India’s second-largest hydropower project, the 2,000 MW Subansiri Lower Hydroelectric Project (SLHEP), is in the final stages of construction at Gerukamukh on the Arunachal Pradesh and Assam border.
Out of the five 9.5-meter diameter diversion tunnels of the SLHEP, the only diversion tunnel being used was closed due to a landslide at 11:30 am on October 27, 2023. The remaining four diversion tunnels had already been closed. As a result, the flow of the river is reduced for about 10-12 hours. The level at the entrance of the diversion tunnel is EL101 meters and the level of the dam’s spillway is EL145 meters (from sea level).In the present time due to a lack of rain in the catchment, the flow of the river depends mainly on the snowmelt waters of the Himalayas along with some base flow. At that time, 997 cubic meters per second of river inflow was in the reservoir and the water level in the reservoir was rising. As the water level reached EL145 meters at 10:40 pm the river again started flowing back downstream through the spillway of the dam.
The diversion tunnels are temporary arrangements to divert the river during the construction of the foundation of the main dam. According to the project plan, the diversion tunnels were likely to divert 1 in 25 years of return flood of 4550 cubic meters per second non-monsoon flood to work for 6/7 months from October to April.
One tunnel was designed to divert a flow of 950 cubic meters per second. The need for diversion tunnels has been over as the dam is almost completed and all diversion tunnels are to be plugged.The construction work of the Subansiri Lower Hydro Electric Project (SLHEP) which started in January 2005 had been stopped for eight years from December 2011 to October 2019 due to the anti-dam agitation which has adverse effects on prolonged working of the temporary structures like diversion tunnels. A landslide on May 26, 2020, led to the closure of diversion tunnel No. 5 near the outlet. Heavy rains on September 15, 2022, created a crater near the entrance to diversion tunnel no.2. As a result, landslides started due to the loss of stability of the hill slope above it. Several minor landslides since September 24, 2022, have caused considerable damage to the inlet areas of the diversion tunnels.
Arrangements have been made to permanently close all the diversion tunnels except diversion tunnel no.1 as the work of the dam was almost complete except for some work on the installation of the spillway gates.
The works for hill slope stability have been started after all the diversion tunnels were closed because of the landslide on October 27, 2023.
Since 1955, several organizations investigated multipurpose projects on the Subansiri River for flood control in Assam. In 1983, the Brahmaputra Board planned a 257m high Rockfill Dam for 4800 MW of electric power and flood control at Gerukamukh on the Assam-Arunachal Pradesh border. Arunachal Pradesh objected to the huge submergence in the State as flood control projects necessitate high dams for creating large reservoirs.As a result, three cascaded projects of 265m high-2000MW Subansiri Upper, 213m high-1800MW Subansiri Middle and 116m high-2000MW Subansiri Lower Hydroelectric Projects were planned on the Subansiri River.
The Subansiri Lower Project was transferred to National Hydroelectric Power Corporation (NHPC) Ltd. in March 2000. After receiving all statutory clearances, NHPC Ltd. started the construction of SLHEP in January 2005.
Since then, apprehending adverse downstream effects, SLHEP has been facing an anti-dam movement by various civic groups of Assam spear-headed by the All Assam Students Union (AASU). Due to the recommendations of the Assam Experts Group (AEG) in March 2011 against construction of the project due to its geologically and seismologically sensitive location, construction of the project stopped in December 2011 after completion of 50 per cent works.The Government of India (GoI) constituted several expert committees to examine the recommendations of the AEG. Technical Expert Committee (TEC) constituted by the Planning Commission with Experts C.D.Thatte and M.S.Reddy (both were Ex-Chairman, Central Water Commission and Secretary, Ministry of Water Resources, GoI) in July 2012 allayed the apprehensions of EG related to downstream impacts and recommended for an independent Dam Design Review Panel (DDRP) to review dam design features keeping in view the suspected foundation competency. DDRP in June 2013 recommended a few modifications in design for additional safety of the dam.
An increase in dam width from 171m to 271m, extension of the upstream concrete cut-off wall and additional wall downstream to stop any seepage to the dam foundation, grouting in dam foundation, provision for rock anchor, cable tendon for stabilising abutments were major recommendations. The National Committee for Seismic Design Parameter (NCSDP) reviewed the Seismic Design Parameters (SDP). However, AEG raised concerns about seismological issues. In January 2015, GoI constituted a Project Oversight Committee (POC) with experts from Assam and GoI to examine issues raised by EG.
POC members agreed that the reduction of dam height as proposed by AEG was not possible with probable maximum flood (PMF) considerations, essential for dam safety.
Besides, it would reduce or eliminate its 15-meter flood cushion. Thatte-Reddy Committee cited Agitation against the dam and demand for flood control are self-contradictory. ‘It is difficult to understand the agitation against dams, so long they are designed with state-of-the-art procedures.’240 cubic meters per second environmental flow would be maintained by running one turbine continuously to ensure the sustenance of aquatic life. There would be no significant impact downstream due to fluctuation of water level during power peaking operation. The establishment of an independent authority for reservoir control would eliminate any probabilities of panic release and dam-induced flood downstream. Downstream river protection up to 30km and development schemes for Rs. 658 crore have been taken up. The POC (GoI) agreed that the revised dam design would adequately address the dam safety issues. However, POC (Assam) could not concur on seismic design parameters despite endorsement from National earthquake experts. Thatte & Reddy said, ‘the opposition to the construction of dam usually is from reservoir displaced people, forest and wildlife conservationists. There has hardly ever been opposition from people living in the downstream. In fact, they are the beneficiaries while the displaced people are the adversely affected people of the project.”’ Stating the beneficial impacts of the SLHEP downstream viz. flood control, irrigation, employment, health, education, industries, trades, electricity, infrastructure development, tourism, etc., said “The government of Assam, the majority of local populations are aware of these benefits. The AEG is too knowledgeable to miss them. That is why AEG is not really objecting to the Dam, per se. The EG and the local people not only want a Dam, but a safe Dam. That is what NHPC will build, taking into account various recommendations made in the report, GoI and GoA will ensure that. AEG and local people should feel rest assured that a safe Dam will be built and minimum flow ensured. It will not be long before they start wondering why SLHEP did not happen earlier.”The disputes had been shifted to the Hon’ble Court as two social activists approached the National Green Tribunal (NGT). In pursuance of the October 2017 judgment of the NGT for resolving various issues, MoEF & CC constituted a 3-member Expert Committee (EC) comprising experts from the Hydrology, Geology, and Seismic fields. Visiting Subansiri Lower and nearby Ranganadi and Pare projects, and having in-depth discussions with several independent experts, the Committee submitted a report in March 2019 and concluded that “downstream issues such as maintenance of continuous minimum flow for the sustenance of flora and fauna, flood control and flood forecasting mechanism, sediment management, protection of Subansiri banks, provision of social security to the riparian population, dam-break analysis, etc. have been taken care of as per the recommendations of various committees”. Reviewing reports of various expert groups related to foundation competency, slope stability, seismotectonic-seismic design parameters, dam design and downstream impacts of the project, the committee opined that “SLHEP has been designed for expected extreme conditions and all the concerns have been adequately addressed to the extent practically feasible.” “The Eastern Himalaya holds enormous hydropower potential, the harnessing of which can usher in rapid overall growth in North East India, in particular. The Commissioning of Subansiri Lower Project can, not only prove to be a boon for the people of the region but also open the gates for the accelerated development of water resources projects in the entire Brahmaputra Basin.”
NGT pronounced its final judgment on 31.07.2019 allaying all apprehensions on dam safety and adverse downstream impacts and construction resumed in October 2019 with all modifications as recommended by the committees after eight years. Ninety-two per cent overall progress of the project has been achieved till October 2023.