Earthquakes be dammed

One of the great uncertainties about building high dams in the Himalaya for hydropower and flood control is the threat that they would pose to the plains in the event of a major earthquake. The grandeur of Himalayan peaks and their stupendous height deludes observers with an image of permanence. This is actually a gigantic pileup resulting from the collision of the Eurasian and Indian plates that began 50 million years ago. India continues to bulldoze under the Tibetan plateau, creating tremendous tectonic tension under the mountains. Most areas of the Himalaya where future dams are planned are rising or slipping at between 10-20 mm a year.

The major Himalayan rivers are older than the mountains and have their headwaters in the Tibetan plateau, behind the main chain. The rivers rose as the mountains were formed, cutting deep gorges and they store great amounts of potential energy—which is what makes them so ideal for power generation. As the mountains rose, they formed a monsoon trap giving the southern slopes one of the highest precipitation rates on the planet. The annual rains turn this steep and seismically unstable mountain range into a crumbling, shattering mass that erodes faster and washes down more sediment than any other mountain system.

It is the debris from the erosion of the young Himalaya that filled up the Tethys Sea and turned it into what is now the Gangetic plains. This process of mass wasting that deposits debris in the plains continues, so the notion that floods in northern India and Bangladesh can be 'controlled' is wishful thinking. The rainfall volume, sedimentation levels, and the size and frequency of earthquakes in the Himalaya far outstrip parameters laid out in engineering textbooks prepared for comparatively docile climes. Many of our specialists have been trained for technological solutions based on case studies that greatly underestimate the Himalayan dimensions of cloudburst, glacial lake outburst floods, and earthquakes in this part of the world.

The rock strata bent by the enormous forces beneath the Himalaya trigger thousands of small tremors every year. But every once in a while there is a major crack as the pressures are too much for the elasticity of the rocks, and the strata snap. When that happens, there is a magnitude Richter 8+ earthquake. Geologists now agree that there occurs a high intensity earthquake once every 100 years along any section of the Himalayan chain.

The stretch between Dehra Dun in India and Kathmandu in Nepal is one area where there has not been a magnitude 8 earthquake now for at least a century, and the big one is long overdue. This 'seismic gap' makes a major earthquake in the central Himalaya inevitable in the near future. "Such an earthquake can have a ground acceleration of more than 1g. What this means is that, if the ground is moving downwards, anything that is lying loose on its surface—a boulder for instance or a high dam and the massive volume of water behind it—will be left up in the air," write cultural theorists Michael Thompson and Dipak Gyawali in a recent paper.

The catastrophic impact of the failure of a dam like Tehri or Pancheswar with 20 cubic kilometres of impounded water on the downstream plains is unthinkable. But there are failures of natural dams caused by landslide blockage of rivers in the past that give us an indication of the scale of such a disaster. In 1893, a rockslide on a river in the Garhwal Himalaya burst, causing a huge flashflood and great loss of life all the way down to the plains. In 1970, debris flow on the Alkananda River created a 60-metre high dam on this tributary of the Ganga. When this burst, it caused a flashflood that thundered down all the way to the plains of Uttar Pradesh, destroying settlements, bridges and highways.

Some scientists believe that as long as the dangers are known, there are engineering measures that can be taken to make the catastrophic failure of a high dam less likely. But the question is how much is it going to cost and if the risk, however minimal, is acceptable. Thirty years after they happened, reports are filtering out now of dam bursts in south-central China that killed tens of thousands of people.

Besides the geological uncertainties, there are the geo-political complexities posed by a 300-metre high dam in a neighbouring country upstream. In an interview in the Kathmandu newspaper, Jana Astha, in 1996, minister Gajendra Narayan Singh had this to say: "If they (India) go against international norms (by not buying our electricity) we can destroy the dam. If we release all the waters, it will drown India." The fact that Singh belongs to the pro-Indian Nepal Sadbhavana Party may or may not have a bearing on his remarks.

Dam failures then become like nuclear war, you don't want to think about them. Designing earthquake-proof storage dams is a question of how much risk countries are prepared to take. The Great Bihar-Nepal Earthquake of 1934 registered 8.4 on the Richter scale and virtually destroyed Kathmandu killing about 4,000 people—about one in every ten inhabitants. Kathmandu's population was a lot less then, and there were fewer lethally unstable concrete structures. If an earthquake of similar intensity were to occur today, the National Society for Earthquake TechnologyNepal estimates that as many as 40,000 people could be killed in Kathmandu Valley alone. And any dams in the vicinity could be severely damaged.

Earthquake prediction is still an imperfect science, but there are ways to reduce risks from earthquakes and their aftermath by preparedness. Provided basic data are correctly accumulated, earthquake zonation maps could be drawn up to show more vulnerable areas. Li Tianchi, a geologist and natural disaster expert with the Kathmandu-based International Centre for Integrated Mountain Development (icimod), is working on zonation. He says: "We can identify which areas are more sensitive to earthquakes and take measures accordingly. But so far in Nepal, Bhutan, India and Pakistan—nations south of the Himalaya—no such map exists."

In a candid report, Nepal's Department of Mines and Geology assesses the risks to high dams from earthquakes: "A high dam can cause colossal downstream consequences in case of failure by an earthquake. Assessment of the seismic risk should be carried out for such structures."