When the French government decided to build a dam to improve the water supply and irrigation for the notoriously dry agricultural region around Frejus on the country’s Mediterranean coast, it chose the site of a narrow gorge on the Reyran River a few kilometres inland. Geological tests indicated that the surrounding rock was suitably solid, although some concerns were expressed when construction started. The dam was of the double-curvature arch type, built of concrete. At the time of its completion in 1954 it was the thinnest of its kind in the world.
The prevailing climate meant that it took several years before the reservoir was filled to its full capacity of some 50 million tons of water. This was only achieved late in 1959 when unusually heavy rainfall had raised it to unprecedented levels.
On December 2 officials released excess water through the dam in an attempt to control the level, but with limited success. Shortly after nine in the evening the dam came away from its foundations on the left bank and burst, releasing a wall of flood water over 40 m (130 ft) high which raced down the valley at speeds approaching 70 kph (44 mph). After passing through the outskirts of Frejus, the deluge of mud and water finally discharged into the sea. In just half an hour the entire landscape had been devastated.
In spite of its thinness, the design of the dam was not at fault. The cause of the collapse seems to have been a combination of exceptional water pressure induced by the torrential rains; the composition of the rock on the left bank; and a fault line a little distance downstream.
When: December 2 1959
Where: Near Frejus, Cote d’Azur, France
Death toll: Estimates vary, but it is generally accepted that more than 420 people were killed.
You should know: The name of the site – Malpasset – derives from the French c’est mal passe (‘bad passage’) since in former times the road through the narrow gorge had been a notorious spot for ambushes by highwaymen.