Vajont Dam — the Dam Survived, the Mountain Didn’t: a Slide That Killed ~2,000

On the night of 9 October 1963, the Vajont Dam — a 262-metre concrete double-curvature arch in the Vajont gorge above Longarone, in the Dolomite region of northern Italy — did not break. It stands today, almost undamaged. What failed was the mountain on the reservoir’s left flank: roughly 270 million cubic metres of Monte Toc detached along an ancient slide plane and dropped into the lake in some forty-five seconds, displacing water that rose more than 250 metres up the opposite wall and overtopped the dam crest by about 250 metres. The resulting wave poured down the gorge and obliterated Longarone and the villages around it, killing close to two thousand people. The structure was sound. The reservoir slope was not, and no one had treated the slope as part of the structure.

The dam was the work of Carlo Semenza, among Italy’s foremost dam engineers, and it was a feat of arch design — at completion in 1959 one of the tallest dams in the world, a thin double-curved shell anchored into the limestone walls of a deep gorge. The arch behaved exactly as designed; the post-event survey found damage confined to the topmost metre or so of the crest. The fatal element lay outside the engineers’ frame of attention entirely. The left abutment of any arch dam carries reservoir thrust into rock, but at Vajont the rock above the waterline on Monte Toc was a stack of limestone interbedded with thin clay seams, dipping toward the gorge, and crossed by the buried floor of a prehistoric landslide. The reservoir was about to wet exactly the surface that had failed once before.

Filling proceeded in stages, and the mountain answered each rise in level. In November 1960 a minor slide of roughly 700,000 cubic metres slipped on the south bank, and an M-shaped tension crack opened along the slope, tracing the head of a far larger moving mass. From then on the operators tried to control the slide by manipulating the reservoir — raising and lowering the level to coax the rock to creep slowly and safely. In the final weeks the creep accelerated instead, from about a centimetre a week to nearly a metre a day, and on 9 October the mass went all at once.

Italy’s criminal courts pursued the case for years. The L’Aquila Court of Appeal in 1970 and the Court of Cassation in 1971 confirmed that the disaster had been foreseeable and that the dam’s managers were liable for what they knew, concealed and failed to act on. Vajont remains the textbook proof that a dam’s reservoir is part of the dam, that an old landslide will move again when its failure plane is flooded, and that a reservoir slope must be investigated to the same standard as the foundation. It changed how dam sites are surveyed and made slope stability around the rim a mandatory subject of dam safety.

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