CD and DOC action reduces Ruapehu lahar risk
25 November 2002
Civil Defence and DOC action reduces Ruapehu lahar risk
Action already undertaken by Civil Defence and the Department of Conservation have reduced the risk to life if Ruapehu’s Crater Lake produced a lahar (mudslide), a report released today shows.
‘The Ruapehu Lahar Residual Risk Assessment Report’, commissioned by the Ministry of Civil Defence and Emergency Management, showed the risk to life in the most extreme lahar event ranged from 1 per cent to a high of 10 per cent, following precautionary measures undertaken by the two departments.
Without such measures, the risk to life could be up to 40 percent, the report stated.
More than 60 lahars (mudflows) have been recorded at Mt Ruapehu since the 1860s. Another is expected after a makeshift dam around the rim of the Crater Lake, made up of up to 7.5 metres of debris and sandy ash deposited at the rim after the mountain erupted during 1995 and 1996, collapses.
Scientists predict the dam will fail sometime between late winter 2003 and the summer of 2005/06. Under the most extreme scenario, once water in the Lake reached the rim, the dam could give way and release approximately 1.5 million cubic metres of water and ash down the mountain.
“We’ve taken the approach that it’s best to plan for a worst-case scenario, however unlikely, because where human safety is concerned we’d rather take too many precautions than too few,” Mr Hawkins said.
“A lot of work has been put into being prepared for the lahar,” said Mr Carter.
“An embankment has been built at the base of the mountain to prevent a large lahar from spilling into the Lake Taupo catchment, and an early warning system, known as ERLAWS, is in place to provide as much advance notification of the lahar as possible,” he said.
Other precautionary measures included automated warnings that would broadcast through police communication centres to close off vulnerable places on the road and rail networks, simultaneous triggering of warning signals on the Desert Road and barrier arms at the Tangiwai road bridge.
Several issues remained around the extent of the potential lahar.
The report could not predict the potential size of any lahar and risks associated with an extreme level event could not be discounted. Consequently, ways to further reduce the risk, including the feasibility of managing the timing and nature of the lahar, were being investigated.
On the other hand, it was also possible a less dramatic lahar could occur, with some evidence that the dam was more permeable than previously thought and water could leak away. This possibility was also being considered by an independent science advisory panel.
No private homes stood in the lahar’s expected route and risks were mainly to structures such as roads and bridges. Civil Defence was working with local authorities, asset owners and emergency services on developing appropriate response plans.
The report’s findings would be reviewed to ensure the government was acting on the best information, the Ministers said.
Mt Ruapehu is in a National Park and has World Heritage status.
Note: full report available on