Ruapehu crater lake meddling ruled out

By JO-MARIE BROWN

The Government has ruled out intervening at Mt Ruapehu's crater lake to prevent a potentially disastrous mudslide from occurring.

Conservation Minister Chris Carter yesterday told a public meeting organised by the Ruapehu District Council that engineering options, such as building a trench to drain the lake, had been reviewed and discounted.

A mudslide, known as a lahar, is expected to occur early next year when an ash dam surrounding the volcano's crater collapses.

Numerous lahars have crashed down the side of Mt Ruapehu in the past, including one that claimed 151 lives at Tangiwai in 1953.

"After careful deliberation, the Government has decided not to proceed with any of the crater lake intervention options and will instead continue to bolster the extensive safety measures already in place around Ruapehu," Mr Carter said.

Those measures included strengthening the Tangiwai road bridge to enable it to withstand lahars and installing an alarm system which gives a 90-minute warning that a lahar is on the way.

Response plans have also been prepared to evacuate the area and close all at-risk bridges, roads and railway lines, and a 300m-long stop bank has been built to stop the lahar spilling into Lake Taupo's catchment.

Mr Carter said the Government believed there was a greater chance of people being hurt while trying to carry out intervention work than there was in allowing the lahar to occur naturally.

Building a trench to drain the lake would have placed workmen in danger and would have left behind most of the existing ash dam, increasing the risk of large lahars occurring.

Engineering consultants had also looked at sluicing the ash dam to trigger the lahar and siphoning the lake.

"Sluicing was rejected because it was likely to be even more risky for workmen than excavation and a trial questioned whether it would even work," Mr Carter said.

"Siphoning is simply impractical because the reduced air pressure at that altitude restricts the ability to lift the water over the ash dam."

Chris Lester, chairman of the area's regional council, said he accepted that Mt Ruapehu was on Crown land and it was up to the Government to make decisions relating to it.

But both his council and the Ruapehu District Council wanted an assurance they would be indemnified against any legal action if something went wrong, and some financial assistance from the Government to prepare for the actual event.

National's environment spokesman Nick Smith condemned Mr Carter's decision.

"In my view it's a cop-out and I'll be holding the minister responsible for any loss of life or damage to property from this eminently preventable large lahar."

Lahar-busting

The options:

* Building a trench to drain the lake: rejected (too dangerous to build, could increased lahar risk in future).

* Sluicing the ash dam to trigger the lahar: rejected (even more dangerous, might not work).

* Siphoning the lake: rejected (technically difficult because of reduced air pressure at altitude).

* Strengthening Tangiwai road bridge and installing 90-minute warning system: approved.