Red alert on lethal lahar flow

JO-MARIE BROWN reports on the first early-warning system for volcanic torrents set up on Mt Ruapehu.

An alarm system to give early warning of a potentially disastrous volcanic torrent has just been set up on Mt Ruapehu.

Scientists predict that a lahar - a deluge of ash, water and mud - will crash down the mountain by 2005.

The warning system will trigger pagers held by the police, Ruapehu District Council, Department of Conservation and others.

They will then evacuate the area to prevent a repeat of the 1953 Tangiwai disaster, in which 151 railway passengers died after a lahar swept away a rail bridge.

Barrier arms, flashing lights and signs will soon be put up along State Highway 1 (the Desert Road) and on State Highway 49 between Waiouru and Ohakune to stop motorists when a lahar occurs.

"We're treating it with some urgency and devising safety measures that will be in place well ahead of any expected major event," said Transit New Zealand's Wanganui regional manager, Errol Christiansen.

Dr Harry Keys, advisory scientist for the DoC's Taupo-Tongariro conservancy, said a lahar was expected to occur on Mt Ruapehu between next year and 2005.

The crater lake was now only two-thirds full, but melting snow and ice eventually would eventually cause the rim to collapse and send a lahar down the Whangaehu River.

More than 60 lahars have occurred on Ruapehu since records began in 1860. The next one - which scientists say could be almost twice as big as the one that destroyed the Tangiwai bridge - was at first predicted to occur as early as this year.

But Dr Keys said the crater lake's temperature reached 39C over the summer. Water evaporated and the level dropped.

A $200,000 bund (embankment) of compacted gravel and ash was completed last month to help to stop a lahar spilling into the Waikato Stream and heading to the Tongariro River and Desert Road.

The $300,000 East Ruapehu Lahar Alarm Warning System was finished on Saturday.

"Ruapehu is quite an active volcano and very destructive lahars have come down," Dr Keys said.

"Tangiwai was the worst we have seen but Tangiwai is actually a small one compared to what it has done in the past and there's been no warning system for people.

"So this is the first time people are going to have a system which is protecting them, and that's fantastic."

A major emergency response exercise will be held in the coming months to test the alarm system and ensure that the police, local authorities, Transit New Zealand and others have time to clear the area.

"The prime aim is to avoid loss of life," Dr Keys said. "Structures like the toilets and bridges and stuff can all be rebuilt."

The alarm system includes sensors that detect vibrations at three points - the crater lake, the New Zealand Alpine Club hut and the Tukino skifield. The time it takes for the sensors at each spot to activate will confirm whether a lahar is on its way and how fast it is moving.

Radio signals from the sensors will then be relayed to a computer in Tokaanu, 5km northwest of Turangi, which will set off pager alarms.

Dr Keys said the system was expected to give up to one hour's warning before a lahar struck the Desert Road and up to two hours before it arrived at the Tangiwai Bridge.

Work to install the bund and alarms was approved by Conservation Minister Sandra Lee in 2000.

The option of carrying out engineering work at the crater rim to drain the lake before a lahar occurs was ruled out because of the risk to workers.

Tranz Rail already has an alarm system to stop trains before a lahar reaches the Tangiwai bridge, and the Genesis power company has an alarm for its water diversion and hydro power operations.

Dr Keys said regular updates were now being given to emergency services and others about the crater lake's level.

"By knowing what the crater lake is doing we will be able to give people an indication of the worst that could happen and they can get more ready accordingly," he said. "Normally you wouldn't have that luxury."