On October 27, the long delayed Subansiri Lower Hydroelectric Project on the border of Arunachal Pradesh and Assam suffered its latest setback after a large part of the hill on the left side of the dam collapsed into its reservoir. The deposits blocked the only functional diversion tunnel and stopped the flow of water downstream of the dam into the Subansiri river, a major tributary of the Brahmaputra. Construction of the project, which when complete will be India's largest, began in 2005, but suffered a long stoppage between December 2011 and September 2019 due to local opposition and major changes in the design of the dam. After work resumed, the deadline for commissioning the first two of the project’s eight 250-MW units has been extended five times: to March 2021, August 2022, March 2023, June 2023, and now March 2024. How a dammed river flows Typically, once a location is selected for a dam, a temporary earthen barrier (coffer dam) and a few diversion tunnels are built immediately upstream to bypass the dam construction site. (See illustration) Once the dam is ready, the diversion tunnels (DTs) are closed, and water starts to flow through the multiple spillways — which are gated holes in the dam wall that ensure regulated release. In a hydel project, a set of tunnels are also built to carry water from the reservoir to the powerhouse. The water turns the turbines, and then goes back into the river. Once the power house is operational, this becomes the main path of the water, and the spillways are used occasionally. What happened at Subansiri On October 27, a spokesperson for the Subansiri Lower Project said the flow downstream was “highly reduced” — to an estimated 5-10 cubic metres per second (cumecs) — after a landslide “at about 11.30 am” blocked the only DT in use. “The dam spillway bay level is at 145 m [above] msl (mean sea level). At 1 pm, the water level reached 139 m msl. At the present river flow of 997 cubic m/sec, the water level of the reservoir is expected to reach 145 m msl this evening and the river will again flow through the spillway.normally,” the spokesperson said. The water eventually started flowing out of the spillways around 11 pm. As of now, “three spillway gates are complete and the remaining six were to be ready by December. It was perhaps a mistake to depend on a single DT,” an engineer associated with the project said. The 'mistake' NHPC committed The Central Electricity Authority (CEA), the statutory body that advises the government on policy relating to electricity systems, had recommended in April 2022 that the impact of the DTs on the slope stability of the project site should be examined. NHPC Ltd, which is implementing the Subansiri Lower Project, ignored the recommendation. In April 2023, it said that “no more assessment of landslide hazards and their impact was needed”, and that “necessary stabilisation measures have been carried out”. The site's history of landslides The landslide on October 27 was the sixth to hit the dam site since the CEA sounded the warning last April. Major landslides took place in June, September, and October 2022, and in April and September 2023. There had been landslides before the CEA's alert as well — probably the reason why the CEA raised a flag in the first place. “A landslide blocked the outlet of one diversion tunnel in 2020. Another tunnel wall collapsed last June. In all, four of the five diversion tunnels suffered damage, or were blocked or sealed over the last two years,” a project official said. Yet, the project authority relied on the sole functional diversion tunnel to release water downstream, and to lower the dam reservoir level to complete the construction of the spillway gates. What happens here onward A team of the NHPC's board of directors, and top officials of the CEA and Geological Survey of India visited the site last week. The project will now focus on slope stabilisation, NHPC project consultant A N Mohammed said. Biswajit Basu, project director of NHPC, said, “We will complete the work before the (March 2024) deadline.” He declined to comment on how the spillway gates would be built, and the potential time and cost overruns. The likely option would be to build a temporary sheet pile dyke — a metallic barrier with steel bracing — inside the reservoir to isolate the under-construction spillway gates, and to finish the work after the slope stabilisation process is over, a project official said. Basu also declined to comment on the potential impact on the riverine ecosystem and aquatic life forms. “Whenever DTs are closed, the water level goes down. But a river is never dry and water levels anyway fall in the winter,” he said. While the Subansiri's lowest-ever winter discharge on NHPC records was 188 cumecs in 1979, for almost 12 hours on October 27, the flow went down to 5-10 cumecs. The project clearance conditions require Subansiri Lower to maintain a minimum downstream flow of 240 cumecs at all times.
