The evening of October 9, 1963over 260 million cubic meters of rock detached from Monte Toc, on the border between Friuli-Venezia Giulia and Veneto, hitting the artificial basin produced with the waters of the Vajont torrent from what was there at the time largest dam in the worldcausing a tsunami of 50 million cubic meters of water which went over the dam, hitting 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 a 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 awarded to a company called SADE (Adriatic Electricity Company). 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, which included a German engineer named Leopold Muller and two Italian geologists, Eduardo Semenza And Franco Giudiciwho 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 10.39pma part of the mountain from the top of Monte Toc slipped into the basin of the Vajont dam. Let’s talk about 263 million cubic meters of rock, more than double the volume of water in the reservoir, which precipitated a 110 km/h into the basin, causing the water to rise up to 250 meters. This generated a huge tsunami wave of at least 50 million cubic meters of water which 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 turning the land below the dam into a mud plain with a deep impact crater 60 meters and wide 80 meters. 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 area of water accumulation, especially on the left bank, included uncompacted 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 they began to grasp the first signs of instability on the banks just above the artificial lake. The southern ridge slid approximately 3.5 cm/day and a long crack was created 2km. 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 how the sliding surface was very deep; later a seismographic laboratory was set up for earthquake monitoring near the dam itself. These studies have shown that the area was very unstable and there was therefore the 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 summit. The Visitor Centers of Erto and Casso can also be visited.
