Subansiri Lower Dam

Subansiri Lower Dam bridge
Coordinates	27°33′13″N 94°15′31″E
Carries	vehicles
Characteristics
Total length	150 m (492 ft)
Width	150 m (492 ft)
Location

The concrete gravity dam is designed to be 116 m (381 ft) tall, measured from the river bed and130 m (430 ft) from foundation. Its length will be 284 m (932 ft) and the dam will have a structural volume of 2,250,000 m³ (2,942,889 cu yd). The reservoir created by the dam will have a gross storage capacity of 1.37 km³ (1,110,677 acre⋅ft), of which .44 km³ (356,714 acre⋅ft) can be used for power generation or irrigation. At normal level, the reservoir's surface will cover 33.5 km² (13 sq mi). The surface powerhouse, located on the left bank, will contain eight 250 MW Francis turbine generators.[1][2]

There will be eight horse shoe shaped head race tunnels, each being 9.5 m (31 ft) in diameter and having a length from 608–1,168 m (1,995–3,832 ft). There will be eight horse Shoe shaped surge tunnels, each being 9.5 m (31 ft) in diameter and having length from 400–485 m (1,312–1,591 ft). There will be eight horse shoe/circular shaped penstocks with varying diameters of 7–9.5 m (23–31 ft) and lengths of168–190 m (551–623 ft). The tail race channel, which will transfer water discharged by the turbines back to the river, is 206 m (676 ft) wide and 35 m (115 ft) long.[2]

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 (from mid-April to mid-October), 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.[6]

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 10 m (33 ft) 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.[6]

As of November 2011, the dam reached an elevation of 138 m (453 ft), just below the spillway elevation of 145 m (476 ft). On 16 December 2011, construction equipment was halted by protests.[7]

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.[8]

Commissioning of the first generator is loosely expected to occur in 2016 with the final generator in 2018.[9]

Some environmental impacts unique to very large dams will result from completion of the Subansiri Project, both upstream and downstream of the dam site. These impacts will include ecosystem damage and loss of land.

The reservoir of the Subansiri Project will submerge a 47 km (29 mi) length of the Subansiri river and occupy 37.5–40 km² (14.5–15.4 sq mi) which includes Himalayan subtropical pine forests, Himalayan subtropical broadleaf forests, part of the Tale Valley Wildlife Sanctuary, an elephant corridor and some subsistence agriculture fields.[10]

Thirty eight families will be displaced if the dam is completed, according to official data.[11]

Downstream

Water flow downstream will be regulated by the dam which is expected to result in low releases (6 m³/s) during winter and very high releases (2,560 m³/s) when energy is being generated.[12]

The project has met stiff resistance from several groups including All Assam Students’ Union and the Krishak Mukti Sangram Samiti, who are apprehensive about safety and the project’s downstream impact.[8]