Calcutta Notebook


In the context of the recent disaster in Uttarakhand there is increasing concern about safety issues concerning the construction of more and more dams. One of the main safety violations pointed out in Uttarakhand has been that very large scale use of explosives at the time of heavy construction activity had damaged the fragile geological formations in these hills. So hills and houses were tumbling down to a much greater extent than would have been the case if hill-stability had been maintained. This happened at the time of Uttarkashi earthquake earlier and also in the recent disaster.

The situation in Sikkim was found to be quite similar at the time of a recent earthquake. Here a series of tunnel dams and hydro-electric projects were being constructed. Local villagers had been upset with this heavy construction work in these fragile hills and the large scale use of explosives for the same. The earthquake came at a time when hills were already badly destablised and highly vulnerable. This is why the earthquake caused a lot of damage in villages and settlements close to the construction sites. Hills which were hollowed and weakened by tunnel construction were particularly vulnerable.

Now that the hazardous nature of these projects in a high seismic zone has become more apparent in a very tragic way, it is hoped that the authorities will agree to a careful review of the series of dam projects in the Himalayas with a special emphasis on the potential of hazards and ecological disruption.

In other cases of big storage projects, some of which are planned in the North-East region, an additional factor that has to be carefully considered is RIS or reservoir induced seismicity. This should be adequately considered first at the evaluation stage, and later when a project is completed, there should be adequate arrangements for monitoring any evidence of RIS.

Geologists and seismologists have debated for long the link between the impoundment of water in reservoirs created by dams and earthquakes. The generally accepted view is that in certain favourable conditions these artificially created lakes can cause quakes of relatively smaller magnitude. This is called the phenomenon of reservoir induced seismicity or RIS.

However more recent studies based on a 7.9 intensity quake near Zipingpu dam in China have revealed the possibility of RIS related quakes of higher intensity as well. Above all, it is the recent experiences of the giant Three Gorges Dam Project in China which have focused attention on how serious the threat of RIS can become. A study by the seismologists at the China Earthquake Administration revealed that monitors around the reservoir and in Hubei province registered 3429 earthquakes between June 2003 and December 31, 2009. According to Patricia Adams of Probe International, "This represents a 30% increase in frequency over the pre-dam period."

Available studies indicate that RIS tremors in Three Gorges areas triggered so many landslides that nearly 300000 people had to be evacuated. Even though so far the RIS quakes of this area have been of lower intensity, a senior engineer of Sichuan Geology and Minerals Bureau has warned that "strong earthquakes could occur in the future as the reservoir fills because the micro-fractures, caused by the large number of micro-earthquakes, could make the area dangerously prone to a strong earthquake."

In India the possibility of these hazards was recognised following the devastating earthquake which struck Koyna-nagar (Maharashtra) on December 10, 1967. This region had earlier been a stable non-seismic one, but experienced a series of tremors after the impounding of the Koyna reservoir in 1962. In this earthquake nearly 200 lives were lost, 1500 people were injured and thousands were rendered homeless and more than 80 percent of the houses in Koynanagar were either completely destroyed or became uninhabitable. Bombay and its suburbs were also rocked, industry was paralysed and generation of electricity was disturbed.

The controversy of dams-induced quakes later received a new impetus following a series of tremors in Khardi village (North Konkan area of Mahara-shtra), just eight kilometres away from Bhatsa Dam. After the Koyna earthquake of 1967 and its preceding and succeeding tremors, Khardi was the first major instance in India of seismic activity in a region which had long been regarded as a non-seismic zone. The sequence of events seems to suggest that there was a strong connection between the rising water level in the Bhatsa lake and, the tremors at Khardi.

According to noted geologist Dr S K Guha, from a review of case histories of induced seismicity in the world, it has been observed that significant seismic activity follows large fluctuations in lake levels. Larger earthquakes in the Koyna region had occurred in the post-monsoon months of October to December. In Khardi, the activity showed an enhancement following large changes in the level of the Bhatsa during the month of July.

One view emerging from research on reservoir induced seismicity is that the massive weight exerted by the impounded water on land hitherto unused to anything like this sets in motion a series of geological changes which may culminate in destructive earthquake. Another contributing factor may be the existence of active faults.

However the biggest issue in dam safety is the possibility of dam failure. During the last 65 years nearly 50 dams have collapsed in India. One of the biggest tragedies caused by dams in India, the Machu dam disaster of August 1979 killed several hundred people and destroyed Morvi town as well as several villages. According to the Consumer Education and Research Centre, Ahemdabad, Machu dam was designed on the basis of outdated empirical formula. Although in the course of the project study conducted before constructing this dam the Central Water and Power Commission had thrice asked the state government to check on a more scientific formula for the construction of the dam, the government went ahead with the construction of dam without having carried out this exercise. It was only at a very late stage when a major share of the dam had already been constructed that this exercise was conducted, and then somehow the findings were tailored to fit in with the almost already constructed dam.

Vol. 46, No. 44, May 11 -17, 2014