Expecting A Disaster: The 1963 Landslide of the Vajont Dam

The valley of Vajont (or Vaiont) in the Italian Dolomites is characterized in the upper part by a broad catchment area, eroded by ancient glaciers, and a narrow gorge, carved into limestones by the river Vajont, in the lower part. This peculiar shape 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 the time it was the highest double-curvature arch dam in the world, rising 261.6 meters above the valley floor, with a capacity of 150 to 168 million cubic meters of water. Filling of the reservoir began in February 1960 and in October the lake was already 170 meters deep. Soon afterward, first fissures were noted on the slopes of Mount Toc and November 4, with the lake 180 meters deep, a landslide with 700.000 cubic meters occurred. Alarmed, technicians decided to slightly reduce the rate the reservoir was filled. This strategy was successful until mid-1963 when, between April and May, the depth of the reservoir was rapidly increased from 195 to 230 meters. By mid-July, the depth was 240 meters, another slight increase in movements was noted. In early September the depth of the lake was 245 meters and the movements had accelerated until 3.5 centimeters per day. In late September, the water level was lowered in an attempt to slow down the entire slope. October 9, the reservoir's depth had finally been lowered to 235 meters. Even so, the slope continued to move, reaching more than 20 centimeters per day, enough to open large fissures along the slopes of Mt. Toc.

October 9, 1963, at 10:39 p.m. local time, the entire slope of Mt. Toc collapsed. Within 30 to 40 seconds estimated 240 to 270 million cubic meters of rock plunged into the reservoir, filling the 400 meters deep gorge behind the dam. The wave generated by the impact of the landslide traveled 140 meters up on the opposite shore, reaching some buildings of the village of Erto. At the moment of the impact the reservoir contained 115 million cubic meters of water. The landslide pushed part of the water out of the lake, producing a wave with a maximal height of 230-240 meters.

David Bressan

View of Mount Toc with the landslide of Vajont. The small lake on the left is what remains of the reservoir.

A 100 to 150 meters high wave rushed into the gorge of the Vajont, in direction of the larger and inhabited Piave valley. There, the wave destroyed the villages of Longarone, Pirago, Villanova, Rivalta, and Fae, in less than 15 minutes more than 2,000 people were killed. This animation shows the landslide and the generated waves.

The valley of Vajont is characterized by a succession of Jurassic/Cretaceous to Eocene marl and limestone-formations, forming a large fold, with the valley following the axis of the fold. Sedimentary layers found along the slopes of the mountains, especially on Mt. Toc, plunge into the valley, forming possible sliding planes for a mass movement.

David Bressan

Limestone with chert nodules, from the Vajont site. In the same geological formations also thin layers of clay can be found. If wet, such layers form a perfect sliding plane.