The Subansiri Lower Dam, officially named Subansiri Lower Hydroelectric Project, is an under constructiongravity dam on the Subansiri River in NorthEastern India. It is located upstream of Gerukamukh village in Dhemaji District and Lower Subansiri Districton the border of Assam and Arunachal Pradesh. Described as a run-of-the-river project by NHPC Limited, the Project is expected to supply 2,000 MW of power when completed. The project has experienced several problems during construction to include landslides, re-design and opposition. It was expected to be complete in 2018. It is notable that, if completed as planned, it will be the largest hydroelectric project in India. As of early 2019, work was not progressing on either of the two major dam projects in the Assam region, the Dibang and the Lower Subansiri. After clearance of ‘Subansiri Lower Project’ by Hon’ble NGT vide its Order dated: 31.07.2019, Main works of the Dam has been resumed on 15.10.2019.
Design
The concrete gravity dam is designed to be tall, measured from the river bed and from foundation. Its length will be and the dam will have a structural volume of. The reservoir created by the dam will have a gross storage capacity of, of which can be used for power generation or irrigation. At normal level, the reservoir's surface will cover. The surface powerhouse, located on the left bank, will contain eight 250 MW Francis turbine generators. There will be eight horse shoe shaped head race tunnels, each being in diameter and having a length from. There will be eight horse Shoe shaped surge tunnels, each being in diameter and having length from. There will be eight horse shoe/circular shaped penstocks with varying diameters of and lengths of. The tail race channel, which will transfer water discharged by the turbines back to the river, is wide and long.
Construction
Construction of Subansiri Lower Project involves many challenges. These include land not being available when construction was scheduled to commence, a limited annual construction time because of monsoons, the need to handle high flood flows and poor rock conditions. The design of the dam has undergone drastic and repeated revisions that have affected the schedule and planning of the construction work. In December 2003 the contract to build the dam and its associated structures was awarded to a consortium of Boguchandgesstroy, Soyuzgidrosptsstry and Soma Enterprise Ltd. Due to difficulties acquiring land around the site, construction could not begin in earnest until 13 months after the contract was awarded. Unexpected geological conditions at the dam site led to landslides and slower tunnel excavation. By November 2007, the river was successfully diverted and in April of the next year, the foundation was clear for construction. Before the foundation was fully prepared it was discovered that bedrock was reached sooner than expected. This led to an alteration in the dam's design for stability. While the dam was being re-designed, concrete was placed over the foundation to protect it from the upcoming monsoon floods as the cofferdams stood a good chance of not protecting the foundation from the strong floods. The re-design was completed in October 2008 and soon after the foundation was once again cleared. In May 2009, work was suspended because of the monsoon season and re-commenced in November of that year. As of November 2011, the dam reached an elevation of, just below the spillway elevation of. On 16 December 2011, construction equipment was halted by protests. The construction cost has gone up by about 1,200 crore owing to forced suspension of work since December 2011. NHPC has already spent about 6,600 crore, according to a status report prepared by the company. Commissioning of the first generator is loosely expected to occur in 2016 with the final generator in 2018.
