Subansiri Dam: Is a disaster waiting to happen in Eastern Assam?

Lower Subansiri Hydroelectric Power Project (LSHEP) | Photo courtesy: Special arrangement

Assam’s unending tryst with floods could just get worse after the completion of a controversial hydel dam along the state’s border with Arunachal Pradesh despite repeated warnings from experts on the project’s design.

Dr Nayan Sharma, Adjunct Professor with IIT-Roorkee speaks to The Probe

The project is the 2,000 MW Lower Subansiri Hydroelectric Power Project (LSHEP) at Gerukamukh on the Subansiri river, which is being constructed by the state-owned National Hydro Power Corporation (NHPC). The hydel dam is being developed as part of India’s 50,000 MW hydropower programme launched in 2003, out of which a total of 5,600 MW was planned on the Subansiri river that has been envisioned with three components.

The LSHEP will be the single largest hydroelectric plant in India when completed. It consists of a concrete gravity dam, which will be 116 metres high from the river bed level and 130 metres from the foundation. The length of the dam will be 284 metres, and the gross storage capacity of the reservoir will be 1.37km3.

The powerhouse will be equipped with eight 250 MW Francis turbines on the left bank of the river, eight horseshoe-shaped headrace tunnels (diameter 9.5m and length varying from 608m to 1,168m), eight horseshoe-shaped surge tunnels (diameter 9.5m and length ranging from 400m to 485m) and eight circular penstocks (diameter varying between 400m and 485m and length between 7m and 9.5m). A tailrace channel (35m-long and 206m-wide) will take the water discharged by the turbines back to the river.

In early 2013, the project was more than 50% complete when the construction was halted for an indefinite period owing to a fierce agitation in Assam. Several rounds of protests have been held since then, with the latest being organised only last year at several districts in eastern Assam on the north bank of the Brahmaputra river following comments by Union Minister of Power R K Singh that the dam would start generating 500 MW from next year.

Anti-dam activists and civil society organisations such as the powerful All Assam Students’ Union (AASU) have alleged that the dam is located in a high seismic zone and is under-designed to resist earthquakes. The fluctuation of the water level in the river is also feared to have an adverse impact on the ecology in the lower Subansiri region after the project is completed.

Singh’s optimism stemmed from a judgement by the Supreme Court in 2019 when it dismissed a petition filed by a Guwahati-based NGO challenging an order passed by the National Green Tribunal that had given the green signal for the resumption of the dam’s construction.

How serious are the allegations levelled by anti-dam activists against the project? Was the National Green Tribunal’s assessment justified in giving the go-ahead to the jumbo scheme?

A section of NHPC and government officials believes that the fears are misplaced and that no disaster would overwhelm Assam in the wake of the hydel project. Further, in reply to a query filed under the Right to Information Act, the government-owned PSU pointed out that during most of the monsoon period, the reservoir would be maintained at 190 metres which is 15 metres below the ‘rule curve’.

“The reservoir capacity between EL 190m (metres) AND EL 205m (metres) is 442 MCM (million cubic metres) which will cause regulated flow in the d/s of Subansiri lower dam during floods in the downstream of SLHEP dam by about 30% to 40%,” the reply said. It adds that other measures like disaster management plan, emergency action plan, flood forecasting, raised platforms, and river bank protection have been envisaged as per the recommendations of the Joint Steering Committee (JSC). There will also be an Independent Reservoir Regulation Committee to examine reservoir regulations.

However, there were a couple of incidents recently that raise doubts over the manner in which the project is being executed. Last year, the guard wall in the project collapsed following incessant rains in the region resulting in the submergence of the powerhouse by the river. One worker was killed and another seriously injured when the roof of an under-construction tunnel collapsed and fell on them early this month.

Professor Nayan Sharma, an expert on hydel dams who is currently associated with IIT-Roorkee, says that stationary operating Rule Curves lack adaptability. “Hence, under changing environmental conditions, stationary Rule Curves cause inefficient operation of reservoirs with multipurpose objectives such as – hydropower and flood control as in the case LSHEP. For LSHEP, in the context of sharply growing uncertainties in climate change-induced hydrometeorology, reservoir operation Rule Curve has to be compatibly dynamic and flexible. This is required to fulfil the commitment of moderating peaks of ‘medium floods’ as claimed by NHPC,” he explains.

He adds, “Due to climate change, flood inflows are expected to significantly vary with monsoon rains, and this would make it quite challenging to hold the reservoir level at EL 190m for required flood storage space with a stationary operating Rule Curves lacking adaptability. Of late, the concept of Dynamic Flood Control Rule Curves (DFCRCs) has come for increasing adoption globally. This is because it balances the use of conservation space between flood control and conservation purposes that are necessary for the operation of a hydro-power based reservoir like LSHEP.”

No less critical of the project is the former chief engineer of Brahmaputra Board, D J Borgohain, who is convinced that there is no option to prevent floods except with an adequate provision to store excess water. “The Subansiri dam is meant for the production of hydroelectricity. In reply to the RTI application, NHPC said that measures had been incorporated to control floods of moderate-intensity only. A storage reservoir would be necessary to store excess rainwater, which was found mentioned in the earlier proposal submitted by Brahmaputra Board, but the government of Arunachal Pradesh did not accept it,” he explains.

The concept of a storage reservoir as a remedy to the recurring floods in Assam was first elucidated in 1955 when the Brahmaputra Flood Control Commission envisaged a 125-metre high dam on the Subansiri river at the same spot. The proposal was put in cold storage owing to constraints of funds.

In 1983, the Brahmaputra Board came up with a detailed project report for a 257-metre high rock-fill dam at the same site. But this time, the concept met with a dead-end following vehement opposition from the Arunachal Pradesh government. In 2000, the Centre instructed the National Hydroelectric Power Corporation to take over the project from the Brahmaputra Board, triggering controversies and a series of agitations in Assam.

Rajeev Bhattacharya is a senior journalist in Assam in India’s northeast. He has worked with The Telegraph, The Indian Express, The Times of India and Times Now, and was the managing editor of Seven Sisters Post. He is a Chevening Fellow and author of “Rendezvous With Rebels: Journey to Meet India’s Most Wanted Men” and “Lens & The Guerrilla: Insurgency in India’s Northeast.” He reports on India’s northeast and its border regions with Myanmar, Bhutan, China and Bangladesh.