yesterday morning appeared to have contributed to the lahar.
DoC spokesman Dave Wakelin said staff were able to fly up early this morning to inspect the site. He said: "The dam did actually wash right down to the old hard rock rim."
Gushing
Yesterday, as Margaret Wickens stood on a hill watching more than a million cubic metres of water gushing down the Whangaehu Valley, she could see boulders the size of small cars being smashed against rocks.
Trees and logs were being thrown about in waves up to 4m high and the Whangaehu River was transformed into a sea of mud and debris.
As a spectacle, the Ruapehu lahar did not disappoint.
"It was really quite unbelievable to be standing on top of this hill and watching it all unfold like a movie," said the Tangiwai resident.
"It was amazing. The river went from a dwindle to three, four, five metres in a heartbeat. It was like boom, down it came and all you could hear was the volume going up of the water coming and the trees and the boulders."
Mrs Wickens and her husband Scotty were in Ohakune at midday when a fireman told them Mt Ruapehu's Crater Lake had breached.
The couple returned to their home on State Highway 49, near Tangiwai, to check stock before going up a hill on a neighbour's property to watch the lahar as it worked its way down the Whangaehu River.
"We were there only 10 minutes when we got a phone call from the next house up to tell us that it was six minutes away, and we pretty much turned around and saw it coming.
"This mass of logs and boulders came around the corner. It was, like, 'Oh my God, look at it now'.
"You could hear the boulders hitting rocks as they came down. It was echoing through the bottom of the river. The water is usually a sulphury ugly-looking grey, and it just turned into a sea of mud.
"To see it all unfolding, it was incredible. I don't think I'll see that again in my lifetime. It was amazing."
The Wickens live near Strachan's Bridge, which is the only access across the river for a handful of residents, including Hamish Blackburn and his family.
Mr Blackburn was alerted to the lahar by several sources, including a pager given to residents so they would have enough time to escape if needed.
As emergency services closed State Highway 1 and State Highway 49 and Overlander trains carrying 200 passengers were stopped at Ohakune and Waiouru, the Blackburns moved stock from the river's edge and waited for the lahar to hit. That happened just after 2pm.
"We were waiting and the river looked normal and all of a sudden she rose quite quickly," said Mr Blackburn. "It would have kept flowing violently for about an hour and a half. At its peak, the water was flowing over the bridge."
Strachan's Bridge survived the torrent, but not unscathed, and the family are housebound until it is repaired.
"All the rails have been smashed off the side and there's logs and silt on it. The approach to the bridge took a bit of a hiding but it's nothing a digger won't fix."
Other minor damage has been reported, including the loss of a small footbridge and a toilet block knocked off its foundations at the Tangiwai Memorial, but the lahar appears to have caused no injury or major destruction.
The main highways opened again at about 4pm and Ontrack opened the Tangiwai Bridge at 4.30pm, although with a speed restriction, after inspectors found no signs of damage.
Taihape residents Jan and Alan Thomas waited until the roads reopened before driving up to see the aftermath of the lahar.
"It's a once-in-a-lifetime thing, isn't it?" said Mrs Thomas.
Mr Thomas said the roar of the water was phenomenal, and must have been deafening at its height. He was surprised at how quickly the water receded.
"It's amazing it's gone so quickly. One minute it was there, and two to three hours later it's almost disappeared."
Conservation Minister Chris Carter last night praised all the agencies involved in managing the lahar, saying he was delighted with the way things went.
The lahar travelled down the path as predicted, and the early warning response system worked as planned.
"I am pleased that we now have a robust system to manage this kind of natural event," Mr Carter said.
"To do so with a great degree of advance warning, with the least amount of damage to people and property, is the ideal situation."
Department of Conservation staff hope to be able to get up in helicopters today to see what, if anything, is left of the crater dam, which was holding about 1.3 million cubic metres of water when it breached.
- Additional reporting Simon O'Rourke
