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Heavy La Niña rains prompt landslide

February 20, 2006 By Mark Peplow This article courtesy of Nature News.

Roughly 1,000 people are still missing, feared dead, following a massive landslide that covered the village of Guinsaugon in the Philippines on Friday 17 February. Here, news@nature investigates the science behind the disaster.

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Are landslides like this unusual?

A slide of this scale is quite unusual, says Diarmad Campbell, a landslide expert from the British Geological Survey in Edinburgh, UK. But, he adds, smaller landslides are very common in the Philippines. "They happen all the time."

The islands that make up the Philippines archipelago tend to have very steep-sided mountains, with people living on the flat plains beneath. The tropical climate means that these slopes see sporadic, heavy rainfall that soaks surface soil until it loosens and slides down the mountain. In the rainy season, lasting from June to November, landslides can be at least a weekly occurrence, says Campbell.

"This is an unusual time of the year for it to happen, because we're outside typhoon season," says Simon Dadson, an expert on geological risks at the Centre for Ecology and Hydrology in Wallingford, UK.

So what caused this one?

Unconfirmed reports say that some 50 centimetres of rain fell in this area in the first two weeks of February - more than twice the average for the entire month. It would certainly have left the soil completely sodden and prone to sliding.

The rain has been linked to a periodic weather pattern known as La Niña, he adds, which usually leads to a shift in jet-stream winds that delivers more rain to the Philippines. The weather system of the Pacific Ocean oscillates between La Niña and El Niño, which sees a warmer surface temperature, every three to five years.

Although a magnitude 2.6 earthquake was recorded just before the landslide, Campbell believes it was too small to play a major role in the disaster. Earthquakes of this size are extremely common in the Philippines.

Vegetation can help to hold a slope together, and deforestation by loggers has been blamed as a contributing factor to previous major landslides in the Philippines. But in this case, aerial photos seem to show that the slope was well covered with plants, says Campbell.

Why did it do so much damage?

Campbell estimates the landslide saw millions of cubic metres of mud flowing from near the top of a large mountain. The mass would have been moving at tens of metres per second when it reached the village, he adds, and people would have had little chance of getting away.

Could the landslide have been predicted?

Earlier this month, the US National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center said that an unusual cooling of the Pacific surface temperatures was likely to trigger La Niña this year. So observers did have a way to know that rainfall was likely to be high. But knowing if and when that would lead to a disastrous landslide is beyond current powers of prediction.

Many countries in the Pacific region have sophisticated risk-assessment programmes that can help to prevent major loss of life in the event of similar disasters. In Hong Kong, for example, typhoons are a major trigger for landslides, so weather forecasts can help to warn people away from particularly dangerous areas. Slopes can also be reinforced with steel and concrete to prevent slides, and planning regulations can stop houses being built in the likely paths of future landslides.

But for a landslide of this scale "there are no engineering solutions," says Campbell. The best answer would simply be to not build houses there.

So why couldn't the disaster be averted?

"In an ideal world you wouldn't want to live near a steep-sided mountain in the Philippines," says Campbell. "But the Philippines is a large country with limited resources," he adds, which forces people to make their living wherever they can.

"It's very hard to stop people living where they want in that region," agrees Dadson.

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