The Malpasset Dam was an arch dam on the Reyran River (fr), located approximately 7 km north of Fréjus on the French Riviera (Côte d'Azur), southern France, in the Var département. It collapsed on December 2, 1959, killing 423 people in the resulting flood. The damage amounted to a total of $68 million.
The dam was a doubly curved equal angle arch type with variable radius. It was built to supply drinking and irrigation water for the region. Construction began in April 1952 and was finished in 1954. Another source reports that construction began as early as 1941. Delays due to lack of funding and labor strikes interrupted construction a few times. Construction cost 580 million francs (by 1955 prices), and was led by well-known French engineer André Coyne. The project was funded and owned by Var département. Concurrent with the dam construction, the A8 autoroute was also being built 1,400 meters further down the course of the Reyran from the dam location.
The dam was breached at 21:13 on December 2, 1959. The entire wall collapsed with only a few blocks remaining on the right bank. Pieces of the dam are still scattered throughout the area.
The breach created a massive dam break wave, or wall of water, 40 metres (130 ft) high and moving at 70 kilometres (43 mi) per hour, destroying two small villages, Malpasset and Bozon, the highway construction site, and in 20 minutes, still standing 3 metres (10 ft) high, reaching Fréjus. Various small roads and railroad tracks were also destroyed, water flooding the western half of Fréjus and finally reaching the sea.
Geological and hydrological studies were conducted in 1946 and the dam location was considered suitable. Due to lack of proper funding, however, the geological study of the region was not thorough. The lithology underlying the dam is a metamorphic rock called gneiss. This rock type is known to be relatively impermeable, meaning that there is no significant groundwater flow within the rock unit, and it does not allow water to penetrate the ground. On the right side (looking down the river), was also rock, and a concrete wing wall was constructed to connect the wall to the ground.
A tectonic fault was later found as the most likely cause of the disaster. Other factors contributed as well; the water pressure was aimed diagonally towards the dam wall, and was not found initially. As a consequence, water collected under a wall and was unable to escape through the ground due to the impermeability of the gneiss rock underneath the dam. Finally, another theory quotes a source stating that explosions during building of the highway might have caused shifting of the rock base of the dam. Weeks before the breach, some cracking noises were heard, but they were not examined. It is not clear when the cracking noises started. The right side of the dam had some leaks in November 1959.
Between November 19 and December 2, there was 50 cm of rainfall, and 13 cm in 24 hours before the breach. The water level in the dam was only 28 cm away from the edge. Rain continued, and the dam guardian wanted to open the discharge valves, but the authorities refused, claiming the highway construction site was in danger of flooding. Five hours before the breach, at 18:00 hours, the water release valves were opened, but with a discharge rate of 40 m³/s, it was not enough to empty the reservoir in time.
Until the Malpasset incident, only 4 other incidents of arch-type dam breaches were recorded:
- Manitou dam, Manitou Springs, Colorado, 1924 at
- Moyie Dam (the Eileen Dam), Moyie Springs, Idaho, 1925 at
- Lake Lanier, North Carolina, 1926
- Purisima dam, California, 1930
- List of natural disasters by death toll - Floods and Landslides
- List of wars and disasters by death toll - Flood disasters
- Vajont Dam
- J. Bellier, Le barrage de Malpasset, 1967
- Max Herzog, Elementare Talsperrenstatik, 1998
- Max Herzog, Bautechnik 67 Heft 12, 1990