Lower Subansiri is one of the biggest hydropower projects in India. Image courtesy of NHPC.
The Lower Subansiri hydroelectric project will have an installed capacity of 2GW. Image courtesy of NHPC.
The Lower Subansiri hydroelectric project is being developed near the borders of Arunachal Pradesh and Assam. Image courtesy of NHPC.

The Lower Subansiri hydroelectric project being built on the Subansiri River, a tributary of the Brahmaputra River, on the border of Assam and Arunachal Pradesh will be one of the biggest hydroelectric facilities in India.

The construction of the mega hydropower project has been underway since 2005.The 2GW hydropower station is being developed by India’s state-run National Hydro Power Corporation (NHPC) with an estimated investment of £2.21bn (Rs203.7bn).

Scheduled for commissioning in August 2023, the Lower Subansiri hydroelectric power station, intended for base-load operations, will generate up to 7.4 billion kWh of electricity annually.

Location and site details

The project is located near North Lakhimpur, in the Lower Subansiri district of Arunachal Pradesh, in the north eastern part of India.

The project site is approachable from the Nagaon railway station and the Lilabari/Dibrugarh airport.

Lower Subansiri hydroelectric plant make-up

The Lower Subansiri hydroelectric project includes the construction of a surface powerhouse and a concrete gravity dam which will be 116m-high from the river bed level.

The powerhouse will comprise eight Francis-type turbines capable of generating 250MW of electricity each. It also comprises a control block and a transformer hall.

Eight horseshoe-shaped head race tunnels of 9.5m-diametre will be built to carry water from dam to the powerhouse. The length of the head race tunnels will vary from 608m to 1.1km.

The project also includes the construction of eight 9.5m-diametre horseshoe-shaped surge tunnels with length varying from 400m to 485m.

There will be eight vertical circular pressure shafts with varying length from 195m to 215m. The shafts will be installed vertically at a depth of 48m.

The tail race channel of the power station will measure 206m in width and 35m in length.

Contractors involved

ÅF Pöyry (AFRY), a Swedish engineering, consulting and design company, was contracted for setting up the management information system (MIS) for the construction management support, in addition to providing the design review services for the temporary structures, the basic design and planning services for the river diversion, as well as the geotechnical and claims preparation services for the project.

Larsen & Toubro (L&T) was awarded the engineering, procurement and construction (EPC) contract for the civil and structural works pertaining to the water intake structure, head race tunnels, surge chamber, pressure shaft, powerhouse and the tail pool in January 2004.

L&T, however, was released from the project work in 2015 while Patel Engineering was awarded a £160m (Rs15.6bn) contract to complete the remaining civil works in October 2020.

Alstom (now GE) was awarded the turbine supply contract in April 2005 while Texmaco was contracted for the supply of the hydro-mechanical package for the project in June 2006.

Subansiri Lower hydroelectric project background

Although the construction works were started in 2005 with completion expected in 2010, the project development was stopped in 2011 due to stiff resistance over the alleged adverse environmental impact of the project.

The development activities were restarted following the execution of an memorandum of agreement (MOA) between the Government of Assam and NHPC in August 2019.

NHPC previously signed an MOA with the Government of Arunachal Pradesh in January 2010.