June 25, 2008

Mt Taranaki Lahars a Fabulous Force of Nature

By WINDER, Virginia

This article was written by a contributor. It is not to be reproduced without permission from the Taranaki Daily News and charges may be incurred.


IT could be happening right now, this moment, as cloudbursts turn the mountain into a torrent. A rain- triggered lahar may be forming, ready to slow- roar down the slopes in a tumble of rock, sand, water and shingle. This is all part of the ongoing erosion highlighted on the front page of the Taranaki Daily News yesterday.

Two lahars, caused by exceptionally high rainfall on April 28, 29 and 30, are now being studied by Massey University scientists. The team, led by Vince Neall, a professor in earth science, says one flowed down the Little Maketawa Stream on the northeastern slopes while the other followed the Maero Stream on the northwestern side of the mountain.

They both formed high up and swept down the cone at 40km per hour or so, ending just before the Egmont National Park boundary - "fast enough that it would be difficult to get out of the way very quickly", Neall says.

Both lahars cut deep channels as they went, but it was the Maero Stream flow that caused the most damage. Heading down towards the Stony River, the stream makes a sharp right. The lahar couldn't do that.

"This flow simply went straight ahead up a bank, which varies from five metres to eight metres in height, and has then destroyed a swathe of forest about 1km long and a couple of hundred metres wide," Neall says in awe. "There are boulders there which were transported in this event which were up to three metres across.

"All around the edge of this thing is one big logjam, where all the trees have been bulldozed to either side and there are just a few trees that still stand and you can see where the bark has been removed from them by the boulders coming along."

The bark damage clearly shows that in some places, the lahar was 4.2m above ground level.

Anyone who walks up the Puniho Track for an hour will be able to see the destruction.

"It's well worth going to see - it blows your mind away just to see it."

But don't venture on to the mountain in wet weather. Trampers caught out should hunker down in huts, he advises. And they should avoid crossing swollen streams at all costs. If someone had attempted to ford rivers the day the lahars came down, lives would have been lost, he says.

Rain-triggered lahars, which are smaller than those that occur during volcanic eruptions, happen through a combination of conditions.

First up, there needs to be a great deal of rain, just like at the end of April when North Egmont was swamped with 270mm over three days. When the rainfall was at full force, the Visitor Centre site recorded 45mm in one hour. "The water came down off the higher part of the cone and it just cut into these gravels like a knife through butter," Neall says.

Sediment builds up in the rivers and when it reaches high concentrations, the thick liquid begins to "bulk up", a little like a snowball, collecting massive rocks, some weighing up to a tonne. "Water can't transport boulders that weight. You have to have this concentrated flow of sand and water. If you get a high enough mix of water and sand, then these larger boulders become more buoyant and can be either carried or rolled along in the flow and transported."

That's what happened in both cases, with the growing flow of debris taking out vegetation on either side of the water channel.

"This certainly happened on the Little Maketawa Stream, making a sharp trim-line on both sides of the valley floor to heights of up to seven metres," Neall says.

"Then it began to behave like wet concrete in a hydro- slide." Just like a person slipping down a slide, the lahar tipped up, or became "super-elevated", on the bends. It did this on a 90-degree corner right below the Maketawa Hut, where the debris flow was seven metres higher on one side than the other.

"It just climbed up the valley wall due to the momentum of the flow."

Then the bend went the other way and the lahar slid seven metres up the other bank. "This is at the very point where the Round-the Mountain Track goes across the river."

On Friday, the scientists took a helicopter flight to get an overall picture of the damage, and could see how the lahar travelled about 5km downstream strewing boulders in the leatherwood scrub. "By the time it got to the park boundary, it would have been a flood with a lot of logs floating in it."

The latest lahars are part of the mountain's ongoing pattern of collapse.

One rain-triggered flow occurred during heavy rains in the Oaonui catchment on September 15, 1998, and began when a section of lava flow collapsed. The lahar overflowed into the Waiaua River and affected Opunake's water supply because of the sand that got into the rivers. Neall says there will be more lahars and is quick to say when: "next intense, heavy rainstorm."

Which could be right now, possibly this moment.

Meanwhile, at Massey, the calculations go on.

"We've got fairly sophisticated precise surveying type gear that uses a radio frequency."

The scientists are using this to map where the overflow occurred in the Maero Stream and have also taken a detailed survey of the damaged area.

"We've done cross-sections of the river both up- and downstream of these points so we can work out the speed and the volumes involved to build up a picture of what happened. But we can't do that overnight."


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