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Lower Subansiri Hydroelectric Project


A landslide has damaged part of the under-construction 2000 MW Lower Subansiri Hydroelectric project in Arunachal Pradesh and Assam which has affected the flow in the Subansiri river downstream.


GS I: Geography

Dimensions of the Article:

  1. Subansiri Hydel Project Incident
  2. About the Subansiri River
  3. About the Subansiri Lower Hydroelectric Project (SLHEP)

Subansiri Hydel Project Incident:

Hydel Project Operations:

  • Hydropower projects involve the construction of tunnels to channel water from a reservoir to a powerhouse. This water, upon reaching the powerhouse, is used to turn turbines that generate electricity, after which it is returned to the river.
  • Once operational, the powerhouse becomes the primary route for water, while spillways (gates) are used intermittently.

Recent Setback:

  • On October 27, the Subansiri Lower Hydropower Project (SLHEP) faced a significant setback.
  • A substantial portion of the hill on the left side of the dam collapsed into the reservoir.
  • This incident marked the sixth landslide at the dam site since April 2022.

Blockage of Diversion Tunnel:

  • The landslides resulted in deposits blocking the sole functional diversion tunnel (DT).
  • As a consequence, the flow of water downstream of the dam into the Subansiri River was interrupted.

Central Electricity Authority’s (CEA) Recommendation:

  • The CEA, which serves as the statutory body advising the government on electricity system-related policies, had suggested that the impact of the diversion tunnels on the slope stability of the project site be assessed.

NHPC’s Response:

  • However, NHPC Ltd, the entity responsible for executing the Subansiri Lower Project, disregarded this recommendation.
  • NHPC claimed that no further evaluation of landslide hazards and their effects was necessary and that essential stabilization measures had already been carried out.
  • The incident thus raises concerns regarding the project’s safety and environmental impact.

About the Subansiri River:

  • Trans-Himalayan River: The Subansiri River, known as Chayul Chu in Tibet, is a trans-Himalayan river that flows through Tibet’s Lhuntse County in the Shannan Prefecture. It serves as a tributary on the right bank of the Brahmaputra River.
  • Geographical Extent: This river extends its flow into the Indian states of Arunachal Pradesh and Assam. The Subansiri River stretches approximately 518 kilometers (322 miles) in length and covers a drainage basin of around 32,640 square kilometers (12,600 square miles).
  • Significant Contributor: Among its distinctions, the Subansiri River holds the status of being the largest tributary of the Brahmaputra River and contributes about 7.92% of the total flow in the Brahmaputra.

About the Subansiri Lower Hydroelectric Project (SLHEP):

  • Hydroelectric Project Location: The Subansiri Lower Dam, officially known as SLHEP, is an ongoing gravity dam project located on the Subansiri River. It is situated in North Eastern India near the borders of Arunachal Pradesh and Assam.
  • Gravity Dam: A gravity dam is so named because it relies on its own weight to withstand the pressure of the water it retains.
  • Projected Capacity: The National Hydroelectric Power Corporation (NHPC) Limited categorizes the Subansiri Lower Hydroelectric Project as a run-of-the-river project. It is designed to provide 2,000 MW of power, featuring eight units each with a capacity of 250 MW.
  • Run-of-River Hydro Project: Run-of-river hydro projects leverage the natural flow of rivers and micro turbine generators to harness the kinetic energy carried by water.
  • Construction and Delays: NHPC Limited initiated construction work on the SLHEP in 2005 following the acquisition of forest clearance. However, the project encountered various challenges during construction, including landslides, redesigns, and local opposition. Due to protests and demonstrations by local stakeholders, construction work on the project was halted between 2011 and 2019. It subsequently resumed in 2019 following clearance by the National Green Tribunal (NGT), with operations anticipated to begin from March 2024.
  • Prominence: Upon completion as planned, the Subansiri Lower Hydroelectric Project will rank as the largest hydroelectric project in India.

-Source: Indian Express

December 2023