Why did Vaiont Dam suffer only superficial damage after a massive landslide overtopped it?

The Vaiont Dam survived the 1963 disaster with only superficial damage primarily due to its structural design as an arch dam.1

Arch dams are engineered to derive significant strength from their curved profile, which transfers the force of water pressure directly into the sound rock of the valley walls or through concrete abutments. Because of this geometry, arch dams possess a high degree of inherent resistance to failure, even when subjected to extreme events like overtopping.1

In the case of the Vaiont Dam, this robust structural integrity allowed the concrete shell—which measured approximately 3.4 metres (11.2 feet) thick at its crest—to remain largely unaffected by the massive displacement of water caused by the landslide. While the resulting wave caused catastrophic destruction and loss of life in the valley below, the dam itself successfully withstood the immense physical stress, demonstrating the great reserves of strength characteristic of the arch dam design.12