1.1.1 The Past: Lessons We Learned

On the evening of October 9, 1963, after a period of heavy rain, a block of rock of some 270 million m3 detached from the mountainside above the reservoir impounded by the Vajont Dam in the Italian Alps (see Figure 1.1). The rock mass reached an estimated velocity of 110 km/hr by the time it reached the reservoir. The wave of water displaced by the landslide destroyed the town of Casso, 260 m above the reservoir on the opposite side of the valley, and then sent a wave of water 250 m high over the top of one of the world’s tallest dams and crested at 262 m. In Longarone and other hamlets downstream 2,500 unsuspecting villagers lost their lives that evening. The dam remained intact.

The geology of the reservoir area was incompletely understood and mapped. The analysis conducted after the disaster found that the massive slide occurred along an unrecognized clay layer in the limestone bedrock. The lack of knowledge of the geology and a misunderstanding of the

FIGURE 1.1 The Vajont landslide looking from upstream (image courtesy of Professor E. Bromhead, Kingston University; used with permission).

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