The Tehri turnaround

Published : May 12, 2001 00:00 IST

The issue of purity of Ganga jal puts a bigger question mark on the Tehri dam than what the seismic controversy has done to the progress of the dam in the last two decades.

YET another review of the Tehri dam project has begun following a government order of April 10. Perhaps no other national project has seen as many technical reviews as the 260.5 metre high Tehri rock-fill dam project on the Bhagirathi has. The current one, which has been set up under pressure from the Vishwa Hindu Parishad (VHP), almost borders on the farcical. When the issue of seismicity seemed to have been settled after a February 1998 report of a five-member Group of Experts (GOE) was considered by the government and the go-ahead for resuming construction was given in February 1999, the VHP raised the spectre of the dam sullying the Ganga.

The anti-dam lobby led by environmentalist Sunderlal Bahuguna found a convenient ally in the VHP. Having lost the premise of the seismicity argument after the GOE - members of which were nominated on the suggestion of Bahuguna himself - unanimously concluded that "the present design of the dam is expected to be structurally safe to withstand the maximum credible earthquake (MCE) during the economic performance life of the dam-reservoir system", Bahuguna has now begun to play the "Gangatva" card of Ashok Singhal, the VHP leader (Frontline, February 16). It is significant that in February Bahuguna, along with VHP leaders, had met Prime Minister A.B. Vajpayee seeking his intervention to get the project reviewed.

The formation of the new review committee can be traced to Singhal's threat on March 26 to go on an indefinite fast. His tactics worked and the government assured him that a new committee would be constituted under Murli Manohar Joshi, Minister for Human Resource Development (Frontline, April 14). Bahuguna had also threatened to go on fast from April 12 if work on the dam continued.

The notification announcing a fresh review just two days before that date clearly indicated that the government was giving in to the VHP's threats. According to the Tehri Hydro Development Corporation (THDC), however, since the terms of reference of the review do not involve stoppage of construction, work has continued though it had slowed down in between when protesters blocked work. They were arrested and removed.

The earthquake of January 26 has also come in handy for Bahuguna and others to raise the spectre of seismicity again. In recent times Bahuguna has also begun to raise the issue of security. Since the dam site is in a border State, the security aspect needed to be looked into as well, he said. While the government seems not to have attached importance to the last of the points, it has yielded on the other two, namely the purity of Ganga jal and the earthquake.

The April 10 notification by the Ministry of Power (the apex Ministry for the THDC) says: "In view of the continuing concern relating to the seismic safety of the Tehri dam, a Group of Experts was constituted in June 1996 to further examine the issue of seismic safety of the Tehri dam. This Group in its report concluded that the design of the dam was safe to withstand the MCE. However, in the wake of the Bhuj earthquake of January 2001, the need for review of seismic safety of the Tehri dam is felt necessary. Further, concern has also been expressed about the possible impact of Tehri dam on the self purification quality of Ganga jal."

These two new issues constitute the terms of reference of the new 11-member review committee that includes the Director-General of the Council of Scientific and Industrial Research (CSIR); the Chairman of the Central Water Commission (CWC); the Director-General of the Geological Survey of India (GSI); the chairman of the Central Pollution Control Board (CPCB); the director of the National Environmental Engineering Research Institute (NEERI); C.D. Thatte, former Secretary of the Ministry of Water Resources; K.S. Valdiya, geologist at the Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore; T. Shivaji Rao, Professor of Environmental Studies, Geetam College of Engineering, Visakhapatnam; U.K. Chowdhury of Varanasi; and Guru Das Aggarwal of Vridda Sewa Sadan of Chitrakoot, Madhya Pradesh. The committee is expected to submit its report within four weeks.

Curiously, considering that it is a major project of the Ministry of Power, there is no one from the Ministry in the committee. It is also equally curious that a Minister should chair a technical review committee.

From a technical perspective this review is particularly farcical because the apparent purity of Ganga jal is not a new issue. It has been part of the Hindu psyche and belief for thousands of years. If the impact of the dam on purity was an important issue, it should have been considered at the stage of the dam proposal in 1969 itself.

The earthquake can have no relevance whatsoever for the seismicity of and the impending earthquakes in the Himalayan region. This is particularly so in the case of Tehri because the tectonic features of the two regions are totally different. If the Bhuj earthquake warrants a review of the Tehri project, any major earthquake anywhere else in the world should be reason enough for a review. Obviously that would be unreasonable. It would then seem that the purity of Ganga jal is the dominant issue for the present government, and the earthquake has been thrown in for good measure to give the re-appraisal the respectability of a technical review.

THE Bhuj earthquake is of no consequence to the Tehri dam design. The seismic belt along the Himalayas lies on the boundary of the peninsular Indian tectonic plate, which is incessantly thrusting northward under the Eurasian plate. As a result of this under-thrusting, geological features known as "thrust faults" have formed all along the plate boundary which cause the built-up stress to be released at various points along this belt in the form of earthquakes. That is, Himalayan earthquakes are linked to "inter-plate" tectonics. In contrast, the Kutch region is within the Indian plate and the stress release within the plate occurs along "intra-plate" tectonic fractures known as "strike-slip faults".

The mechanisms of energy release in the two are quite different and in intra-plate earthquakes large magnitude earthquakes (8+ on the Richter scale) are unlikely. In fact, the intensity of the recent earthquake (of magnitude 7.7) is itself regarded as extremely rare. Kutch is in earthquake Zone V (the most quake-prone with an intensity rating of IX and above on the Modified Mercalli Scale) whereas Tehri falls in Zone IV (corresponding to an intensity rating of VIII) as per the classification of the Bureau of Indian Standards (BIS). Indeed, while the Kutch region has had a major earthquake in 1819 to which the present one is believed to be similar, Tehri has no history of a large earthquake. Both the Uttarkashi earthquake (magnitude 6.6) of 1991 and the Chamoli earthquake of 1999 (magnitude 6.8) occurred north of Tehri.

The most important point, however, is the fact that the Tehri dam design has been tested for much more severe ground motion than what was experienced during the Kutch earthquake. Based on an exhaustive field survey carried out independently by the GSI and the Gujarat government under the guidance of experts on disaster mitigation, an upper bound of rating X has been suggested for the intensity of the Bhuj earthquake. Empirically, an intensity of X corresponds to a horizontal ground motion acceleration of about 0.6 g or 60 per cent of the acceleration due to gravity. The Tehri dam has been tested for 'the worst case scenario' as postulated by the GOE, which had far more severe ground motion and has been found to be safe.

Indeed, since 1989, when the Department of Earthquake Engineering first tested the dam for 7.2 magnitude earthquake with an effective peak ground acceleration (EPGA) of 0.25 g, the seismic stability of the dam design has been tested repeatedly against different seismic parameters, each time the worst case scenario being revised upwards. A recall of the various tests the dam design has been subjected to over the years culminating in the most severe ground motion test as desired by the GOE would place the safety design of the Tehri dam in the proper perspective.

Until the constitution of the GOE, the most stringent test was the one carried out in 1991 against the actual accelerogram of the Gazli earthquake of 1976 which had a peak ground acceleration (PGA) of 1.36 g in the vertical and 0.72 g in the horizontal, both acting simultaneously. This was done at the suggestion of geophysicist V.K. Gaur, a member of the high-level committee (HLC) of 1990 that went into the seismic safety of the dam. Gaur was the sole dissenter to the conclusions of the committee which held that the dam was safe. When the dam was found to be safe against the severe ground motion of the Gazli earthquake too, the government cleared the project on March 15, 1994.

But Bahuguna, aided by the remarks of Gaur and other opponents of the dam, continued to raise the bogey of seismic risk to the dam, and exactly a year after the government approval went on a two-and-a-half month fast at the dam site. Gaur had been critical of the Gazli test saying that the test had been carried out only for 12 seconds shaking whereas vibrations in a 8+ magnitude earthquake at Tehri could last much longer. The Gazli test was repeated in deference to Gaur's views in July 1995 by applying the Gazli accelerogram three times so that the effective duration of the shaking lasted for 42 seconds. The dam was found safe even against this test.

Supported by political leaders, Bahuguna continued to demand a technical review - to which the government yielded and constituted the GOE consisting of members recommended by Bahuguna himself. They included Gaur, K.N. Khattri of the Wadia Institute of Himalayan Geology, the Director of the Central Building Research Institute (CBRI), Roorkee, R.N. Iyengar, and Ramesh Chandra of the Department of Earth Sciences of the University of Roorkee. None of the four is an earthquake engineer and all of them are opposed to the Tehri dam. However, in order to offset the biased character of the GOE, the government later included, again on Bahuguna's suggestion, N.C. Nigam, an expert in earthquake engineering and former Vice-Chancellor of Roorkee University.

After considering the performance of the dam in various earlier studies with varying seismic parameters, the GOE concluded that a comprehensive appraisal of the seismic safety of the dam required the completion of two more key studies, "employing recent advances in conceptual and computational capabilities": (i) quantitative estimate of seismic hazard at the Tehri dam site; and, (ii) evaluation of the performance of the dam if it was exposed to the estimated seismic hazard at the site. Towards this, the seismic parameters for the MCE were evolved by Gaur, Khattri and Ramesh Chandra, three of the five members of the GOE, in a base paper.

These, according to A.R. Chandrashekaran, former Professor of Earthquake Engineering at Roorkee University, are the most severe ground motion parameters at any dam site in the world. For these parameters, the dam sections were analysed for two-dimensional linear and non-linear behaviour and the entire dam analysed for 3-D linear behaviour.

Based on the results, the GOE unanimously concluded that the present design of the dam is safe against an MCE as defined by Gaur, Khattri and Ramesh Chandra. However, there was difference of opinion among the members when it came to the final recommendations. As a result, two reports were submitted: one signed by only Nigam and the other signed by opponents of the dam: Gaur, Khattri, Iyengar and Ramesh Chandra (Frontline, June 6, 1998). The latter included two additional recommendations, with which Nigam disagreed, for conducting additional studies as a matter of abundant caution: (i) 3-D non-linear analysis of the dam to evaluate its performance against the MCE; (ii) a simulated dam break analysis to ensure that in the unlikely event of an uncontrolled release of water, the consequences are minimum.

The report(s) of the GOE, submitted to the government on February 18, 1998, was considered by the National Committee on Seismic Design Parameters (NCSDP), a standing technical body constituted by the government that clears seismic design parameters for all water resource projects. The NCSDP, in fact, observed that the 'worst case' earthquake postulated as MCE by Gaur and associates was against the accepted principles for design of dam structures as per International Code of Practice.

The NCSDP concluded that since the dam has been tested for very severe seismic hazard, carrying out 3-D analysis was not necessary. In any case, 3-D analysis is not a fully developed technique, particularly for rock-fill dams, and is not a global practice. According to Chandrashekaran, in the case of Tehri a 2-D analysis is bound to be more conservative because of the peculiar S-shaped canyon at the site because of which the abutting mountains on the sides offer extra stability to the dam by bearing a substantial part of the load. In any case, he says, theoretical studies on non-linear analysis have indicated that 2-D behaviour gives results comparable with 3-D analysis.

The NCSDP ruled out dam break analysis (DBA) as well for good reason. To carry out such a study one needs to know the failure mode. Since the dam was not breaking even for the most severe ground motion, it can only be hypothetical and it is up to the critics to come up with a credible failure mode and the nature of dam breaking. DBA is usually carried out in cases where a review indicates insufficiency of original design and a failure mode can be identified.

The report of the GOE and the NCSDP recommendations were considered by the Committee of Secretaries on January 11, 1999, and it was decided that the NCSDP recommendations be accepted and the additional studies recommended by four members of the GOE need not be undertaken. On February 1, 1999, an official notification accepting the unanimous recommendation of the GOE was issued, after which work on the dam has been going on in full swing. In the light of the above, the turnaround on the seismic issue is clearly unwarranted.

The issue of purity of Ganga jal can be addressed if the contention is made in scientific terms. But the arguments on this issue are likely to be made on the grounds of faith and religion rather than any scientific reasoning. If reason was to have any role in the matter, the VHP (and Bahuguna too) would first try to work towards ensuring that Tehri town does not disgorge garbage into the Bhagirathi. But reason is at a premium where emotions, and that too religious, rule. Indeed, it is not unlikely that, like in the Babri Masjid affair, the VHP will claim that purity of Ganga jal is an integral part of the Hindu belief and cannot be decided through scientific deliberations, committees and technical reviews. If that happens, there will be a bigger question mark on the Tehri dam than what the seismic controversy has done to the progress of the dam in the last two decades.

Sign in to Unlock member-only benefits!
  • Bookmark stories to read later.
  • Comment on stories to start conversations.
  • Subscribe to our newsletters.
  • Get notified about discounts and offers to our products.
Sign in


Comments have to be in English, and in full sentences. They cannot be abusive or personal. Please abide to our community guidelines for posting your comment