Left: The 5.2m high flood gauge in Whangaehu valley upstream of Tangiwai and right: 25 minutes later
Scientists say the Mt Ruapehu lahar was larger than the 1953 debris flow that caused the Tangiwai disaster.
But last Sunday's lahar lacked the violent force of the December 24, 1953, surge that wiped out the rail bridge at Tangiwai and led to the train plunge that cost
151 lives.
That was because of the comparative lack of debris it picked up on its way down the Whangaehu Valley last Sunday, said Dr Vern Manville of GNS Science.
"Our initial interpretation is that the 95-96 eruption did a good job of scouring out loose material from the gorge, whereas prior to the 1953 event there had been 30 years of no lahars and there was a much bigger accumulation of debris in the channel."
The lahar travelled at different speeds throughout its descent, the fastest of which was 60km/h, reached between the Crater Lake and the Whangaehu alpine hut. Those points were 2.5km from each other, Dr Manville said.
Another 2km separated the alpine hut from the Tukino skifield, where other measuring instruments indicated the lahar was travelling about 30km/h.
Further down, the debris flow slowed to 20km/h.
"They're back-of-the-envelope calculations," Dr Manville said.
History indicated the flow would have been running at 15km/h by the time it reached Tangiwai, but it was too early to accurately say what the speed was last Sunday.
"There is the average speed and there are local speeds, depending on the depth and flow. The more water, the more diluted it is [which applied to Sunday's event] and the slower it will go," he said.
"A build-up of sediment means it is more concentrated and it will go faster with all the energy that builds. It puts all its energy in to rushing along rather than splashing around."
According to Department of Conservation instruments, the "trip wire" indicated the lahar began at 11.22am. It reached the Tangiwai road bridge, 39.4km away, at 1.30pm.
Researchers from GNS Science and Massey University have spent the week gathering data from the lahar site as part of a $1 million research programme.
Dr Manville said this would ensure maximum scientific value from the event.
The money had come from a combination of the Marsden Fund ($700,000, administered by the Royal Society of New Zealand), the Earthquake Commission and the Foundation for Research, Science and Technology.
A plethora of digital information was captured by instruments installed along the Whangaehu Valley in advance of the tephra dam's gradual demise, Dr Manville said.
It would take months to sift through and interpret the data, but researchers were excited because this time around 10 times the amount of information was available.
"In 1995 there was just one measuring swipe taken on the whole river. This [current] data will be very valuable as it will mean the lahar will be extremely well characterised."
This meant the country would be in a better position to predict and plan for lahars, as well as determining how they might behave.
"All our years of effort have paid off. We have collected a world-class dataset that will make this the best-studied lahar ever."
Teams from Massey University and GNS Science have been on the mountain all week surveying deposits and taking samples for analysis. They have been joined by scientific groups from Hawaii, Japan, France and Britain.
The material collected will be studied in combination with photographs and television and video footage of the event.
A laser scan survey of the dam-breach site is planned over the next few days.
Lahar numbers
* Maximum speed as it tore down Whangaehu Valley high on the mountain: 60km/h
* Time from its start to Tangiwai road bridge, 39.4km away: 128 minutes
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