Living under the volcano

This summer or next, the Blackburn-Doolan family of Tangiwai is likely to lose their only road access to the outside world.

If she gets enough warning, Carla Doolan will take 2-year-old Logan and baby Scarlett, who turns one this month, across Strachan's Bridge to stay with her partner's parents on the other side of the treacherous Whangaehu River.

Her partner, Hamish Blackburn, will stay alone on his side of the river, minding the farm that his great-grandparents carved out of the bush.

Just downstream, Doug and Puti Goodwin have been deputed to block the Whangaehu Valley Rd in case the road bridge at the Tirorangi Marae is washed out, too.

At Tangiwai itself, where the main road bridge is being raised 2m at a cost of $4.5 million, the 50-year-old rail bridge alongside it is also vulnerable.

The three bridges, and the people who depend on them, are the most obviously at-risk when a wall of water, silt and rocks roars down the river some time in the near future.

Mt Ruapehu's crater lake, about 2000m above Tangiwai, is expected to break through a 7.4m layer of soft ash and sediment laid down in eruptions in 1995-96 across the lake's former outlet into the Whangaehu River.

If the lake fills to the top of the new sediment before leaking, it is likely to burst out with all the force of the high lake level behind it, sending a mudflow down the river that will be up to 80 per cent stronger than the one that destroyed the last Tangiwai rail bridge and killed 151 passengers on Christmas Eve, 1953.

Yet the Blackburn-Doolan family, the people at Tirorangi Marae and even Toll Rail can do nothing to stop it.

They go about their normal lives constantly aware that, at any moment, they could have to face what the Indonesians call a "lahar" - a surge of rock, silt and water out of a volcano.

"It hacks you off a bit, actually," says Blackburn, with rural understatement.

He reckons the Government should have cut a hole in the soft sediment on the crater rim so that the water from the lake would dribble out slowly, rather than building up to a catastrophic breakout.

"That would save building another bridge. If we have to replace it, it might cost $100,000 to $150,000," he says.

However, others in this tough farming district that had seven snowfalls this winter are willing to let the lahar throw whatever it can at their vulnerable bridges.

Doug and Puti Goodwin, who returned to Puti's home area a decade ago to help renovate the marae, believe they are "pretty self-sufficient", with a wood stove and battery radio to keep them going even if the marae bridge is washed out.

Marae secretary Debbie Te Riaki says Ngati Rangi, the iwi of the land between Ohakune and the Rangitikei, are united in opposing any crude cuts in the mountain summit. "It's our maunga tapu [sacred mountain]," she says. "We have a traditional burial ground up there. So we have a cultural and spiritual perception.

"But we also have a very practical view as well, once we looked at some of the strategies such as sluicing the crater lake. It was impractical."

It is, says another farmer, Bruce Mazengarb, "a touchy subject". Hamish Blackburn's father is Ngati Rangi, as is Lorraine Wilson, who looks after young Logan and Scarlett Blackburn while their mother works at the local Winstones pulp mill.

"Myself, I think they should leave things alone and not tamper with them," says Wilson.

Pakeha farmer Cyril Wise, who farms the land his grandfather bought just south of the marae, agrees.

The short side road to his farm is often blocked anyway by heavy rain such as the storm that swept away houses further down the Whangaehu in February.

"I don't dwell on it. If it happens, it happens, I guess," he says.

The debate about what to do about the impending lahar has smouldered ever since the 1995-96 eruptions.

Dr Tony Hurst of the Institute of Geological and Nuclear Sciences (GNS), who presented a paper on the issue to a conference in London in September, says volcanologists around the world are watching this unusual case of a totally predictable disaster.

Only a few dozen volcanoes, mainly around the Pacific rim, have crater lakes high up in active volcanic vents full of hot acidic water - "an accident waiting to happen," as GNS colleague Vern Manville put it at this week's New Zealand Geological Society conference in Taupo.

After the Government yet again ruled out cutting through the crater rim this year, Hurst told the London conference: "It appears that in New Zealand, where engineering-type measures that could be undertaken to reduce natural hazards run into cultural obstacles, decisions will be taken to a high level and made on political grounds."

But in part this is because even the scientists cannot be sure what will happen.

Massey University earth scientist Vincent Neall, nicknamed "Professor Lahar" by other geologists because of his lifetime study of Ruapehu, says there won't be a lahar at all if the water in the crater lake cuts its own channel through the soft sediment before building up to a high level, or seeps out slowly.

Turangi-based Conservation Department geologist Dr Harry Keys reckons there is a 5 to 20 per cent chance of a lahar this summer, about a 50 per cent chance of one next summer, and a reasonable chance that it will come even later or not at all.

The crater lake, drained completely in the 1995 eruptions, is about 150m deep and is rising by about half a metre a month, fed by melting snow and ice.

Keys measured the water level on Tuesday at 2528.1m above sea level - just 1.4m below the rim of pre-1995 hard rock and 8.8m below the top of the soft material deposited in the eruptions. This means it has reached the first warning level in a five-graded risk scale.

But no one can know in advance either how fast the water will rise, or at what level it will break out through the soft upper rim.

This year's early summer has been colder than usual because of weak "El Nino" conditions bringing cold, dry weather.

"We've had three significant snowfalls since November 1," Keys says.

"In the previous El Nino we had eight over the whole summer. We've had three already.

"Every time it snows, it stops melting. If melting stops, the lake doesn't rise."

Nevertheless, he expects that the lake will reach the pre-1995 hard rim level, defined as "warning level 1b", between late December and late January. The question then is how soon it will seep or cut through the soft stuff above.

In the worst case, if the break occurs near the top of the soft layer, it is likely to tunnel quickly down through the soft material and release the whole soft rim plus about 1.5 million cu m of warm water from the lake into the Whangaehu valley in a few minutes.

Keys estimates that that would produce a lahar about 10 to 30 per cent higher, and carrying 80 per cent more rock and water as it passes Tangiwai, than the lethal Christmas Eve lahar of 51 years ago.

Professor Neall and others, in an article on the GNS website, describe lahars as resembling "wet concrete" and moving at between 10 and 90 km/h.

"Buildings and vehicles may be demolished, buried or carried along with the flow," they write.

"People caught by lahars are unlikely to survive as they become sucked into the flow and drown."

Big lahars are capable of climbing up river banks at bends in the valleys.

One of the lahars caused by the 1995 eruption flowed over an approach road behind the Blackburns' bridge at Tangiwai, leaving the bridge half-marooned in the middle of the mudflow.

The deafening noise of such a huge volume of rocks and debris being hurled down the river at high speed could be heard 7km away in Waiouru at Christmas 1953.

A Government report, commissioned by National's Conservation Minister Nick Smith in 1998, considered 23 possible measures to reduce the risks of a lahar. Smith himself, on his last day on office after National lost the 1999 election, minuted his opinion that the Government should cut through the soft rim to prevent a lahar occurring - an operation that would have cost only $200,000.

"The reasons for not proceeding with the engineering works within the ash deposited in the 1996 eruption have largely been as a consequence of putting far too much weight on the cultural objections of Maori and the difficulties of doing work within a national park," he said this week.

"The issue of public safety must always outweigh those other considerations."

But public submissions on Smith's options, mainly from mountaineering, skiing, environmental and Maori groups, overwhelmingly opposed cutting a ditch at the crown of the country's oldest national park.

Only the roading agency Transit and the Ruapehu and Taupo district councils, representing the people and bridges in the likely paths of the lahar, supported engineering works.

The incoming Labour Government went with the majority. Instead, it approved a $300,000 alarm system with vibration sensors to detect lahars at five places, alert key agencies via pagers and trigger flashing warning signs on the Desert Road and at Tangiwai.

Signs have been put up on walking tracks across the Whangaehu warning: "Extreme lahar risk. Do not stop in this area. Do not proceed past here if you hear a loud roaring noise upstream."

Three years ago the Government also built a 300m-long stopbank near where the Whangaehu emerges from a steep gorge to fan out across the Rangipo Desert. It hopes this structure will stop lahars crossing into the Tongariro River and damaging the river's trout.

Finally, Transit is lifting the main Tangiwai road bridge by 2m and drilling piles 30m into the rock. It is running two months late but says the bridge should now be finished by the end of January.

Keys says these measures are enough to make him "absolutely confident no one will die" in the lahar.

In contrast, he says, there would have been a real risk of deaths if the Government had ordered workers to dig a trench on the crater rim.

"It's not Queen St," he says. "It's the top of a mountain, with a lot of bad weather."

In any case, he says, totally unpredictable lahars associated with sudden eruptions and storms have happened on Ruapehu for thousands of years, making the preparations that have been implemented for the current lahar a valuable long-term insurance policy.

Neall and his Massey colleagues have found evidence of seven huge lahars, each between five and 25 times as big as the 1953 event, during the past 1900 years - an average of one every 270 years.

Even though we have not seen one of these for 400 years, history suggests there is a real chance of such a huge lahar occurring in the lifetime of the bridges we build today.

In his London paper, Hurst noted the irony that New Zealanders seemed to be more worried about the prospect of a relatively small "overflow" lahar than about the much more serious damage that will almost certainly be caused by future eruptions.

"The hazard related to a lake filling to overflow seems to be easy for politicians and the public to grasp," he told his audience. "Perhaps if volcanologists could see into magma chambers and report on their state of filling, they would find it easier to get attention when they talk about volcanic risk."

But Hurst may underestimate the hardy New Zealanders who live around Ruapehu. Doug Goodwin, for one, sees the relative risks clearly. "We've had lahars before," he says. "We've had boulders rolling down the stream and the water went up a bit.

"It didn't affect us. If it gets past Tangiwai without taking anything out, it will probably be okay around here. You just take precautions. People who live under volcanoes are dicing with death anyway. I think we are reasonably safe here, but you never know."