Subansiri

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 (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.

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.

Design

The concrete gravity dam is designed to be 116 m (381 ft) tall, measured from the river bed and 210 m (690 ft) from the deepest foundation. Its length became 284 m (932 ft) and the dam has a structural volume of 2,250,000 m3 (2,942,889 cu yd). The reservoir created by the dam now has 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 covered 33.5 km2 (13 sq mi). The surface powerhouse, located on the right bank, will contain eight 250 MW Francis turbine generators using 86 m design head out of 91 m gross head available.

Environmental impact

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.

Thirty eight families will be displaced when the dam's reservoir will be fully filled, according to official data.

Power generation

The dam is intended for hydroelectric purposes and is part of Stage I of the Ranganadi Hydro Electric Project and supports the 405-megawatt (543,000 hp) Dikrong Power House. The 68 m (223 ft) tall dam diverts water south into a 10.1 km (6.3 mi) headrace tunnel which is then transferred into a 1,062 m (3,484 ft) penstock before reaching the three 135 megawatts (181,000 hp) turbines. Since commissioning, the power house has been generating much less than its capacity because of drought.

Stage II of the project is designed to provide water storage for Stage I and includes a 134 m (440 ft) rock-fill embankment dam with a 523,000,000 m3 (1.85×1010 ft3) storage capacity. This dam will support an additional 180 megawatts (240,000 hp) power station.