January 18, 2020
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When Mountains Move

A scientific understanding of the destructive forces of nature can help avert landslide disasters

When Mountains Move

JUST as the Uttarkashi earthquake forced a serious rethink about Himalayan tremors, the barrage of landslides in Garhwal and Kumaon has had geologists and planners fumbling for answers to questions that have long been lying idle.

Namely: Have landslides increased over the years? Are they just epileptic fits of nature or are they a violent reaction to its tinkering by humans? Is climate change behind the recent fury of the mountains? Are some regions of the Himalayas more prone to slips than others? Is there a way science can forewarn us? Do developers take into account the fragility of the Himalayan geology while building houses, dams, felling trees, and engineering roads?

One simplistic theory that has gained much popular currency needs to be debunked—that cutting trees is the prime trigger of landslides. The oft-cited texbookish argument goes like this: trees act as binds on the soil; remove them and the soil is loosened which is then whisked away by a downpour; the more the erosion, the weaker the hold of soil on the underlying rocks; and then one day a gushing torrent rips apart the mountain's back, hurtling down tonnes of debris.

"Deforestation," explains Jayanto Bando-padhyaya, a geologist at IIM Calcutta, "does hasten the debacle, but only nominally. Landslides have been occurring for aeons, even when mountains were fully clothed in green. The Himalayas being young, the tree roots do not go more than 2-3 feet into the soil. So when seismic tumults join forces with intense rains, the trees do not stand a chance in the ensuing tug of war."

Humus, the thick undergrowth of leaf-litter and other deadwood, plays a far more crucial role in checking soil-hijacking. Says geologist G.L. Narula, formerly with the Geological Survey of India (GSI): "When there is no humus, raindrops crash thuddi-ngly into the bare soil, virtually cutting it open. The trees sway helplessly. Yes, we must not allow more trees to be cut, but more importantly, the villagers must be told of the importance of humus so that they don't take it away for compost or fuel."

But humus pales into insignificance in comparison to nature's own landslide triggers. The Himalayan realm is trembling all the time as the Indian plate presses into the Asian plate. Says Bandopadhyaya: "Every day there are several small earthquakes jerking the Himalayan belt. With each quake, boulders underlying the hill slopes slip against each other violently. Usually there is enough friction to prevent a dislocation. But during intense rains, water can seep into rock crevices and make them slip more freely so that even a mild tremor is enough to dislodge them, thus forcing a slide."

Nature's suicidal tendencies can hardly be stopped. But we can refrain from abetting them. Says Bandopadhyaya: "Blasting a hill to make roads upsets the slope equilibrium beyond repair. That's why slopes are most vulnerable where a road is cut into it and why most landslides occur on these slopes. Unfortunately, we still follow outdated and dangerous road engineering techniques. We have the choice of better technologies but they are expensive. Nevertheless we should do a cost/benefit analysis in the long run and introduce safe engineering methods wherever the risks outweigh the costs."

But how do we assess landslide risks? Understandably, this is a tricky exercise. For instance, nobody knew that the Malpa landslide in Kumaon would result in so many deaths. "That it happened when lots of people are around was sheer coincidence. It didn't happen last year simply because there weren't enough rains. Nobody could have predicted the monsoon to be so fierce this time," says Narula. He empha-sises the need to prepare maps indicating major landslide hazards as well as areas where the risk to life and property is very high. The GSI is preparing such danger zone maps, though it doesn't seem to be high priority. In 1993 landslides had caused a loss of more than $7 billion. But the maps still haven't been made.

Another question that should strike the curious: why has the monsoon been so ferocious this year? Sanjeev Prakash, an economist, studying the implications of climate change, believes that global warming could be a reason. "There is a trend towards more precipitation in the hills. If we assume this intensity will increase in the future, it would be foolish not to take landslides seriously." The need, clearly, is to understand the forces of nature better. So that, rather than contributing to its destructive designs, ways can be devised to escape it.

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