Trip to Fréjus: Part II

On a recent trip to Fréjus, we visited its Roman ruins. In the gardens in front of the remains of the arena is a monument to the collapse of the Malpasset Dam, considered France’s greatest civil disaster. Malpasset was an arch dam on the Reyran River, about 7 km (under 5 miles) north of Fréjus which collapsed on 2 December, 1959 and, in the resulting flood, 423 people lost their lives. It was very moving reading the names of the deceased on the memorial, particularly those where entire families had been wiped out, including many children.

The five monumental stainless steel columns and row of desks bearing the names of the 423 identified victims were designed by local artist Michel Mourier and inaugurated 50 years after the Malpasset dam burst.

I’d never heard of this disaster, and knowing how often the Var experiences flooding, I was keen to learn more about how it happened, and what lessons had been learned to prevent it occurring again. Apologies that some of this is a bit technical but it’s something which I found interesting.

I learned that the Andre Coyne designed dam was completed in 1954 and heralded as being the thinnest arch dam for its height.  It was equipped with one gated, notched spillway at the centre of its long crest wall. Because the planned valley-side left abutement of the dam (from an upstream perspective) was higher than site topography, a large dihedral thrust block was placed below the dam to raise it to the necessary height, and spread the pressure.

Construction of the dam began in 1952 with filling of the reservoir starting in April, 1954. Approximately five years later, when the debut filling was almost complete, Malpasset Dam failed after the area experienced several days of heavy rain and high winds. The sudden failure of the dam resulted in the death of many people when emergency rescue attempts were thwarted due to the inaccessibility of the town’s flooded roadways and access routes.

Post-disaster studies revealed that a series of foundation deficiencies and human oversight led to the instability issues at the Malpasset Dam. Prior to construction, insufficient effort had been devoted to analysing the geology of the foundation on which the dam was to be located. Geological investigations that took place after the dam’s failure revealed that it had been built on a gneiss formation (banded rock) with a foliation structure (repetitive layering) exhibiting a slope of thirty to fifty degrees in the downstream direction of the dam. In addition, a fault oriented perpendicular to the river was discovered just downstream of the dam.

The foliation pattern of the foundation in combination with the presence of the fault and the forces associated with the water accumulating behind the dam caused the gneiss along the left abutment to enter a compressive state in which the permeability of the formation decreased with the increasing pressure behind the dam. Uplift pressure at the abutment caused by this phenomenon increased with the filling of the reservoir until it was great enough to dislodge the thrust block. Failure of the left abutment led to the ultimate failure of Malpasset Dam as cracks resulting from the uplift pressures that moved the thrust block propagated quickly across the dam face.

Today only bits of the Malpasset Dam remain but the memorial ensures that the devastating tragedy will never be forgotten and, importantly a lot has been learned. Specifically:-

  • Three-dimensional computer analyses were developed to study the cause of failure; these types of analyses are now used to design new arches under new standards.
  • Greater testing of foundations tested for different qualities.
  • Further development of the study of rock mechanics.
  • Recognition of the need for safety monitoring of arch dams.

If the Malpasset dam were designed today, almost no changes in the shape of the arch would have to be made: in other words, the design of the dam itself was not to blame.