After the Gujarat earthquake, the Prime Minister has ordered a review of the Tehri Dam, which lies in a seismic area and could also suffer a major earthquake measuring 7.9 on the Richter scale. I think the review should conclude that the Tehri dam could be an environmental saviour in the event of a quake.
This will astound traditional critics of the dam. Critics believe a quake could breach the dam, causing a huge wall of water to flood into the valley below, destroying everything in its path.
After Gujarat, the critics need to think again. This quake breached no dam, yet 100,000 people may have perished, and entire villages and towns have been levelled. This demonstrates that the main danger from an earthquake is to populated areas, few of which are anywhere near a dam. Tehri\’s critics warn that a major quake could cause floods that destroy downstream towns like Rishikesh and Hardwar. In fact major quake will flatten Dehra Dun, Mussorie, Naini Tal and other cities far bigger than Rishikesh and Hardwar. Only a tiny fraction of Garhwalis lives along the river. So if indeed the critics are serious about saving human lives, they should focus on improving building standards in Dehra Dun and all other towns in Uttranchal, rather than just oppose the dam. I know of no prominent dam critic who has led a movement for earthquake-proof building codes. Which convinces me that the critics do not care much for saving lives, they only care about stopping dams.
They cannot see that dams can also be saviours in a quake. Let me explain this in some detail. Readers may recall that in June 2000, a sudden flood in Arunachal Pradesh destroyed every bridge and village in its path, but only a few lives were lost in this sparsely populated area. First reports speculated that a nuclear bomb or dam burst in Tibet might have caused the flood. But then satellite photos showed that a cloudburst in Tibet had led to a major landslide, which blocked a tributary of the Brahmaputra. The subsequent bursting of the landslide-induced dam unleashed the flood.
This is not uncommon in the Himalayas. In the 1970s, a landslide in the upper Ganga created a temporary dam, which then burst and flooded Rishikesh and Hardwar. Just think: if floods of this sort can occur even without an earthquake, will not a major quake produce far bigger landslides and floods?
One answer to this comes from the 1950 Assam quake, 9.2 on the Richter scale. An entire hill, around 300 feet high, fell into the Brahmaputra. When this mud dam finally burst, it caused terrible havoc in the valley below. Only the scarcity of population prevented a major human disaster.
Nor is the problem confined to big river valleys. Vinod Gaur, former director of the National Geophysical Research Institute, has written about the 1819 quake in Gujarat which created a huge natural bund (Allah Bund) as well as a natural dam 6.5 metres high.
What is the biggest difference between man-made dams and nature-made dams? Nature-made dams are weak and bound to burst, whereas man-made dams can be designed to withstand extremely severe shocks. So, if indeed there is a major quake of 8.0 on the Richter scale at Tehri, it is entirely plausible that the dam will remain intact while the walls of the upper Ganga gorges collapse, causing calamitous floods. The only thing that can stop such floods is – you guessed it – the Tehri Dam.
When a wall of water floods into the big reservoir of the Tehri Dam, its force will be dissipated in raising the level of the reservoir. In the lean rainy season, the reservoir alone will hold the flood water. In the peak season, when the reservoir is full, the flood water will overflow the dam, but will slow down as it does so. That will reduce the force and damage-causing capacity of the water.
So strident have dam critics been that some readers may not know that flood control has always been a major aim of dams. They yield electricity and irrigation, but also tame floods. This could be all-important in a quake. Some readers might think the Tehri Dam might is a concrete wall that could collapse in a quake, just as skyscrapers did in Ahmedabad. Not so. The Tehri Dam is a rock-filled dam, not a concrete one. A rock-filled dam is a gradually sloping underwater hill whose crest peeks above water level. Even if a quake disturbs part of a rock-filled dam, it will resettle and remain in the shape of an underwater hill. Indeed, such a dam is infinitely more stable than the sides of the valley, which are likely to crumble.
The Himalayas are mostly composed of brittle rock that crumbles easily. So weak is the rock that engineers cannot build conventional arched concrete dams in the area. As all engineers know, the principle of an arch is that pressure applied on the curve is transferred to the two ends. So, a dam curved in the shape of an arch transfers the pressure of water to the two ends embedded in the sides of the valley. This makes possible low-cost, thin concrete dams provided the sides of the valley are made of strong rock that can take the pressure. But Himalayan rock is generally too weak, forcing engineers to build thick straight-gravity dams (as at Bhakra) or rock-filled dams (as at Tehri).
In sum, the main threat comes from the environment, not the dam. Rock-filled dams are far stronger than Himalayan hillsides. The major flood threat comes from not from man-made dams but landslide-induced ones. The silver lining is that the Tehri Dam could tame such floods.
In such circumstances a dam could be an environmental saviour, not a hazard. I hope the official review of the Tehri Dam emphasises this.
