The Tehri Dam Project (TDP) constructed in Garhwal region of Uttarakhand is one of the most discussed dams of the world in the context of the debates on dam safery. The salient features of this project are :-
A 260.5 meters high earth and rockfill dam, across the river Bhagirathi at Tehri just after its confluence with river Bhilangana for creating a storage reservoir to generate power and provide irrigation facilities.
An underground power house of 1000 MW capacity with reversible pump turbine sets under Stage II and Tehri (pump storage plant).
A 103.5 meter high concrete dam (which will provide a balancing reservoir) with a surface power house of 400 MW (4×100 MW) capacity at Koteshwar, 22 KM downstream of Tehri Dam Site, across river Bhagirathi.
Transmission system for evacuation of power generated at Tehri and Koteshwar projects, through two single circuits 765 KV lines to Meerut for further utilisation in Northern region.
However, it is important to point out that the benefits of TDP mentioned above by project implementation authorities have been questioned by important persons and committees within the government.
The Tehri Dam Project was comprehensively assessed by the Environmental Appraisal Committee (River Valley Projects) of the Ministry of Environment and Forests, Government of India. This report was submitted in February 1990. This Committee included experts in the fields of water resource management, soil conservation, agriculture, ecosystems management, forest ecology, anthropology, civil engineering etc.
The Committee was guided by the following concerns :
- a) Whether the risks and hazards attendant to the project are acceptable in terms of their magnitude and in terms of the level of preparedness of the project authorities either to prevent them altogether or at least to minimise them to an acceptable level.
- b) Whether the ecological and social impacts of the project have been adequately studied, and the environmental and rehabilitation action plans adequately developed for prevention and mitigation of adverse impacts and
- c) Whether the project, as proposed, it conducive to the optimal use of natural resources foe economic development and human welfare.
After examining the available information and data in detail, meeting several experts and social activists, and visiting the dam site area and villages the committee concluded –
From the assessment of the data furnished and detailed discussions held with concerned agencies and experts, it became evident that answer to all these questions is in the negative. The committee has come to the unanimous conclusion that the Tehri Dam Project, as proposed, should not be taken up as it does not merit environmental clearance.
Although the EAC Report examined all important aspects of Tehri Dam project ) it gave its opinion that safety factors alone are important enough to stop the clearance of this project. It says,
“Taking note of the unacceptable risk involved, extremely poor status of readiness to deal with the hazards and unprecedented damage in case of a breach or overtopping the Committee reiterates its considered view that it would be irresponsible to clear the Tehri dam as currently proposed.”
More specifically, the committee says about the hazards of TDP,
“Therefore, considering the almost total certainty that a strong earthquake of magnitude greater than 8.0 on Richter scale will occur in the region during the life of he dam, and considering that the dam design does not provide for such an earthquake the Committee has no option but to conclude that construction of Tehri dam, as proposed, involves totally unjustified risks. The magnitude of the disaster that would follow, if the dam collapsed, strengthens the Committee’s opinion that approval to the construction of this dam, as proposed, and at the present site, would be irresponsible.”
The Tehri Hydro Development Corporation (THDC) had not given the necessary attention to the hazards and risks of TDP. This report says, “Though despite repeated requests, the THDC did not provide the committee with risk analysis in terms of the impact of dam failure on the life, property and cultural heritage, our own tentative calculations suggest that if the Tehri dam collapsed, it would cause flood wave which would wipe out Rishikesh and possibly Haridwar. This wave would wash away most of the settlements around this region.”
The stability of adjacent hills is also a crucial factor from the point of view of safety. This report says.
The stability of the abutments, adjacent hills and the reservoir rim have an equally important role to play.
“It is not clear whether an earthquake of magnitude 8.5 could be tolerated by the hill sides and the reservoir rim. This, in any case, has not been tested or established. The question of rim stability is especially important because if the hillsides collapse due to an earthquake the implications can be as bad as if the Dam collapsed.
A famous example is that of the Vajont Dam, in Italy, when collapse of a hillside into a reservoir caused a huge wave which over-topped the dam and instantly killed a large number of people even though the concrete dam itself survived.”
Regarding the reservoir induced seismicity (RIS) this report says, “Frequent occurrence of tremors caused by RIS would become a source of hazards to the population in the neighborhood in the reservoir.”
Yet another dam-related risk relates to the possibilities of flash floods. The report warns,
“One of the benefits claimed from this multipurpose project is the moderation of flood and the Tehri dam is, therefore, stated to be provided with surplusing arrangements for a discharge of about 15400 cumecs, even after providing 5 M.ht. for flood moderation. The normal flood in the valley below is now reported to be in the region of 3500 cumecs. Therefore, the release of 15400 cumecs of water will certainly bring much larger areas under the fury of floods downstream. The situation would get further aggravated if Alaknanda also happens to be in spate at the same time.”
But preparations to cope with the new threat are not being made or provided for. As the reports observes,
“The increased intensity of flood flow calls for a critical examinations of the safety, and for remodelling, if necessary, of existing public structures like bridges, roads, regulators, barrages etc. downstream. The committee was not informed of any such scrutiny for modifications to existing structures, with financial implications, by the project authorities.”
On the whole the project poses serious risks for downstream people – Since the Tehri Dam site is located in a geologically unstable area and has a high hazard potential, the community downstream would live in perpetual apprehension and fear for their life and property unless all efforts are made to effectively demonstrate that such fears are totally unjustified.
According to a prominent seismologist Prof. James N. Brune,
“We have to conclude that the proposed Tehri Dam’s location is one of the most hazardous in the world from the point of earthquakes. There is little question that in terms of the hazard rating of the international Commission on Large Dams (ICOLD), its hazard class is extreme.
No large rock-fill dam of the Tehri type has ever been tested by the shaking that an earthquake in this area could produce, and thus we have little basis for confidence as to how the dam would perform. Given the number of persons who live downstream, the risk factor (in the ICOLD classification) is also extreme. In such circumstances the ICOLD declares that a fully state-of-art dynamic design analysis of the dam, in response to specified acceleration time histories, is mandatory.”
A critical factor on the basis of which clearance was given to Tehri Dam Project by a special High-Level Committee of the Government of India (after the Environment Appraisal Committee had refused to clear the project) was the calculation of the Peak Ground Acceleration (PGA). This calculation was based on the work of Prof. James N. Brune of University of Nevada, USA. However, when this calculation was shown to Prof. Brune, he responded, “The calculation used …… is unfortunately used in an incorrect, out of context and out of date manner and, therefore, in my opinion no weight can be given to the resulting calculation of a maximum acceleration of 0.22g.
“Further, I would like to add my concern over the simple geologic interpretation given in the report forwarded, namely that the risk can be assessed by considering only the simple case of the main fault. In thrust faulting situation, there are often multiple intricate faults and thus a branch fault of the main fault is likely to come much closer than the main fault and might cause high accelerations at high frequency and possibly local ground rupture. It was the possibility of a local ground rupture which led to cancelling plans for Auburn development in the US.
“I believe there is no chance that a dam in similar circumstances as Tehri dam could be licensed for 0.22g in the US. I have discussed the matter with several of my colleagues and they all consider 0.22g as too low a design acceleration for Tehri Dam.”
On the basis of these comments a prominent expert involved in assessing TDP (Prof. Gaur) wrote to the High-Level Committee which finally cleared TDP,
“It is quite clear to me that the issues raised by Prof. Brune are extremely critical. They are ‘new’ in the sense that they did not receive a critical scrutiny at time of our earlier deliberation or else their far reaching implications would not have been missed. And in case they were to be discounted, a scientific critique would have been warranted to justify their omission. This as you know, did not happen.”
Hence it is clear that the TDP was constructed by the authorities on the basis of ignoring the reports of their own experts committees and misrepresentation of the views of prominent experts. Therefore very important precautions regarding its safety aspects are needed.
Bharat Dogra is a veteran journalist and author. Before the construction of the TDP he wrote over a hundred articles and several booklets in English and Hindi on why this project should be given up. His recent books include Planet in Peril and Vimla and Sunderlal Bahuguna—Chipko Movement and the Struggle against Tehri Dam Project.