At 10:39 pm, the landslide of a portion of Mount Toc sent 260 million m3 of earth and rock hurtling at more than 90 km/h into the full reservoir adjoining the Vajont Dam. The falling rock nearly instantaneously filled the storage reservoir. Two waves, containing 25 million m3 of water each, unfurled both downstream and upstream of the reservoir. The water rose above the dam crest, forming a wave higher than 150 metres that rushed into the gorge in the direction of downstream, town of Longarone.
The thrust of this water mass into the very narrow valley caused some phenomena of tremendous violence: the air brutally propelled by water formed a pressure wave that started to ravage the city, then the water carrying considerable debris led to major destruction. The water mass devoured the cities of Longarone, Pirago, Rivalta, Villanova and Faè along with many surrounding villages. According to estimations, between 1,900 and 2,100 people lost their lives. The 261-m high, reinforced concrete arch dam with double curvature was left practically undamaged, as only the adjoining installations (control room, machinery, etc.) were actually destroyed.
From geological studies and model tests conducted after a partial rockslide that had occurred in November 1960, the causes of this accident were rather easy to identify, i.e. a combination of several unfavourable factors, as confirmed by the follow-up investigation:
- the site’s complex geological configuration and Mount Toc’s northern slope featuring a unique “chair”-shaped sliding surface along a curved stratification plane;
- storage reservoir level variations;
- rainfall intensity – heavy rains during the week before the accident had weighed on the ground and partially lubricated the slipping plane.
One of the facility managers blamed for the disaster would later commit suicide. Most individuals with either political or technical involvement in the project were acquitted due to a lack of evidence, with the exception of the lead project engineer, sentenced to 5 years in prison in 1977, although he was pardoned after serving just 1 year.