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The Vajont dam tragedy - the massive tsunami from the dam wiped out many towns and villages, killing more than 2,000 people. Yet, the next morning, authorities were shocked to find the dam intact. If the dam remained undamaged, how did such a devastating tragedy occur?
That night, a massive landslide occurred on the south side of Mount Toc, moving at speeds over 90 km/h. You can see here, the soil which plunges into the water body at high speed, generates an enormous wave that overtops the dam with ease, propagating downstream and wreaking destruction.
The authorities decided to construct a dam between these two mountains. The area’s strong rock formations were seen as suitable for supporting the dam, while the steep angles and narrow gaps promised significant savings in construction materials. But, there was a problem.
Mount Toc had a grim nickname—locals called it the "mountain that walks". This region with vertical cliffs was infamous for landslides, sometimes shifting inches per year and other times moving at speeds of tens of kilometers per hour. However, the authorities were convinced that the narrow region was ideal for dam construction.
The Vajont Dam featured a beautiful double-curvature arch design, also known as a shell arch design. The curvatures in both directions efficiently resisted the immense hydrostatic pressure, much like the thin shell of an egg withstands applied force. To grasp how slender the Vajont Dam’s structure was, compare it to the Hoover Dam, which is a gravity arch dam. The initial proposed height for the Vajont Dam was 150 meters, but this was later increased to 200 meters, and finally to 260 meters, making it the tallest dam in the world at the time. It had the capacity to hold 168 million cubic meters of water and was estimated to generate 200 MW of electricity from different hydroelectric power plants—an impressive feat.
Filling of the dam began in February 1960. When the water level reached 170 meters, authorities noticed a large crack on Mount Toc—an ominous sign that a major landslide was imminent. The rate of displacement was measured at 3.5 cm/day. To understand this phenomenon, we must delve into some crucial geotechnical details.
Mount Toc appeared calm, stable, and strong due to its limestone rock, but a hidden danger lurked beneath. Beneath the thick limestone layer lay a 5–10 cm thick clay interbed. The high slope of the mountain, combined with the limestone’s permeability and the clay layer acting as a slip surface, created a perfect setup for disaster. As the dam filled, water percolated through the limestone, weakening the clay’s shear strength significantly. Heavy rainfall between 1960 and 1963 further exacerbated the situation. This is why the landslide began to mobilize in 1960, just nine months after filling commenced.
On November 4, 1960, when the reservoir depth reached 180 meters, a large failure occurred. Around 700,000 cubic meters of material slid into the lake in about ten minutes. After this incident the movement got reduced to just 1 mm/day. This incident caused no immediate damage to the dam or downstream villages.
However, this incident foreshadowed greater risks. Authorities feared that a faster landslide could generate a wave capable of overtopping the dam.
Despite warnings, authorities did not abandon the hydroelectric project. Engineers from SADE conducted detailed model-based experiments to assess the impact of a larger landslide and devise solutions. Their July 1962 report warned that a high-speed landslide could be catastrophic, especially if the reservoir was full. The dam might survive such an event, but the resulting water wave would overtop it, destroying downstream villages in a tsunami. The management ignored this warning. The next main finding was that the filling of the reservoir due to the landslide happens in 10 minutes or so , the reservoir water won’t over-top the dam. In this case, they can still operate the hydroelectric facility.
This is a Vajont model rebuilt. You can see here when I introduce the landslide mass very slowly in the reservoir no water is overtopping the damp.
In the second experiment I’m dropping the mass into the reservoir, interestingly small waves of water here and it is able to over top the damp.
The third experiment in fact I am throwing the mass with a high speed. Here a huge wave followed by many small waves easily over topping the damp a great extent and extremely dangerous condition. Interestingly in this case the volume displaced by the water is much higher than the volume of the mass I introduced. However the authorities wanted a very slow land slide in that case the tunnels on the left side will be blocked, so their solution “build the tunnels on the right side.
Their solution to control the rate of landslide was by controlling the level of water level using drainage tunnels.
More the water height, more will be the joint water thrust in the rock hence a higher movement of mass. If they want a controlled landslide, just reduce the water height. The engineers were obviously playing with human lives!
The Vajont dam mountains had impressive internal tunnel arrangement. Some tunnels would fill the reservoir while the other tunnels drained it out. Here is the main power production facility of the Vajont dam - Colomber power plant. This was a Francis turbine based power generation facility. By controlling the valves of these tunnels authorities were able to alter the reservoir water height easily.
The authorities installed electric poles and light to observe the movement of the crack even at night.
These are the sequence of their water level experiments and its impact on the landslide. At first they reduced the reservoir height. The movement reduced from 8 cm/day to 3 mm/day at a level of 185 m of water. When the level was 135 m, the movement was just 1 mm/day. Remember, by this time the main landslide mass had moved an average of about 1 m.
In October 1961 they started the second filling of the dam and reached a water level of 235 m. The movement velocity increased to 1.2 cm/day.
In November 1962 the authorities started the second draw-down of the dam. By April 1963 when the water level height was reduced to 185 m, the movement rate wa s effectively zero. These experiences increased the confidence of engineers. They believe that the landslide can be controlled by adjusting the water level.
You can guess what they have done next. The authorities went for filling the dam once again - the third filling. Until July 1963 when the height was 240 m everything went smooth, the rate of movement increased only by a small amount – half a centimeter per day. In September the height was increased to 245 m and there was a drastic increase in land movement - 3.5 cm/day. The authorities took immediate remedial action - bring down the water lever further. They brought down the water level to 235 m, but the movement rate kept on increasing. On October 9, 1963 the movement rate was 20 cm/day.
There was excessive onsite monitoring at the dam when the rockslide occurred - 20 technical people at the left abutment and 40 people at the right abutment.
At 10:38 PM on October 9, the catastrophic landslide occurred. The size of the forest rock which was moving down was 260 million meter cube, with a speed of more than 90 km/hr. The landslide was completed in mere 45 seconds. Remember the engineers wanted this landslide to happen in 10 minutes. All the technical people perished under this excessive wave. On that day the dam had 115 million cubic meters of water. The village of Casso situated on the opposite bank was completely destroyed by the huge wave of water . The other nearby village on the bank - Erto had a lucky escape. This village was situated after a curvature and the high power water could not reach this region. The other half of the wave over-topped the dam by a whopping 245 meters. The 350 m tall Tsunami first reached the lake Longarone. Here it drops its speed to 80 km/hr and wave height 170 meters. The Tsunami just needs 4 minutes to reach the town. In many bars of Longarone people were enjoying a football match. Suddenly the power failed and they received heavy wind with mist and sand in it. Most of the people came out to understand what was going on. The entire town of Longarone was wiped out by the mega Tsunami. The villages of Pirago, Villanova, Rivalta and Fae were totally decimated. The overall death toll of this disaster was more than 2000.
The Vajont River was permanently diverted due to one of the biggest landslides Europe has ever witnessed. The dam itself sustained only minor damage, standing as a testament to its robust design and construction. The dam overcame the pressure of such a massive wave. Today, tourists visit the site, walking along the dam’s crest road. When you look from here all you can see is a huge chunk of rock and soil - no river.
The landslide caused massive earth tremors that were even recorded in distant cities such as Vienna and Brussels. The reservoir, with a volume of 115 million cubic meters, was effectively a water bomb. It was estimated that the shockwave from the air displacement was twice as powerful as the nuclear bomb that hit Hiroshima.
The Vajont Dam disaster serves as a stark reminder of the importance of geotechnical engineering also the risk of prioritizing business interests over human safety. Authorities ignored clear warnings about the risks of high-speed landslides, instead they chose to play with water level experiments. This disaster was not a natural calamity but a avoidable tragedy caused by human negligence.