Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
Location: 48o 16' 02" N, 012o 19' 45" E.
The narrow and steep sided Vajont Valley in Italy, just an hour's drive north of Venice, was considered a prime location for a hydroelectric dam as early as the 1920's. The narrow, steep sided mouth of the gorge was considered an ideal place to locate a dam. Opposite the mouth of the gorge is the town of Longarone.
Topographically, the Vajont Valley was a perfect place for a dam. Geologically, it would be hard to find a worse place. The rocks in the Vajont Valley form a trough called a syncline. The steep and deep gorge means there is nothing to hold the rocks on the sides of the valley except the friction between layers. The south side of the valley was considered especially dangerous because the layers are nearly parallel to the mountainsides and because there is a very wide expanse of weak rocks just below the surface .
Despite warnings the the mountainside south of the dam was unstable, construction of the Vajont Dam began in 1957 and was completed in 1959. Filling of the reservoir began in 1960 and was interrupted several times when there were indications that the mountainside was beginning to slide. The owners of the dam even planned for the likelihood that the mountainside would eventually slide into the reservoir and cut it in half. They built a diversion tunnel to allow water to flow around the probable location of the slide. But they believed that any slide would offer enough warning to allow the water level in the reservoir to be lowered.
By 1963, the reservoir was full. The Vajont Dam was 262 meters (860 feet) high, the highest dam in Europe. As the reservoir neared full, there were numerous indications that the mountain slope was becoming unstable. The deep gorge meant that there was nothing at the bottom to hold the yalley walls in place except the friction between rock layers. Filling of the dam did three things to make the mountainside even more unstable. First, the water supported some of the weight of the rock. Second, water seeped into the weak rock layers, weakening them further and making them slipperier. Third, water deep in the reservoir was under a great deal of pressure, so water within the adjacent rocks was also under pressure, helping to lift the overlying rocks.
By September, 1963 it was becoming obvious that the mountainside was going to slip and the dam owners began lowering the water level in the reservoir. In October, heavy rains soaked the rocks on the mountainside, weakening the weak layers even more. Finally, at 10:38 P.M. on October 9, 1963, the mountainside gave way. The mountainside dropped into the reservoir at about the speed of a car on a freeway. It pushed water up the far side of the valley almost as high as the dam itself and also pushed water up-valley, damaging and destroying several villages. The village of Casso, on the mountainside north of the reservoir, was narrowly missed by the wave although many Web sites incorrectly claim it was destroyed.
The most remarkable thing about the Vajont dam disaster was that much of the water went around the dam rather than over it. There was only a small amount of damage to the top of the dam and the rest of the dam remained intact. The town of Longarone lay directly in the path of the flood spilling out of the gorge. About 1400 people died in Longarone when the flood hit. It was night and there was no warning. After destroying much of Longarone, the flood rushed upstream and downstream, destroying a number of villages and claiming yet more casualties. Eventually it flowed into another, larger reservoir downstream and was contained. Altogether an estimated 2000 people died in the disaster, now considered one of the largest human-caused disasters in history.
The view here looks southwest toward the rear of the dam. The dam is largely undamaged but the landslide fills the valley upstream from the dam. The flat floor of the valley is sand and gravel washed in from the landslide debris. The circular building with the spire to the right of the dam is a memorial chapel to the victims of the disaster.
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Created 8 October 2009, Last Update 15 January 2020