Controversial Subansiri hydropower project hit by massive landslide

On October 27, 2023, at around 11:30 am, the controversial 2000 MW Lower Subansiri Hydropower project on the Assam-Arunachal Border was hit by a massive landslide. This incident has caused significant damage to the project, further delaying the commissioning of the first unit and increasing the cost and time overruns of this expensive, much-delayed project.

What exactly happened?

A senior official from the National Hydroelectric Power Corporation (NHPC) stated that one of the five diversion tunnels in use, specifically diversion tunnel number 1, was blocked by a landslide on Friday around 11:30 a.m. The other four diversion tunnels had already been blocked post-construction, leading to a significant decrease in downstream river flow. The mean sea level at the dam spillway bay currently stands at 145 meters.

The official added that the reservoir is currently retaining its river flow of 997cum/sec, causing the water level to rise. By 1:00 p.m., the water level had risen to 139 meters. It is anticipated that by evening, the water level would reach 145 meters, allowing the river to resume its regular flow over the spillway and downstream.

The spillway gates of the dam were still under installation and yet to be tested. The water could overflow from the under-construction spillway gates once the reservoir reached a level of 145 meters above Mean Sea Level.

The majestic Subansiri river was completely without any flow until the river water started flowing through the spillway gates about twelve hours later. Diversion Tunnels (DT) are constructed to take river water from upstream of the coffer (temporary) dam to downstream of the dam site so that dam site can remain dry for work to continue.

For Lower Subansiri HEP, five such diversion tunnels each of diameter 9.5 meters have been constructed on the left bank at about 100 m above mean sea level. A senior official of NHPC said:

“The other four numbers of diversion tunnels had already been blocked earlier”, partly due to landslide.

This last functioning DT was also hit by a landslide in April 2023 and October 2022. This incident raises serious concerns about the safety and feasibility of such large-scale projects in ecologically sensitive areas.

Controversy behind Subansiri Hydropower Project

The Subansiri Lower Hydroelectric Project (SLHEP), straddling the border of Assam and Arunachal Pradesh, has been a subject of controversy for several years.  So far, India has undertaken the project, which is the largest hydroelectric project, but it has faced opposition due to concerns over its environmental impact, safety, and administrative issues.

Critics argue that the dam’s location in a seismic zone and its design could pose a significant risk in the event of an earthquake. There are also fears that fluctuations in the river’s water level could adversely affect the ecology of the lower Subansiri region.

Critics accuse the project of contravening the 1980 Brahmaputra Board Act by transferring the work of the Water Resources Department of Subansiri Basin from the Brahmaputra Board to the public and private sector. Concerns also arise about the dam facing increased seismic threat levels.

Recently, a landslide partially damaged the project, impacting the downstream flow of the Subansiri River. This incident has sparked fresh protests. The Asom Jatiyatabadi Yuba Chatra Parishad staged a protest at the entrance of the dam site at Gerukamukh due to lack of response by state government and stakeholders following sudden drying of water in Subansiri river.

A mountain collapse blocked a diversion tunnel in front of the river, causing no water to flow through the large dam and drying up the river downstream.

Violation of the CEA’s directives

The Central Electricity Authority (CEA) had previously stated that reservoir filling should only commence after all Spillway Radial Gates have been erected and tested. However, this process is currently underway despite the gates not being fully installed and tested, which is a clear violation of the CEA’s directives.

The CEA had also specified that civil works should only proceed in dry or lean seasons and that it would take a minimum of 3.5 months for the erection and testing of Spillway Radial Gates after the Civil Contractor hands over the bays to the Hydro-Mechanical Contractor. It is crucial that these processes are followed as directed to ensure the project’s safety.

It remains unclear why NHPC did not use the silt flushing tunnel to maintain downstream river flow, assuming such a tunnel exists at the dam’s base. The sudden drop in water level has reportedly led to the death of lakhs of fish, impacting aquatic biodiversity, habitat, and livelihoods. The Assam Government must assess these impacts and ensure that NHPC compensates for the losses incurred by affected people.

North Lakhimpur District has issued an advisory notice for safety in the downstream area, but we need further measures to compensate affected individuals. We hope that NHPC will adhere to safety measures and that oversight bodies such as CEA, Central Water Commission (CWC), and National Disaster Management Authority (NDMA) will enforce compliance.

Timeline of past accidents

• May 2020: Landslides occurred around 200 meters away on the right side from the dam site on May 27, 2020. Cracks also developed on the hills near the construction site following week-long rains. Landslides also took away a portion of an abandoned road leading to the viewpoint of the dam 300 meters away from the construction site.
• June 2020: The dam site faced a series of natural disasters since May 27, 2020, with landslides and damages to claddings of the diversion tunnel. The overflowing Subansiri also inundated many installed machineries and other infrastructure at the project site. The floodwater continued to flow over the main dam causing fear in downstream villages.
• July 2020: The guard wall of the power house of the dam collapsed due to heavy rainfall and a flood spate.
• August 2021: The guard wall that was built to protect the project’s main dam collapsed due to the rapid surge of overflowing water.
• March 2022: At least three workers were killed in separate accidents at the dam site.
• May 2022: The Central Electric Authority (CEA) asked NHPC to improve its construction safety measures.
• June 2022: A worker lost his life when a steel rib being fixed on to the roof of second headrace tunnel at Subansiri dam site fell on him.
• August 2022: One more worker died due to electrocution at the Block No. 2 of the main dam.
• September 2022: Floodwater entered the powerhouse of the under-construction Subansiri hydro project leading to collapse of a portion of the temporary guard wall.
• October 2022: Incessant rain caused a landslide near the main dam and a large portion of the hill fell on the diversion course of the river.

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