Srisailam Dam

The Srisailam Dam is constructed across the Krishna River in Nandyal district, Andhra Pradesh and Nagarkurnool district, Telangana near Srisailam temple town and is the 2nd largest capacity working hydroelectric station in India.

The dam was constructed in a deep gorge in the Nallamala Hills in between Nandyal and Nagarkurnool districts, 300 m above sea level. It is 512 m long, 145 m maximum height and has 12 radial crest gates. It has a reservoir of 616 km2. The project has an estimated live capacity to hold 178.74 Tmcft at its full reservoir level of 885 ft MSL. Its gross storage capacity is 6.116 cubic km (216 tmc ft). The minimum draw-down level (MDDL) of the reservoir is at 705 ft MSL from its river sluice gates, and corresponding dead storage is 3.42 Tmcft. The left bank underground power station houses 6 ×150 MW reversible Francis-pump turbines for pumped-storage operation (each turbine can pump 200 cumecs) and the right bank semi-underground power station houses 7 ×110 MW Francis-turbine generators.

Tail pond dam/weir located 14 km downstream of Srisailam dam is under advanced stage of construction to hold the water released by the hydro turbines and later pump back into the Srisailam reservoir by operating the turbines in pump mode. The weir portion got breached in November 2015 unable to withstand the normal water release from the hydropower stations. Tail pond weir was completed during the year 2017 and pumping mode operation is being done even when the downstream Nagarjuna Sagar reservoir water level is below 531.5 ft MSL. The tail pond has nearly 1 tmcft live storage capacity.

History
The Srisailam project began in 1960, Initially only as a power project. After several delays, the main dam was finally completed twenty years later in 1980 July 26. In the meantime the project was converted into a multipurpose facility with a generating capacity of 770 MW by its second stage which was completed in 1987. The dam is to provide water for an estimated 2000 km2. Under the right bank branch canal 790 km2 in Kurnool and Kadapa districts will have assured irrigation. From the initial modest estimate of ₹38.47 crore for a power project the total cost of the multipurpose project was estimated to cross ₹1000 crore in its enlarged form. The dam has alone cost ₹404 crore together with the installation of four generating sets of 110 MW each. The right bank branch canal is estimated to cost ₹449 crore and the initial investment of ₹140 crore has been provided by the World Bank. The projected cost-benefit ratio of the project has been worked out at 1:1.91 at 10% interest on capital outlay.. In 1998 a coffer dam was over topped by flooding. The power house required repairs and did not generate power for a year. On 2 October 2009, Srisailam dam experienced a record inflow which threatened the dam. It was built under the engineering leadership of Kanuri Lakshmana Rao.

Irrigation
Srisailam right main canal (SRMC) is constructed with 44,000 cusecs capacity at Srisailam reservoir level of 269.22 m MSL to feed Veligodu reservoir (16.95 tmcft), Brahmamsagar Reservoir (17.74 tmcft), Alaganoor reservoir (2.97 tmcft), Gorakallu reservoir (12.44 tmcft), Owk reservoir (4.15 tmcft), Gandikota Reservoir (26.86 tmcft), Mylavaram reservoir (9.98 tmcft), Somasila reservoir (78 tmcft) and Kandeleru reservoir (68 tmcft) with nearly 235 Tmcft total storage capacity. This canal also supplies water to Telugu Ganga project which supplies Krishna river water to Chennai city for its drinking purpose. This main canal by feeding water to K. C. Canal, Srisailam right bank canal, Telugu Ganga canal and Galeru Nagari canal irrigates vast area in Kurnool, Kadapa, Chittoor and Nellore districts.

Handri-Neeva lift canal by drawing water from the Srisailam reservoir, which supplies drinking water in all the districts of Rayalaseema. Veligonda reservoir receives water by gravity through tunnels to irrigate lands in Nellore, Kadapa and Prakasam districts. Kalwakurthy lift irrigation scheme by drawing water from the Srisailam reservoir, supplies irrigation water in Mahbubnagar and Nalgonda districts

Srisailam left bank canal will receive water by gravity through tunnels to irrigate lands in Nalgonda district. Tunnel work is not complete and the required water has been provided to most of the project area by lifting water from the downstream Nagarjuna Sagar reservoir.

Dam Maintenance and Safety
Systematic Neglect and absence of diversion of funds is evident in maintenance of the dam and lack of any modernization attempts of the Powerhouse. The officers report a shortage of maintenance staff.

Safety concerns to the 2nd largest Hydroelectric project in the country have been raised over the years and have been assessed false subsequently. In 2009, the dam, designed for a maximum flood of 19 lakh cusecs, endured a flood of 25.5 lakh cusecs. According to a survey conducted in the summer of 2018, the scouring resulted in the formation of a huge pit in the apron downstream the dam. And a structure protecting the ‘toe’ and foundation of the main dam has weakened. Efforts are not yet made to repair and maintain the dam.

Power generation optimization
At present, the initial inflows into Srisailam reservoir are stored excessively without being used for power generation. The flood water fills the remaining empty Srisailam reservoir quickly and most of the flood water overflows into the downstream Nagarjunasagar reservoir without being used for power generation. The endeavor shall be to fill the Nagarjuna Sagar reservoir fully with the uniform water released through the power generating units.

The existing right bank power station (770 MW) can be converted into pumped storage hydroelectric power (PSHP) to operate in pumping mode when the downstream Nagarjuna Sagar reservoir level is above 560 ft MSL. The PSHP can be used for energy storage purposes on daily basis and to transfer Godavari water to the Srisailam reservoir during drought years. Alternately, the tunnel of the existing power station can be used for a new PSHP station to pump water from the Nagarjuna Sagar reservoir to the Srisailam reservoir.

Pumped storage hydropower potential
Srisailam reservoir, serving as lower level reservoir, has potential to install nearly 77,000 MW high head pumped storage hydroelectric plants on its right side.