Vajont Dam and reservior. Source TeLL-Net.Vajont Dam is a hydroelectric dam completed in 1960 under Monte Toc, 100 km north of Venice, Italy.  During the filling of the reservoir on October 9, 1963, a block of approximately 270 million m³ detached from one wall and slid into the lake at velocities of up to 30 m sec-¹ (approx. 110 km h^-1).  This generated a wave that topped the dam by 245 m and swept onto the Longarone Valley below, with the loss of about 2500 lives. Remarkably, the event did not destroy the dam.

The dam was built across the Vajont Valley, a deep, narrow gorge. Vajont is located in the south-eastern part of the Dolomite Region of the Italian Alps, about 100 km north of Venice.  It was built as a part of the post-war development of Italy in order to provide energy for the rapidly-expanding northern cities of Milan, Turin and Modena. Excavation of the site began in 1956 and the dam was completed in 1960. The completed doubly curved arch dam was, at 265.5 meters above the valley floor, the worlds highest thin arch dam. The chord of the dam was 160 m, and the volume of impounded water was 115 million m^3. 

Events during the filling of the reservoir indicated that landslides posed a serious risk. Filling was initiated in February 1960, but on three separate occasions the reservoir level had to be drawn down due to the induced movements or "creeps" of material down the left bank above the reservoir.  Engineers believed they could prevent a landslide by controlling the rate and timing of the reservoir filling.  The third filling brought the level of the reservoir to a depth of 235 meters by late September 1963.

The town of Longarone, Italy before and after the disaster. Source: TeLL-Net.

At 22:38 GMT on October 9 1963 catastrophic failure of the landslide occurred. The entire mass slid approximately 500 m northwards at up to 30 m sec^-1. The mass completely blocked the gorge to a depth of up to 400 m , and it traveled up to 140 m up the opposite bank.  At the time the reservoir contained 115 million m³ of water. A wave of water was pushed up the opposite bank and destroyed the village of Casso, 260 m above lake level before over-topping the dam by up to 245 m. The water, estimated to have had a volume of about 30 million m³, then fell more than 500 m onto the villages of Longarone, Pirago, Villanova, Rivalta and Fae, totally decimating them. A total of about 2500 lives were lost. The dam was not destroyed and is still standing today. The by-pass tunnel is used for the generation of electricity.

Research into the causes of the failure suggested that the sliding surface was located in thin (5 - 15 cm) clay layers in the limestone. Increasing the level of the reservoir drove up pore pressures in the clay layers, reducing the effective normal strength and hence the shear resistance. Resistance to movement was created by the chair-like form of the shear surface. Dropping the level of the reservoir induced hydraulic pressures that increased the stresses as water in the jointed limestone tried to drain. Failure occurred in a brittle manner, inducing catastrophic loss of strength. The speed of movement is probably the result of frictional heating of the pore water in the clay layers.

Longarone was rebuilt following the disaster and is now a thriving community of about 4,100 inhabitants.

Sources

• Kiersch, G.A., 1964. Vaiont reservoir disaster, Civil Engineering, 34, 32-39.
• Petley, David, The Vajont (Vaiont) landslide, Land-man.net,  Accessed 26 September 2008.
• Suburban Emergency Management Project (SEMP), Epic Vajont Dam Disaster, Italy, 1963: Manmade or Natural?, Biot #373: June 17, 2006.  Accessed 26 September 2008.
• TeLL-Net (Transfer Live Lessons Network), The Vajont Dam Disaster, Longarone, Italy (1963).
• Voight, B. and Faust, C. 1992. Frictional heat and strength loss in some rapid landslides: error correction and affirmation of mechanism for the Vaiont landslide, Geotechnique, 42, 641-643.