Given the large and increasing number of dams in the world, and the fact that so many of them are ageing,  the question of dam safety is getting more importance. Whenever dam safety is discussed , invariably a significant part of the discussion takes place in the form of earthquakes. However two aspects of the issue need to be clearly distinguished. Both are important.

Firstly, there is the question of dam collapse or extent of damage to dam caused by earthquakes of varying intensity. This is often discussed in terms of the intensity of earthquake anticipated and the capacity of dam to withstand this.

As many large and medium dams are being constructed in regions of high seismicity this issue has become very important. Even in parts of Himalayan region ( or other such regions) where very high intensity earthquake is anticipated, authorities of several countries are going ahead with the construction of big dam projects, or a series of dam projects.

Of course while doing this they have to claim that they have satisfied themselves that technically the dam is safe in terms of its ability to withstand the earthquake of the maximum anticipated intensity. But are these claims always completely correct? In the context of the most discussed Tehri Dam Project the evidence is quite strong that these claims are not completely correct, to use mild words. So the possibility  of very massive loss of lives, not to mention other damage, remains, and this is very disturbing.

While a high intensity earthquake is inevitably going to cause very large-scale loss in terms of its direct impacts, the additional possibility of secondary damage—quake leading to collapse of a large dam and this in turn leading to even more than the direct damage caused by quake—is mind-boggling.

So the question arises why we keep on adding such risks. With China going in for very large-scale dam construction activity on the Brahmaputra this question becomes even more important as those who can experience harm do not even have  access to essential authentic information about these dams and how the more crucial safety questions have been addressed.

The second aspect of the question relates to the possibility of one or more dams in a region themselves becoming the cause of triggering seismic activity in an area. This is referred to as the phenomenon of  Reservoir Induced Seismicity or RIS which has been receiving a lot of attention of geologists and seismologists.

Explaning the basics of RIS, a prominent geologist Prof. K. S. Valdia has written, “Analysis of seismic date from a number of reservoir sites all over the world such as Hsinfengkiang in China (1961, M-6.1), Koyna in Maharashtra (1967, M-6.7), Katiba in Zimbabwe (1963, M-5.8), Gorancarevo in Yugoslavia (M-4.5), Vajont in Italy (1963), Monteynard in France (M-4.5), Mead in California, USA and 300 other cases has established beyond doubt that seismicity is induced soon after the impounding of waters behind the dams, even in essentially aseismic areas. It was noted that the degree of seismic activity increased as the level of the water rose, the strongest shock registered at the peak water level point.”

Dr. Harsh Gupta of the Centre for Earth Sciences, Trivandrum says worldwide statistics have confirmed positive correlation between height of the water column in the reservoir and RIS. For example 6 out of the 20 reservoirs with heights between 150 and 250 meters have witnessed RIS i.e. 30% whereas for reservoirs having heights between 90 and 120 meters RIS has been observed in only 6% cases.

In the case of dams on Himalayan rivers, this controversy has a special significance. Former Chairperson of the Central Water Commission Y.K. Murthy has written, “Earthquakes are known to have accompanied the construction of a number of dams in different parts of the world. While there is no complete agreement among the geologists and seismologists on the exact cause of this seismic activity, there is a consensus that the seepage of water from the impoundments in vulnerable geological formations helps to trigger off seismic activity. The Himalayas being geologically very young and active, the effect of creating lakes on seismic activity in the region would have to be carefully studied.”

Valdiya writes in the context of this region, “A majority of the 200 odd hydel projects that have been built or are being constructed or planned in the southern front and the interior are located not far from the seismically and tectonically active zones of boundary thrusts characterized by strongly deformed and weakened rocks. The hundreds of transverse faults that tear the Himalayan terrain into blocks and segments are seismically more active than the boundary thrusts. Many of the rivers emerge into the plains through narrow valleys controlled by these faults. It is established that larger dams are capable of inducing and increasing seismicity of quite larger magnitudes.”

Another aspect that needs to be considered is the RIS in the context of the impact of not a single project but the combined impact of several projects or a series of dams on a river and in a region, the various projects being located rather close to each other. Are there adequate efforts for studying RIS in this context in important areas where this is badly needed.

Both these aspects of the relationship between dams and earthquakes should receive  more careful attention.

Bharat Dogra is a journalist and author. His latest book is Vimla and Sunderlal Bahuguna—Chipko Movement and The Struggle Against Tehri Dam Project.


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