50 years on, Tangiwai braces for destruction

The railway bridge at the site of New Zealand's worst train disaster faces a new threat from a huge mudflow expected to burst from Mt Ruapehu's crater lake this summer or next.

The Conservation Department geologist responsible for planning measures to cope with the mudflow, Dr Harry Keys, said this week that it was "doubtful" whether the rail bridge at Tangiwai, near Waiouru, would survive.

But he was "absolutely confident" no one would die because a warning system had been set up.

On Christmas Eve in 1953, 151 people died when the Wellington-Auckland express train sped on to the bridge at Tangiwai moments after a similar mudflow from the crater lake swept part of it into the Whangaehu River.

The bridge was rebuilt in 1954 at a higher level above the river, and with piles sunk 20m into the bedrock.

It withstood mudflows in 1968, 1969, 1975 and a series of 26 mudflows triggered by the eruption of Mt Ruapehu in 1995.

But it was designed to survive flows only up to 50 per cent bigger than the one that struck in 1953, and scientists believe the one expected this summer or next may be up to 80 per cent bigger than the one that wiped out the previous bridge.

The mudflow, or "lahar", is expected when the water in the crater lake breaks through a 7.4-metre layer of soft sediment laid down over its previous outlet by the 1995-96 eruptions.

The breakout may be gentle if the water seeps out slowly soon after it rises above the pre-1995 rim of hard rock.

It is expected to reach this level late this month or next month.

But if the water keeps rising through this summer and next towards the top of the soft layer, scientists believe it will eventually break out catastrophically.

The weight of water will throw the entire soft layer and 1.5 million cubic metres of acidic water into the Whangaehu Valley in a matter of minutes.

The mudflow will reach within a few metres of State Highway 1 on the Desert Road south of Turangi in 38 to 72 minutes, and will hit Tangiwai between 57 and 91 minutes later.

The Desert Road is unlikely to be flooded because most of it is on high ground, but solar-powered warning lights have been installed north and south of the the Mangatoetoenui and Waikato Streams, the last two streams that the road crosses south of Turangi.

Eight extra police officers have been posted to Waiouru to stop traffic as soon as the lahar starts.

Dr Keys expects the road to be closed for no more than two hours while police check for damage.

State Highway 49 between Waiouru and Ohakune would be closed for longer while engineers check possible damage to the road bridge at Tangiwai, even though it is being raised 2m and reinforced with piles 30m into the rock.

"It won't be open for at least 48 hours because the structures will have taken the brunt of the lahar," said Ruapehu District Council emergency management co-ordinator Johan Cullis.

But the 50-year-old rail bridge upstream of the road bridge is not being raised, and Dr Keys said it was clearly in danger.

"The railway bridge is interesting because it probably will survive the most likely sized lahar, but it's doubtful," he said.

"It will survive anything up to warning level 2, equivalent to the one in 1975. It's designed for lahars 50 per cent bigger than Tangiwai.

"But the most probable one is an 80 per cent bigger discharge, so the scientific opinion is that the railway bridge may or may not survive."

The bridge was bought a few months ago, as part of the rail network, by the Government-owned Railways Corporation.

The corporation's chief operating officer, William Peet, said the bridge was higher than the pre-1953 structure and had "a significant chance" of surviving.

"From our perspective, the bridge will withstand pretty much anything," he said.

"We have done some work on the approaches, and from our perspective there is no further physical work we could do to protect the railway. We are confident the lahar will pass underneath it."

Warning systems installed upstream will give trains at least half an hour's warning to stop before the lahar hits.

If the bridge is destroyed, Mr Peet said, the corporation had an emergency bridge that it could put in place "within a few days".

Train operator Toll Rail said it would divert freight trains on to the Taumarunui-Stratford line if the main trunk line was cut at Tangiwai.

But Toll communications manager Sue Foley said the Stratford line was not certified to carry passengers, so people on the sole remaining Auckland to Wellington passenger train, the Overlander, would be transferred to buses around any break in the line.