Assam's Lower Subansiri hydel project can learn lessons from Tapovan Vishnugad disaster

It is election time in Assam and the high and roaring waves of the Dhauliganga river that ravaged the Tapovan Vishnugad Hydro Power project at Joshimath in Uttarakhand's Chamoli district on Sunday were too small for the northeastern state's ruling class to bother with. Not that the flash flood in Uttarakhand would have reached Assam, but the state has the perfect recipe for far more severe circumstances in the making right in its own backyard.

The NHPC-owned Subansiri Lower Hydro Electric Project, which is the biggest hydroelectric project undertaken in India so far and is a run of river scheme on the Subansiri, located at Gerukamukh in Assam's Dhemaji district on the border of Arunachal Pradesh and Assam, also holds the dubious distinction of having the highest cost overrun among under-construction hydel projects in the country. The cost overrun is at a massive Rs 13,211 crore for a project whose original estimate was pegged at Rs 6,285 crore when it was conceived in 2002 and now the revised estimate puts the total cost at Rs 19,496 crore. Not just the financial cost alone, but time overrun has also been huge for the project as the original schedule of commissioning it was back in September 2010.

According to the reasons for time and cost overrun provided to Lok Sabha by the Ministry of Power on 25 July 2019, there are many factors behind slowing down the project. They are:

• Delay in the transfer of forest land
• Disruption of works by locals in Arunachal Pradesh side
• Slope failure in Power House in January 2008
• Damage to the bridge on Ranganadi river
• Change in design of surge shafts to surge tunnels
• Stoppage of works due to agitation launched by anti-dam activists in Assam against the construction of the project. Stoppage of work since 16 December 2011
• Issue of D/s Impact
• Case in NGT

However, during a visit to the Subansiri Lower HE Project site by Union power secretary Sanjiv Nandan Sahai on 4 December last year, NHPC CMD AK Singh assured that the project shall be commissioned by March 2022. Contrary to that the Ministry of Power had informed the Lok Sabha that the project will take four more years to be completed from the day work restarts. The work has restarted in the project site in October last year. The company claims that 50 percent work of the project has already been completed.

The decks for the resumption of construction works were cleared after the National Green Tribunal did not "find any merit in the applications" seeking the reconstitution of the Committee constituted by the Ministry of Environment, Forest and Climate Change (MoEF&CC) vide order dated 27.11.2017 in pursuance of the order of this Tribunal dated 16.10.2017 and the same were dismissed on 31 July 2019.

Genesis of the project

The Lower Subansiri HEP consists of one Concrete Gravity Dam of 116-metre high above the river bed level and 123-metre high above the deepest foundation level. It will have a surface powerhouse with an installed capacity of 2,000 MW with eight units of 250 MW capacity each. The project received its environment clearance through Letter No. J-12011/40/2001-IA-I dated 16 July, 2003 and the construction work on the project began in January, 2005.

Authors Swapnali Barman, RK Bhattacharya, SK Sharma and G Tirkey in their joint study Future flow scenario of Subansiri River and its Impact on Power Potential of Subansiri Lower Hydro Electric Project gave a little introduction to the river itself.

"The river Subansiri is one of the major north bank tributaries of river Brahmaputra. It flows through Tibet for 170 km, 250 km through Arunachal Pradesh after which it enters Assam at Dulangmukh in Dhemaji district and flows for another 130 km through the plains of Assam before it merges with the River Brahmaputra near Jamugurighat in Assam. The Subansiri river system has its practical importance as it holds high water resources as well as hydropower potential for the country," the paper said.

Going back on how the initial thoughts for the project came, senior practicing engineer JN Khataniar said it was the Brahmaputra Board that came up with the idea first.

"The possibility of hydropower generation using plenty of water of the river Subansiri was initially investigated by the Brahmaputra Board during 1982-84 and finally submitted the detail project report in 1985 as a multi-purpose 2,000 MW single hydropower project having a 275-metre height Rockfill Dam and an underground powerhouse due to high seismicity and land slid sensitivities of the entire region," said Khataniar from Guwahati.

He said that in 2000, NHPC took over the findings from Brahmaputra Board and prepared a new project concept for a three-stage hydropower project after conducting its own investigation. The NHPC recommendations were:

  1. The upper stage: 2,000 MW with a dam height 214m from bed level and flood cushioning 10.00m
  2. The middle stage: 1,600 MW with a dam height 203 m from bed level with flood cushioning 15.00m
  3. The lower stage: 2,000 MW with a dam height 116m from bed level without having any flood cushioning as upper and middle stage flood cushioning are enough

Barrage of technical issues

The insincerity of NHPC is exposed right from the location of the project as mentioned on the company website with no clear indication of the dam site and rather vague details were given. There is no mention of Gerukamukh at all at least at first sight. From the beginning itself, the project has been engulfed in controversies with serious flaws being pointed out to its design and structure by various experts and panels.

In the 44th meeting of the Expert Appraisal Committee on River Valley and Hydro Electric Projects constituted under the provisions of EIA Notification 2006 which was held on 11 December, 2010 in New Delhi, the safety and stability aspect of the Lower Subansiri HEP dam came under deep scrutiny.

"The site specific seismic design parameter for peak ground acceleration (pga) value of 0.38g considered by NHPC is inadequate as the whole of the North East falls under Zone V – (very high damage risk zone) with probability of earthquakes of MSK IX or more. Previously the earthquakes of 8.7 (Shillong earthquake) and 8.5 (Assam earthquake) on the Richter scale have been experienced. In view of this, the design parameters need to be reconsidered to be at least 8.5 M at pga of 0.5g. The broad scale data provided by NHPC is shockingly lacking and there are gaps in the data. A mega earthquake of 1947 has not been reported. Therefore, lithologically and seismologically it needs deeper investigation, not only for lower but upper Subansiri too. A cumulative effect study should be conducted," the minutes of the meeting noted.

Also Read: Tragedy at Tapovan Vishnugad hydel project: Understanding hydrology, geology of Himalayas is need of the hour

In the same meeting itself, the geological deficiencies in the project were also pointed out.

"The rocks in the foot hills are poorly cemented sand stone on which the dam would be constructed. This also seems unstable even at pga of 0.38 g, especially for the surrounding ground. A fault along the Subansiri at about 2 Km downstream of the dam has also been inferred by GSI. Therefore, the indications of recent tectonics in the area give ample reason for giving a serious relook at the selection of the site for the dam. The Himalayan Foothills, south of MBT (Main Boundary Thrust) is not suitable for any mega hydropower project from the geological, tectonic and seismological point of view," the representatives of the Expert Group from IIT Guwahati, Gauhati University and Dibrugarh University said.

The change in the height of the dam from the originally designed 133-metre above the deepest foundation level including the foundation depth of 17-metre to the newly modified 125-metre by reducing the foundation depth to 9-metre was heavily criticised by the experts. They felt that "the ratio of total dam height to the foundation is inadequate" and "would make the dam highly vulnerable seismically especially when an earthquake of intensity 8.5 in the Richter scale has already occurred in that region in 1950".

NHPC even neglected the flood data of the area where the maximum observed flood at Chouldhowaghat was 21,230 cumecs on 11 July, 1971 but the public sector enterprise only considered 18,790 cumecs as the highest flood level.

What is even worse, the study conducted together by Barman, Bhattacharya, Sharma and Tirkey warned of a looming crisis downstream after the dam comes up fully.

"Devastating flood associated with intense rainfall in the upper catchment of Subansiri is a common phenomenon. There is a possibility that the construction of the dam will further increase the downstream discharge leading to the downstream areas more susceptible to flood and erosion," the study said.

The Thatte-Reddy Expert Committee appointed by the Planning Commission in its report in July 2012 came down heavily on NHPC saying that flood regulation "is the single most important aspect of project planning that has got ignored by the planners while converting single high dam to a cascade scheme of dams (from downstream to upstream)… the very purpose of the Brahmaputra Board Act is defeated and the mandate of the Brahmaputra Board diluted by this action".

The report further said that "The sandstone, which really looks and behaves like a sand rock, on which the dam is founded, has all through the SLP (Subansiri Lower Project) planning been considered very weak. Its adequacy and competence to support the concrete gravity dam is not established satisfactorily."

In no uncertain terms, the report said that the frequency of landslides could grow after the project is completed. What landslide lake outburst flood or a glacial lake outburst flood can do to a hydropower project as the case may be was recently seen in the devastation caused in the Tapovan Vishnugad Hydro Power project at Joshimath in Uttarakhand. A total of 34 people have died so far and a frantic search for the 170 still missing is on in adverse conditions.

With the multi-purpose dam now converted into hydro-power projects, the Thatte-Reddy panel said that only with an independent authority like Subansiri River Basin Authority having operational control of the reservoir can help in mitigating floods when all the three projects — SLP, SMP (Subansiri Middle Project) and SUP (Subansiri Upper Project)— are completed.

"Unfortunately, the execution of the Upper and Middle Stage as was originally being allotted to NHPC Ltd as per their investigation reports was withdrawn from them due to reasons unknown and finally handed over to other two agencies who have not been able to submit the DPR (Detailed Project Report) in a proper scientific manner till date,"
said Khataniar.

Diagram courtesy JN Khataniar

He pointed out a key point that no flood moderation for independent Subansiri Lower HE Project was conceptualised and that the Central Electrical Authority in its Techno-Economic Clearance had made it abundantly clear in 2003 that integrated flood studies including reservoir operation for all three projects — Subansiri upper, middle and lower — shall be done in consultation with the Brahmaputra Board and Central Water Commission before executing the Subansiri Lower Hydro Electric Project.

"There are possibilities of tremendous upstream impact on the most unhealthy 116-metre Concrete Gravity Dam structure in Lower Subansiri Hydro Power Project for the sudden loads of excess water that may occur due to heavy rains, cloud burst, or massive landslide. In the absence of other two projects over the same river as conceptualised as an integrated project, there are insufficient cushioning provisions above the dam structure to take care of probable floods," Khataniar said.

Modified design of Lower Subansiri hydel project concrete dam by expanding the base width up to 271.00 metre with respect to the fixed height 116.00 metre, more than 233% base with against normal 80% with respect to height. Diagram courtesy JN Khataniar

"We have already seen what environmental disasters can do in the recent catastrophic event in Uttarakhand despite the dams being made as per the recommendations. In the case of Lower Subansiri HEP, who will take responsibility for the retrofitting processes at the dam and guarantee the safety of the people living downstream? Who will take accountability for the inadequacies in concepts and design of the dam structure for which we are losing a huge amount of public money and destroying a natural water resource?" he asked.

Water resources engineer Pradip Pujari said that although one dam was completely lost in Uttarakhand, another dam downstream actually prevented a far bigger catastrophe by arresting the massive flow of water.

"The first dam could not absorb the immediate thrust but the second one managed to neutralise the threat because the force of the water had also gone down by then," Pujari said.

This makes the dams in the Upper and Middle Subansiri even more critical but they are yet to be constructed thus exposing the Lower Subansiri dam to grave threats.

In a study, titled Lower Subansiri Hydroelectric Power Project and future of the Subansiri River Ecosystem, the shoddy survey of the environmental impact of the project has also come under severe criticism.

"It becomes evident that no sufficient and detailed study was done in the entire downstream during the preparation of the report. Detailed mention about the probable impacts upon the downstream river health, rare and endangered species, physical resource, people’s livelihood and the options for mitigation etc. did not find place in the report. The EIA report contains insufficient records on terrestrial and aquatic flora, phytoplanktons, fish diversity, riparian flora, zooplanktons, avian fauna, floodplain crops, people’s lively hood etc. besides healthy habitats controlled by the unregulated Subansiri river," the report said.

What is worrisome is that despite the investment of such a huge amount of time and money, there will be a gradual reduction of power generation capacity of the plant and its prime production period is perhaps already over even though not an ounce of electricity has been produced yet.

"Compared to the historical period 1990-2014, where power obtained during this period was 3,72,982 MW, the future power obtained for SLHEP will gradually decrease from 2020-99. Power obtained for 2020-39, 2040-59, 2060-79 and 2080-99 are 29,94,14 MW, 2,83,533 MW, 2,64,851 MW and 2,64,590 MW respectively. From this analysis, it can be said that, as compared to 1990-2014, the power potential during 2020-39, 2040-59, 2060-79 and 2080-99 will decrease by 19.72 percent, 23.98 percent, 28.99 percent and 29.06 percent respectively," the authors claimed in their research.

Prevent the apocalypse

What happened in the Tapovan Vishnugad Hydro Power project was unprecedented and a rare incident that occurred due to natural causes. However, the problem with the Lower Subansiri HEP is that there are many flaws which the scientific community have themselves pointed out raising serious concerns about the safety aspect of the dam. Unless they are fixed, the chase for cheap power may prove disastrous with many lives and a huge amount of public money at stake. NHPC cannot be lackadaisical when it is making the country's largest hydropower project. It must shoulder responsibility, show accountability.



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