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Scientists will try to get to Mt Ruapehu's Crater Lake today after bad weather hampered efforts to determine the exact cause of yesterday's lahar.
The 7m-high soft rock and ash dam, built up from volcanic eruptions on Ruapehu in 1995 and 1996, finally gave way yesterday morning, sending a torrent of water surging down the mountain in waves up to 4m high.
The escaping water gouged out rocks and earth, all hurtling down the mountain in the mudflow known as a lahar.
But it could have been worse. The Department of Conservation believes the dam collapsed slowly over a period of 45 minutes, releasing the pent-up water as a series of pulses rather than as a large volume and thus minimising the amount of destruction.
The lahar flooded the area around the Tangiwai memorial and water topped Strachan's Bridge, a small structure giving access to a farm south of Tangiwai. A footbridge on the mountain track that crosses the Whangaehu River was badly damaged.
An estimated 1.2 million cubic metres of water would have been released yesterday if the temporary dam had collapsed entirely.
"Until someone can actually get up there and say 'yes, we're actually right down to the crater rim', it's pretty hard to say how much water came down," said DoC spokesman Dave Wakelin.
"I guess that's what everybody's waiting to see, just what is actually left. And until they can do that, everything will be supposition."
Before yesterday, lake levels had been just 30cm from prompting an upgrade in warning levels, raising a 5 to 10 per cent chance of a lahar to a 50 to 60 per cent chance. Rain throughout the week might also have contributed, said Mr Wakelin.
"The downstream side of the dam was clearly eroding, and that erosion was first detected at the end of December ... I would say that the heavy rain we experienced probably accelerated the final amount of erosion that was needed in order for what was still there to collapse."
MetService forecaster Bob Lake said 149mm of rain had fallen in the area in the past week, with 40mm falling in four hours yesterday morning.
"It's nothing really exceptional in terms of adding to the Crater Lake volume, but it was probably teetering on the brink anyway."
GNS Science spokesman Brad Scott said the dam's slow collapse was always more likely than a fast, explosive rupture. "It's quite a big structure and you can't expect something like that to be necessarily instantaneous." Another lahar was unlikely, he said.
Mr Wakelin said that by 5pm yesterday, some semblance of normality had returned.
"Everything here shows that we're really recording no other lahar activity, other than maybe just a trickle of water coming down."
The early-warning system had worked well, the bund designed to stop lahar floodwaters going off course had not been damaged, and gates and lights on highways had worked well.
It showed that the decision to let the lahar occur naturally was the correct move, he said.
The Ruapehu District Council, which co-ordinated the roles of the various agencies, was pleased with how its emergency plan worked out.
"Everything went brilliantly from our perspective," said council spokesman Paul Wheatcroft. "It went like clockwork. We're very happy."
Horizons Regional Council emergency manager Shane Bayley said the council did not expect much damage after the lahar, as there were "no significant river schemes" such as stopbanks and pumping stations on the Whangaehu.
Most of the riverbank was privately owned or Defence Force land.
Mr Bayley said the lahar was expected to reach the State Highway 3 bridge at Whangaehu village between 5am and 8am today.
There were fears that the village, at the mouth of the river, would be flooded for the second time in as many years but this was unlikely.
The lahar should be "a very small event by the time it reaches the coast", Mr Bayley said.
Massey University professor of earth sciences Vincent Neall was part of a team taking samples of the lahar before, during and after yesterday's rupture.
He was at Collier's Bridge when it struck. "It's awesome to see it go from the normal river flow to this."
- Additional reporting Simon O'Rourke
