The Holes in the World's Largest Masonry Dam

Nagarjuna Sagar straddles the Krishna river on the border of Telangana and Andhra Pradesh. Its foundation was laid by Prime Minister Jawaharlal Nehru on 10 December 1955. Almost twelve years later, on 4 August 1967, Prime Minister Indira Gandhi released water from the reservoir into the left and right bank canals. At completion, the dam was widely described as the tallest masonry dam in the world. By the Central Water Commission's National Register of Large Dams, the structure is 4,865 metres long and rises 124.66 metres above its lowest foundation level. The reservoir holds 11.55 billion cubic metres of water at gross capacity, with an effective storage of 6.92 billion cubic metres. The spillway carries 26 radial gates, each 13 metres wide and 14 metres tall, designed to discharge 58,340 cumecs. Eight Francis turbines totalling 815.6 megawatts sit at the toe of the dam, commissioned in stages between 1978 and 1985.

This is not an obscure inheritance. It is one of the largest civil engineering works that India ever built.

Through late 2024 and into January 2025, when inspection teams examined the spillway closely, what they found is best described in the language of the field notes that followed. Out of the 26 spillway blocks, holes ranging between one and six metres deep had formed in each block. Additional holes were forming in the spillway floor itself. The mechanism was not a sudden failure. It was the cumulative result of three forces acting on the same surface over decades: routine abrasion-erosion of the spillway glacis and the flip buckets during ordinary operation, the specific cavitation event of the October 2009 Krishna floods when the reservoir received a peak inflow of approximately 14.66 lakh cusecs, and the slow propagation of damage in the years since.

The cavitation mechanism is documented in peer-reviewed work. A pressure analysis of the Nagarjuna Sagar spillway after 2009 found that the as-built profile of the spillway deviated from its design profile. Where the surface departs from the smooth hydraulic curve that the spillway was meant to follow, the velocity over that surface generates local zones of negative pressure. Where the negative pressure drops below the vapour pressure of water, cavitation bubbles form. When the bubbles collapse against the concrete surface, the impact loading is severe enough to progressively detach material from the concrete during a major flood. In 2009, that mechanism is what tore into the spillway over the hours when the dam was passing extreme discharge.

The spillway of Nagarjuna Sagar dam was severely eroded during the floods of 2009 due to cavitation which resulted from negative pressures developed over the spillway.

Peer-reviewed pressure analysis of the Nagarjuna Sagar spillway, ResearchGate publication 273617643

Repair campaigns have followed in sequence. Field documentation describes cracks sealed with low-viscosity cementitious and chemical grouts. The eroded surfaces have been overlaid with high-performance concrete using steel fibres, polymers, and silica fume, materials chosen to bond to old concrete, resist further abrasion, and accommodate the thermal movement of the parent structure. An earlier spillway repair campaign cost the dam owners approximately Rs 16.54 crore. A subsequent rehabilitation programme, reported by Telangana's Engineer-in-Chief B. Nagendra Rao in May 2023, totalled Rs 75 crore, of which Rs 17 crore was directed to the spillway. None of those campaigns ended the deterioration. The 2025 inspection findings are the evidence.

Three responses are now in motion. The Indian Institute of Technology Roorkee was tasked in January 2025 to study the spillway pothole problem in formal academic terms. The Central Water Commission inspected the dam in the same month. The National Dam Safety Authority Chairman directed Telangana to prepare emergency action plans and a fresh dam-break analysis. Through the Dam Rehabilitation and Improvement Project, the state of Andhra Pradesh has proposed approximately Rs 800 crore of works at Nagarjuna Sagar. The World Bank has approved an initial Rs 103 crore as the Phase 1 component of that programme. The dam falls within DRIP Phase II, which is co-funded by the World Bank and the Asian Infrastructure Investment Bank and carries an outlay of Rs 5,107 crore. Phase II together with the complementary Phase III, both operationalised in October 2021, cover 736 dams across 19 states at a combined outlay of Rs 10,211 crore, on a roughly ten-year horizon to 2031.

The lesson here is not a single repair technology. It is the structural one.

The lesson: A 59-year-old composite dam can absorb a sequence of piecemeal repair campaigns over the decades, supply irrigation and power across two states, hold its design storage through repeated monsoons, and yet still quietly carry one-to-six-metre potholes in the surface that is most exposed to extreme discharge. Piecemeal repair is not Comprehensive Dam Safety Evaluation. The December 2026 deadline under Section 38 of the Dam Safety Act 2021 is asking owners of every existing specified dam to find what Nagarjuna Sagar's recent inspections found, before the next major flood does the finding for them. The cavitation that happened over the hours of October 2009 was triggered by a profile deviation that had been present in the as-built dam. That deviation was discoverable by any team that did the work of comparing the as-built surface with the design surface. The CDSE is the structured opportunity to do that work portfolio-wide. Seven months remain.

Read more: DRIP Phase II: What Rs 10,211 Crore in Dam Rehabilitation Means for Concrete Engineers →