KEY POINTS:
It's easy to say, as the New Zealand Herald did in its editorial of March 22, that excavating a channel at the top of Mt Ruapehu would have been the cheap and logical way of avoiding the recent lahar.
I would like to clarify the reasons for installing
an early warning system, a stopbank on the lower slopes of Mt Ruapehu, a system to close sections of state highways SH1 and SH49, and the raising and strengthening of the bridge at Tangiwai across SH49.
The Government had to consider the worst case scenario, not the best case. There was a small but non-negligible chance that the lahar could have been up to three times larger than the 1953 event.
In 2003 an engineering firm, Meritec, produced a report to the Government on a range of possible "engineering solutions" to avoid or prevent a lahar resulting from the filling of the crater lake after the 1995-1996 volcanic eruptions on Mt Ruapehu. The report rejected all the options (including a siphon), except the trenching option, which was not without uncertainties and challenges. The report recommended further work including trials to reduce the uncertainties.
In 2004 Cabinet decided that these and other uncertainties and risks were too large and that overall the risks would be better managed by the works that had already been put in place.
Digging a trench with earthmoving machinery would involve getting heavy machinery onsite, a tricky proposition, let alone the difficulty of operating on top of Mt Ruapehu. Lighter machinery could have been flown in, still a challenge in the high-altitude thin air, as shown by recent events, including a helicopter accident and an evacuation of a worker suffering from altitude sickness. Even if an 85m-long trench could have been excavated, there would still have been a large amount of ash left behind and the next eruption could have produced a larger lahar than that which occurred.
The climate at the top of Mt Ruapehu is harsh and fickle; snowstorms are a possibility at any time of year. You cannot work up there whenever you feel like it. Huge blocks of ice regularly fall into the crater lake, causing mini-tsunamis of acidic water that would harm workers or destroy equipment.
Explosives to blast out a channel might have worked but with a huge risk. The danger is that if the charge was too small, ash would have been left behind, which would not have avoided a lahar. If too big, the "hard rock" rim of the crater lake could have been fractured, in which case there could have been a lahar large enough to destroy Turangi.
Sluicing out a channel with a high-pressure hose carried risks to do with working on the dam while trying to breach it. That would have been like sitting on the outside of a branch while sawing it off nearer the trunk. It would also have been a huge job involving large pumps floating on acidic water.
Siphoning would have simply deferred the problem and it would have left the dam in place. It would have required regular monitoring and maintenance, a poor option given the working conditions at the crater lake. Nor would it have prevented the next lahar caused by a volcanic eruption.
The Government had no option but to accept that an engineering solution on top of Mt Ruapehu was never going to work without creating short- and long-term risks and uncertainties.
A far more effective approach was then taken. The early warning system is based on technology developed after the eruptions of Mt St Helens in the US in 1980 and Nevada del Ruiz in Colombia in 1985, and tested successfully at Mt Pinatubo in the Philippines in 1991. The other users of this technology around the world would have observed events in New Zealand with great interest.
The stopbank along the upper Whangaehu River, in the end, did not come into play because the lahar was not as large as the worst-case prediction. The bund was designed to protect travellers on the Desert Road, the Tongariro Power Station and trout fishery, and the long-term security of the river, Lake Taupo and the Waikato River.
We now know that the bund will protect the Desert Road in much larger lahars that occurred as recently as 450 years ago, and including a lahar more than three times larger than the 1953 event.
The SH49 road bridge performed admirably and the highway was opened within hours. This had benefits for people on the day and will in future when there are unpredictable and much larger lahars created by volcanic eruptions or crater rim collapse.
And the beauty of the whole system is that during the next volcanic eruption, when lahars could go in any direction, including down the Whangaehu, the major lahar path has an emergency management system in place. Any lahar in the future caused by the slow filling of a crater lake or rim collapse would go down the Whangaehu; again, there is a system in place.
On top of all of the above, there was no compromising of the World Heritage site, protected for its natural processes, Maori cultural values and recreational aspirations. The cultural and spiritual values of tangata whenua have been respected with the current system. There has been no need to amend legislation or develop new legislation, which would have been required in light of the national park status of the upper parts of Mt Ruapehu, had the the engineering option been chosen.
* Harry Broad is strategic issues manager, Department of Conservation.
