Nagarjuna Sagar Dam

The Nizam made the British engineers begin the survey work for this dam across the Krishna river in the year 1903.[3] But the project construction was officially inaugurated by Prime Minister Jawaharlal Nehru on 10 December 1955 and proceeded for the next twelve years. Raja Vasireddy Ramagopala Krishna Maheswara Prasad, popularly known as late Muktyala Raja was instrumental in the construction of the Nagarjuna Sagar Dam via active political lobbying, a donation of one hundred million British pounds and fifty-five thousand acres of land. It was the tallest masonry dam in the world at that time, built entirely with local know-how under the able engineering leadership of Kanuri Lakshmana Rao the then Member of Parliament from Vijayawada.

The construction of the dam submerged an ancient Buddhist settlement, Nagarjunakonda, which was the capital of the Ikshvaku dynasty in the 1st and 2nd centuries, the successors of the Satavahanas in the Eastern Deccan. Excavations here had yielded 30 Buddhist monasteries, as well as artworks and inscriptions of great historical importance. In advance of the reservoir's flooding, monuments were dug up and relocated. Some were moved to Nagarjunakonda, now an island in the middle of the reservoir. Others were moved to the nearby mainland village of Anupu.[4]

The reservoir water was released into the left and right bank canals by Prime Minister Indira Gandhi on August 4, 1967.[5] Construction of the hydropower plant followed, with generation increasing between 1978 and 1985, as additional units came into service. In 2015, diamond jubilee celebrations of project's inauguration were held, alluding to the prosperity the dam has ushered into the region.[6]

The right canal (Jawahar canal) is 203 km (126 mi) long with maximum 311.5 cumecs capacity and irrigates 1.117 million acres (4,520 km²) of land in Guntur and Prakasam districts. The left canal (Lalbahadur Shastri canal) is 179 km (111 mi) long with maximum 311.5 cumecs capacity and irrigates 1.008 million acres (4,080 km²) of land in Nalgonda, Suryapet, Krishna, West Godavari and Khamman districts.[8] The project transformed the economy of above districts. 54 villages (48 in Nalgonda and 6 in Guntur) were submersed in water and 24,000 people were affected. The relocation of the people was completed by 2007.

Fishing in Sagar backwaters

Alimineti Madhava Reddy lift irrigation canal draws water from the Nagarjuna Sagar reservoir to irrigate 0.37 million acres (1,500 km²) of land in Nalgonda district.[9] This lift scheme with pump house located near Puttamgandi village on the left bank of Krishna river also supplies nearly 20 TMC water for the drinking water needs of Hyderabad city.[10][11] Nearly 80% of the Nagarjuna Sagar water used in Hyderabad city is available for irrigation use in Nalgonda district in the form of regenerated water/treated sewage water. In addition, the high level flood flow canal drawing water from the left side shore of the reservoir also supplies irrigation water in Nalgonda district.

The hydroelectric plant has a power generation capacity of 815.6 MW with 8 units (1x110 MW+7x100.8 MW). First unit was commissioned on 7 March 1978 and 8th unit on 24 December 1985. The right canal plant has a power generation capacity of 90 megawatts (120,000 hp) with 3 units of 30 megawatts (40,000 hp) each. The left canal plant has a power generation capacity of 60 megawatts (80,000 hp) with 2 units of 30 MW each.[12] The tail pond is under advanced stage of construction to put to use the pumped storage features of 7 x 100.8 MW units.

Nagarjuna sagar dam power generation

Many times, it happens that power generation from the 150 MW canal based units is not optimised when the Nagarjunasagar reservoir is overflowing on its spillway and very less water is required for irrigation from the canals during the monsoon floods. Power generation from canal based hydro units can be optimised by running these units during the flooding period by releasing the water fully into the canals. The unwanted canal water can be released into the natural stream when it is crossing the major stream. Thus run off power can be generated from the water going down unutilised into the river by the canal based power units also.

The water level in the Nagarjunasagar reservoir shall be maintained above the minimum level required for these units in most of the time by releasing water from the upstream Srisailam reservoir to optimise the power generation from the canal based units during dry season.

Water planning for Hyderabad city started in 1920 with the tapping of Musi river for 15 Mgd. It progressed to tapping Esi (Himayat Sagar 1927 – 11mgd) and Manjira (1965–1993 – Majira and Singur dams) for another additional 130 Mgd. It took a huge leap during 1995–2004 with the commissioning of Krishna river water project (Phases I – III) at a total cost of over ten thousand crores to supply an additional 190Mgd to Hyderabad from Nagarjuna sagar away from the Krishna Delta and the Kolleru lake.[15] The project incurs an additional evaporation and leakage loss of 64 Mgd. About 30% of the water naturally flowing to Krishna Delta before 1995 is now diverted to Hyderabad.

The left and right bank canals sill level is fixed at 490 feet (149 m) MSL to supply irrigation water to two million acres. The unutilized storage capacity is nearly 180 TMC below the canals sill/bed level.[16] Nagarjuna Sagar reservoir also meets the Krishna delta water requirements to the extent of 80 TMC by letting water down stream into the river. Nearly 1.3 million acres (5,300 km²) is irrigated under Krishna Delta Canals. There is a possibility to utilize most of this idle dead storage capacity to store the river flood water further and to use as carry over storage. Nearly 150 TMC idle storage up to 380 feet (116 m) MSL, can be used leaving 30 TMC for silt settlement. This is possible by installing Water Powered Pump[17] (WPP) units at the base of the dam.

It is technically feasible to generate power by the existing hydro turbines from lower head (75 to 50 meters) at lower frequency and the lower frequency power can be upgraded / converted to normal grid frequency (50 Hz) by installing HVDC converters before feeding power into the grid.[18] The unused converter stations of HVDC Sileru–Barsoor transmission link can be relocated and utilised for this purpose. With minor modifications to the electrical systems of hydro power units, nearly 100 TMC water available in the dead storage of the reservoir can be put to use every year.

The reservoir dead storage water below the 125 m MSL can be released into the downstream river through the existing diversion tunnel which was in use to divert the river flow during the dam construction.[8]

At present nearly one Tmcft per month or 250 million gallons per day or 350 cusecs is supplied to the Hyderabad city from Nagarjuna Sagar (NS) reservoir.[19] The water supply is nearly 50% of the total city water requirement. This water pumping scheme is part of Alimineti Madhava Reddy lift irrigation project with its foreshore pumping station at Puttamgandi which has nearly 2400 cusecs pumping capacity.[20] The water supply to the Hyderabad city is nearly 15% of its total capacity. The approach channel from the reservoir to the Puttamgandi pump house (PH) is located at 16°34′31″N 79°07′51″E where the Bhimanapalli Vagu tributary is joining the Krishna river.[21] The minimum draw down level (MDDL) of the PH is 502 feet (153 m) MSL below which water can not be pumped from NS reservoir.[22] The reliability / dependability of the PH for supplying assured water supply to Hyderabad city, is not adequate due to meagre inflows into the NS reservoir in some years and the need to deplete the NS reservoir water below 502 ft MSL for other purposes. In these circumstances, adequate water is to be stored above the 502 ft MSL to maintain 100% assured water source without depending totally on NS reservoir.[23]

This is possible by constructing a balancing reservoir by separating some area of the NS reservoir with a new dam across the Bhimanapalli Vagu tributary at 16°34′33″N 79°06′53″E just upstream of the Puttamgandi PH approach channel. This new dam with FRL 590 feet (180 m) MSL, would not submerge any additional area other than the area already submerged by the NS reservoir. The water inflows from the Bhimanapalli Vagu tributary joining the NS reservoir are first impounded by the new dam and if found excess over flows into the downstream NS reservoir. This new balancing reservoir's live capacity is nearly 6 Tmcft above the 502 ft MDDL which is equal to six months water supply to the Hyderabad city. This reservoir would have provision to receive water from the Puttamgandi PH when inflows from the Bhimanapalli Vagu tributary is not satisfactory and water is at adequate level in NS reservoir during monsoon months. When water level of NS reservoir goes below the 502 ft MSL, water is fed to the Puttamgandi PH approach channel from the new balancing reservoir for pumping water needs of Hyderabad city. The cost of this new dam project would be nearly 1.5 billion rupees only which will provide 100% assured water supply to the Hyderabad city without depending on the water availability from NS reservoir during the non monsoon months and drought years.[23]

Nagarjuna Left Canal deep cut before entering the gravity tunnel

The Nagarjuna Sagar left canal supplies nearly 130 TMC of water for irrigation needs in Telangana and Andhra Pradesh states. This is a contour gravity canal with gradual downward gradient (≃ 1:10,000) along the water flow direction. This canal can be used for transferring nearly 80 TMC Godavari river water into the Nagarjuna Sagar reservoir in addition to supplying the Godavari water under its entire command area. Thus a total of 210 TMC of Godavari water can be used in the Krishna basin of Telangana state from Srisailam and Jurala reservoirs for the new projects with 100% water dependability. Godavari water transferred into the Nagarjuna Sagar reservoir and Krishna main river can also be used for the proposed Palamuru lift irrigation and Nakkalagandi lift irrigation schemes in Telangana.

This is possible by re-engineering of the left canal to reverse its water flow direction from the location (near 17°22′13″N 80°21′43″E) where Godavari water would be pumped into this canal. The canal embankments would be raised to facilitate flow reversing towards Nagarjuna Sagar reservoir and intermediate pumping stations (with low head & high flow concrete volute pumps) would be installed near the Paleru balancing reservoir, Pedda Devulapalli balancing reservoir, left canal head regulator on the rim of Nagarjuna Sagar reservoir and the existing major aqueducts across Halia, Musi and Munneru tributaries. The cost of this canal redesigning and the associated pump houses would be one third of a new scheme to transfer Godavari river water into Nagarjuna Sagar reservoir at its FRL 590 feet (180 m) MSL with least possible total pumping head.[24] The above re-engineering of the canal is similar to modifications carried out to reverse the water flow of ancient Grand canal under Eastern Route project of South to North Water Transfer in China.[25]