The Vajont Dam or Vaiont Dam is a disused hydro-electric dam in the valley of the Vajont river under Monte Toc, in the municipality of Erto e Casso, 100 kilometres (60 mi) north of Venice, Italy. It is one of the tallest dams in the world, with a height of 262 m (860 ft).
The dam was conceived in the 1920s and eventually built between 1957 and 1960 by Società Adriatica di Elettricità, at the time the electricity supply and distribution monopoly in northeastern Italy. The engineer was Carlo Semenza (1893–1961). In 1962, the dam was nationalized and came under the control of ENEL as part of the Italian Ministry of Public Works.
On 9 October 1963, during initial filling of the lake, a landslide caused a megatsunami in which 50,000,000 m3 (1.8×109 cu ft) of water overtopped the dam in a wave of 250 m (820 ft), bringing massive flooding and destruction to the Piave Valley below, destroying several villages and towns, causing an estimated 1,900 to 2,500 deaths. The dam remained almost intact and two-thirds of the water was retained behind it.
This event occurred after ENEL and the Italian government concealed reports and dismissed evidence that Monte Toc, on the southern side of the lake, was geologically unstable. They had disregarded numerous warnings, danger signals, and adverse appraisals. Underestimating the size of the landslide, ENEL's attempt to safely mitigate any landslide by lowering the level of the lake came too late, when disaster was almost imminent.
Geography
The dam is located in the valley of the Vajont river under Monte Toc, in the municipality of Erto e Casso, 100 kilometres (60 mi) north of Venice, Northern Italy.
Construction
The dam was built by Società Adriatica di Elettricità (SADE), the electricity supply and distribution monopoly in northeastern Italy. The owner, Giuseppe Volpi di Misurata, had been Mussolini's minister of finance for several years. The "tallest dam in the world", across the Vajont Gorge, was conceived in the 1920s to harness the Piave, Maè, and Boite Rivers, to meet the growing demand for power generation and industrialization. The project was not authorized until 15 October 1943, after the confusion following Mussolini's fall during World War II was resolved.
The dam wall had a volume of 360,000 m3 (13×106 cu ft) and held up to 168.7×106 m3 (5.96×109 cu ft) of water. The dam and basin were intended to be at the centre of a complex water-management system in which water would have been channeled from nearby valleys and artificial basins at higher levels. Tens of kilometres of concrete pipes and pipe bridges across valleys were planned.
In the 1950s, SADE's monopoly was confirmed by post-fascist governments, and it bought the land despite opposition from the communities of Erto and Casso in the valley, which was overcome by government and police support. SADE stated that the geology of the gorge had been studied, including analysis of previous landslides, and that the mountain was held to be sufficiently stable.
Construction work on the dam started in 1957. By 1959, shifts and fractures in the ground were noticed while building a new road on the side of Monte Toc. This led to further studies in which three experts separately told SADE that the entire side of Monte Toc was unstable and was likely to collapse into the basin if the filling were completed, due to the raised water level increasing the instability. SADE took no notice and construction was completed in October 1959. In February 1960, SADE was authorised to start filling the lake. In 1962, the dam was nationalized and came under the control of ENEL as part of the Italian Ministry for Public Works.
Early signs of disaster
On 22 March 1959, during construction of the Vajont Dam, a landslide at the nearby Pontesei Dam created a 20-metre-high (66 ft) wave that killed one person. Throughout the summer of 1960, minor landslides and earth movements were noticed. Instead of heeding these warning signs, the Italian government chose to sue the handful of journalists reporting the problems for "undermining the social order".
On 4 November 1960, with the water level in the reservoir at about 190 m (620 ft) of the planned 262 m (860 ft), a landslide of about 800,000 m3 (30×106 cu ft) collapsed into the lake. SADE stopped the filling, lowered the water level by about 50 m (160 ft), and started to build an artificial gallery in the basin in front of Monte Toc to keep the basin usable even if additional landslides (which were expected) divided it into two parts.
In October 1961, after the completion of the gallery, SADE resumed filling the reservoir under controlled monitoring. In April and May 1962, with the basin water level at 215 m (705 ft), the people of Erto and Casso reported five "grade five" Mercalli intensity scale earthquakes. SADE downplayed the importance of these quakes, and was then authorized to fill the reservoir to the maximum level.
In July 1962, SADE's own engineers reported the results of model-based experiments on the effects of further landslides from Monte Toc into the lake. The tests indicated that a wave generated by a landslide could overtop the crest of the dam if the water level was 20 m (66 ft) or less from the dam crest. SADE therefore decided that a reduced level 25 m (82 ft) below the crest would prevent any displacement wave from over-topping the dam.
In March 1963, the dam was transferred to the newly constituted government service for electricity, ENEL. During the following summer, with the reservoir almost completely filled, landslips, shakes, and movements of the ground were repeatedly reported by the alarmed population. On 15 September 1963, the entire side of the mountain slid by 22 cm (8.7 in). On 26 September 1963, ENEL decided to slowly empty the basin to 240 m (790 ft). By early October 1963, the collapse of the mountain's south side looked unavoidable; in one day, it moved almost 1 m (3 ft).
Tests of the hydraulic model of the Vajont tank
After the discovery of the landslide on the northern slopes of Monte Toc, it was decided to deepen the studies on these effects:
- Dynamic actions on the dam
- Wave effects in the reservoir and possible dangers for nearby locations, with particular attention to the town of Erto
- Hypothesis of a partial breakage of the dam and consequent examination of the rout wave and its propagation along the last stretch of the Vajont and along the Piave, up to Soverzene and beyond
The study of point 1 was performed at the Experimental Institute for Models and Structures (ISMES) of Bergamo, while for the others, the SADE decided to build a physical-hydraulic model of the basin, in which to perform some experiments on the effects of a landslide fall in a reservoir.
The 1: 200 scale model of the basin, which can still be visited today, was set up at the SADE hydroelectric plant in Nove (Borgo Botteon di Vittorio Veneto), and became the Hydraulic Models Centre. The experiments were entrusted to professors Ghetti and Marzolo, university professors of the Institute of Hydraulics and Hydraulic Constructions of the University of Padua, and were carried out with funding of SADE, under the control of the study office of the company.
The study aimed to verify the hydraulic effects on the dam and on the banks of the landslide reservoir, so was directed in this sense rather than reproducing the natural phenomenon of the landslide. The experiments were carried out in two different series (August–September 1961 and January–April 1962), of which the first served substantially to refine the model.
First set of experiments
The first series of five experiments began on 30 August 1961, with a sliding surface of the flat landslide inclined by 30°, consisting of a wooden plank covered with a sheet. The sliding mass was simulated with gravel, held in place by flexible metal nets, which were initially held in position by ropes that were then suddenly released. At the beginning of September, another four tests were carried out intended for orientation purposes. The first always with a 30° inclined plane, the following three with a 42° inclined plane. Having found it impossible to reproduce the natural geological phenomenon of the landslide in the model, the model was elaborated by modifying the movement surface of the landslide, which was replaced with a masonry one (the relative profiles were elaborated by Semenza, who also used the surveys that had already been carried out and that had provided sufficient elements of judgment in this sense), to make it possible to vary the speed of the landslide fall into the reservoir (made difficult by the new "back" shape of the movement surface). To simulate the compactness of the moving material (which in the model remained the gravel), rigid sectors were inserted that were towed by ropes pulled by a tractor.
Second set of experiments
In the second set of 17 experiments, conducted from 3 January 1962, to 24 April 1962, the "collapsing" material was still gravel, this time held in place by hemp nets and cords. Starting from the Muller hypothesis relating to the different characteristics of the mass moving between the downstream part of the Massalezza stream (west) and the upstream part of the same (east), all the experiments were performed by making those two hypothetical parts of the landslide descend separately. In the model, however, the two landslides were initially made to descend at different times, so that their effects were totally separate, and subsequently, when the wave produced by the first came back, so as to obtain a total increase in the water of the even greater lake.
Final Ghetti report
The total increase of the water in the tank (measured by means of special instruments) was broken down into "static increase", which was the nontransient effect of increasing the level of the water left in the tank after the landslide due to the immersion of the landslide in the tank (once the state of rest is reached again), and in "dynamic boost", due to the temporary wave motion produced by the landslide. The static increase depended on the volume of the landslide that remained immersed in the tank, while the dynamic boost depended almost exclusively on the speed of the landslide fall (while it was negligibly linked to the volume of the same).
Based on this simulation (following the disaster object of criticism, as considered approximate by some), placing a reservoir limit at an altitude of 700 m (2,300 ft) predicted that no damage would occur above 730 m (2,400 ft) above mean sea level along the banks of the reservoir, while a minimum quantity of water would have exceeded the edge of the dam (722.5 m (2,370 ft)), causing negligible damage downstream of the same.