"Know, that mountain -
it will not stand still.
Believe me it´s a troublemaker -
you can ask whoever you will."
"La ballata di Longarone", by Beppe Chierici 1969
This geomorphology and the nearby industries made this valley a perfect site for a dam and a hydroelectric power station.
Construction of the Vajont dam started in 1956 and was completed in 1960, at this time it was the highest double-curvature arch dam in the world - rising 261.6 meter above the valley floor it was 190m broad across the top, with a capacity of 150 to 168 million cubic meter of water.
Video.1. A movie was produced to document the construction of the Vajont, considered a wonder of architecture and engineering at the time.
Filling of the reservoir began in February 1960, in October of the same year the lake was already 170m deep. Soon afterwards fissures were noted on the slopes of Mount Toc and November 4, with the lake 180m deep, a landslide with 700.000 cubic meter occurred, however without any consequence.
Since this landslide and the observed movements at Mt. Toc had occurred during the infill of the reservoir, it was assumed that the rising water table and water pore pressure in the slope of the mountain was the most important destabilising factor. By slowly rising the level of the reservoir it was hoped that the movements would also slow down. This strategy was successful until mid-1963 when, between April and May, the depth of the reservoir was rapidly increased from 195 to 230m. Movements increased slightly but never exceeded 0,3cm per day. By mid-July, the depth was 240m and some of the control points indicated a small increase of the movements to 0,5cm per day, then in mid-August to 0,8cm per day.
In early September the depth of the lake was 245m and the movements had accelerated until 3,5cm per day. In late September the water level was again lowered in an attempt to slow down the entire slope - October 9, the reservoir's depth had fianlly been lowered to 235m. Even so the slope continued to move and accelerated reaching more than 20cm per day.
October 9, 1963, at 22.39 local time a part of Mt. Toc collapsed. Within 30 to 40 seconds estimated 240 - 270 million cubic meter of material plunged into the reservoir, filling completely the 400m deep gorge behind the dam. The wave generated by the impact of the landslide travelled 140m up on the opposite shore, reaching some buildings of the village of Erto. At that time, the reservoir contained 115 million cubic meter of water. The landslide pushed part of the water out of the lake, producing a wave with a maximal height of 230-240m. A 100 to 150m high wave overleaped the dam into the gorge of the Vajont, in direction of the larger and densely inhabited Piave valley.
The wave destroyed the villages of Longarone, Pirago, Villanova, Rivalta and Fae, in less than 15 minutes more than 2.000 people were killed - many bodies were never found.
Fig.1. Aerial photo of the valley of Vajont after October 9, 1963 (image in public domain), compare also with an photo taken previously to the landslide.
Video.2. Showing the movement of the final landslide and the directions of the generated wave.
Geological surveys at the construction site had started already in the late 1920s. The area is characterized by a succession of Jurassic/Cretaceous to Eocene marl and limestone - formations, folded into a large fold with the valley following the fold axis. Between 1956 and 1960 it was realized that the slopes of Mt. Toc were more prone to instability than expected, since the mountain's outer flanks consisted of ancient landslide deposits and not solid rock. Research carried out after the event confirmed the presence of interbedded layers of green claystone (5-10 cm thick) in the limestone of the Fonzaso Fm. (an only 10-40 m thick formation), which acted as sliding planes for the prehistoric landslide and were reactivated during the 1963 landslide.
This already unstable situation was then even more destabilized by the rising water level inside the mountain (following the lake level) and strong rainfalls in August, September and October 1963.
Fig.2. Summary of events recorded at the Vajont, modified after MÜLLER 1964 and BELLONI & STEFANI 1992 - geological investigations and proposed models, precipitation, water levels in the reservoir and groundwater levels (measured with piezometers) and rate of movements. The last rise of the reservoir level was accompanied also by stronger earthquakes coming from the slopes of Mt. Toc. Note also how the groundwater level became after 1961 synchronous with the reservoir level, indicating that the previously isolated aquifers in the mountain became (trough the sliding planes?) connected to the lake.
But geology was only one factor causing this catastrophe: After the disaster a legal investigation was initiated to clarify possible human responsibilities.
For more than three years the movements were monitored and various geologists studied the slope. Shear zones with fragmented rocks were discovered during the construction of a tunnel deep inside the mountain.
Some geologists warned afterwards of a deep-seated landslide, like Austrian engineer Leopold Müller in 1960 and later Italian geologist Eduardo Semenza and Franco Giudici, but the most pessimistic views were ignored and a model with more superficial sliding planes, proposed in 1960 by Italian geologist Pietro Caloi, who however changed opinion a year later, adopted.
Minor landslides, as happened in 1960, were in fact always expected, in 1961 even the construction of a by-pass tunnel was started, just in case the reservoir would became partially obstructed by a landslide. In 1961 calculations based on a small model of the entire reservoir suggested that a (small) landslide into the lake could generate a 30m high wave. As results a water level of 700m a.s.l. was recommended (surpassed in 1963 by 10 meter).
The important factor transforming a natural event (a landslide in the Alps is nothing unusual) in a catastrophe was the unclear and chaotic situation of responsibilities and various conflicting interests in the project.
The continuous rejection of the worst case scenario - a gigantic single landslide - by the authorities, by the electric power company and the government was in part a result of the lack of research on such large landslides. In the early 20th century the interests of geologists and engineers focused mainly towards slow mass movements, like slope deformations, better studied at these times. After the tragedy of the Vajont the interests shifted back to the research on rock mechanics and fast mass movements.
There were also a political and financial conflict of interests - a gigantic investment and many politicians directly involved in the promotion and support of the "Great Vajont" project.
Finally also the situation of the ownership of the dam was unclear - the dam was initiated by a private company, but in March 1963 the project was nationalized. The increased movements were observed by engineers of the company and government consultants, but in the end nobody felt responsible for risk communication to the public. When in the last weeks of September and early October 1963 it was realized that the worst case scenario - a mass wasting with over 200 million cubic meter of volume moving as a single block - was occurring, it was already too late.
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