Plateau d'Assy disaster

The site of the landslide was located on the edge of the Désert de Platé, at the foot of the southwest slope of the Pointe de Platé [fr] which overlooks the commune of Passy. The cliff is made up of marl - limestone layers from the Cretaceous belonging to the subalpine nappes.^[1] The sanatorium rests on marl sequences from the Middle and Upper Jurassic which are dominated by thin Tithonian limestones. The slopes below the Tithonian escarpment are covered by a cone of clay scree resulting from the alteration of the upper formations. The escarpment is in turn topped by marly limestones from the Berriasian and black marls from the Valanginian.^[2] These relatively soft strata locally reach around ten meters and are generally covered by scree. The Upper Valanginian limestones then form a well-marked ledge beyond which the series is predominantly limestone and forms the cliffs and the main ridge. The series continues beyond to the Oligocene. The area is known to have been the scene of several major collapses including that of the Dérochoir [fr] in 1751.^[3]

The Plateau d'Assy is a narrow terrace that marks the southern end of the Désert de Platé and whose highest point is the Point de Platé. It was built on a large cone of scree slope overlooking the Arve Valley. This wide dispersion of scree explains the low occurrence of outcrops of the bedrock, especially marl, with the exception of the Urgonian cliffs.

A group of sanatoriums, named Children's Sanatorium of Roc des Fiz [fr], was established from 1926 on the Plateau d'Assy, on the heights of Passy, near Saint-Gervais-les-Bains, then renowned for its thermal spas.

The children's sanatorium at Roc des Fiz was inaugurated in March 1932.^[4] It is managed by the Philanthropic Association of High Altitude Sanatorium Villages, which owns three other establishments. Considered a model of a children's sanatorium, it includes a central three-story general services building from which two series of dormitory pavilions, built on stilts and connected to the central building by covered and heated walkways, depart: the girls' pavilion to the east of the central building and the boys' pavilion to the west. Finally, a fifth pavilion, alazaretto, is located between the central building and the road. The sanatorium had been under the direction of Doctor Philippe Couve de Murville since 1964 at the time of the disaster.

The beginning of 1970 was marked by weather conditions which, although not particularly remarkable, would influence the outbreak of the disaster. The winter of 1969–1970 was marked by heavy snows at high altitudes.^[5] After a January marked by sunny weather that promoted the transformation of the existing snowpack into spring snow, heavy snowfalls occurred in February and March.^[2] The accumulation of fresh snow above this layer of spring snow added to the triggering of avalanches. One of them was triggered on April 5 at an altitude of around 1,450 m (4,760 ft) above the sanatorium, striking the west wing. Damage was observed by the residents of the sanatorium but the director of the establishment decided not to give in to hysteria and only temporarily evacuates one dormitory.^[6]

A week later, heavy rains hit the region from the 12th to the 14th and made the snow heavier. Up to 30 mm fell on the 12th and 13th. On April 15, the day before the disaster, a sudden thaw caused a rapid melting of the snow cover.^[2]

The landslide started on April 16 around 12:15 AM. It began at around 1,630 m (5,350 ft) altitude on the southwest slope of the cliffs overlooking the sanatorium and flowed southwest, descending the slope in a straight line. At around 1,440 m (4,720 ft) altitude, the landslide followed a thalweg and branched off to the southeast, towards the sanatorium.^[2] It swept away the two buildings of the western wing as well as the nurses' and childcare workers' quarters, surprising the occupants during their sleep.

On-site observations on April 21 identified transverse crevasses between 1,650 m (5,410 ft) and 1,540 m (5,050 ft) in altitude, in a steep and unvegetated area, some of which reveal the underlying limestone bedrock.^[7]

Rescue efforts were quickly organized and the ORSEC plan was immediately activated. 500 rescuers, assisted by the local population, participated in the search for possible survivors for four days and three nights.^[8] The access road was first cleared to allow the arrival of rescue services and the evacuation of the injured. Many public works companies provided bulldozers and excavators.

The children who were spared by the disaster were first evacuated to temporary reception centers opened by the municipality of Passy and then transferred to other sanatoriums on the Plateau d'Assy.^[9] For fear of a new landslide and because of the shutdown of the heating system - damaged by the disaster - the children living in the buildings that were not damaged were also evacuated. The latter were not even woken up by the landslide.