On the evening of 9 October 1963, over 260 million cubic meters of rock detached from Monte Toc, on the border between Friuli-Venezia Giulia and Veneto, hit the artificial basin produced with the waters of the Vajont torrent by what was at the time the largest dam in the world, causing a tsunami of 50 million cubic meters of water which went over the dam and hit the towns of Longarone, Erto and Casso and causing 1917 victims. The Vajont disaster, one of the largest man-made natural disasters in history, was an avoidable tragedy.
The ambitious project and the experts’ first warnings
The idea of making a huge dam on the Vajont River first appeared in the 1920s. The contract for its construction was entrusted to a company called SADE (Societa Adriatica di Elettricità). Despite warnings and negative opinions from engineers and the local population, SADE managed to grab the land where the dam was to be built.
During the construction phase, a team was hired to inspect the area, including a German engineer named Leopold Müller and two Italian geologists, Eduardo Semenza and Franco Giudici, who studied the valley and concluded that the slopes above the reservoir were rather unstable and therefore the risk that a landslide could occur was real. However, their warnings fell into oblivion, also because the construction of the dam was now almost completely completed.
What happened on the night of October 9, 1963
At 10.39pm, a part of the mountain from the top of Monte Toc slipped into the Vajont dam basin. We are talking about 263 million cubic meters of rock, more than double the volume of water in the artificial lake, which rushed into the basin at 110 km/h, causing the water to rise up to 250 metres. This generated a huge tsunami wave of at least 50 million cubic meters of water that overflowed over the dam.
The flood caused by the huge wave destroyed villages in the Piave river valley in just 4 minutes, killing 1,917 people and transforming the land below the dam into a mud plain with an impact crater 60 meters deep and 80 meters wide. The town of Longarone, almost directly below the dam, was almost completely destroyed as were several small villages near the landslide.
What were the errors of judgment: an avoidable tragedy?
The need to understand the physical cause of the landslide has stimulated generations of geologists and engineers who have studied the event exhaustively, concluding that an ancient landslide had already affected the slope and the latter was reactivated along the thin layers of clay within the limestone that characterized it. In fact, the sides of the canyon, in the water accumulation area, especially on the left bank, included non-compacted rocks with different internal physical properties, layers of limestone and clay.
The first filling of the storage area began in February 1960, before the dam was completed. From this moment on, the first signs of instability began to be seen on the banks just above the artificial lake. The southern ridge slid about 3.5 cm/day and a 2 km long crack was created. The first landslide weakened the stability of the rock ridge and made it clear that a larger landslide could occur.
Subsequently, more detailed geological tests were carried out on the wells in which the piezometers were placed, which demonstrated that the sliding surface was very deep; later a seismographic laboratory was set up for earthquake monitoring near the dam itself. These studies showed that the area was very unstable and there was therefore a risk that the mountainside could slide at any moment.
Even today there is an ongoing debate on the contribution of rainfall, changes in water levels in dams and earthquakes as triggering factors of the landslide, as well as different opinions on whether it was the reactivation of an old landslide or a completely new one.
The Vajont dam today
Not everyone knows that the dam survived this catastrophe. The mighty reinforced concrete construction still closes the canyon of the Vajont river and has not produced a single kilowatt of energy, moreover it has no longer been filled with water. The dam was partially opened to the public in 2002 with guided tours managed by the Friulian Dolomites Park Authority and access to the walkway along the top. The Visitor Centers of Erto and Casso can also be visited.
