La Valette mudslide 

Introduction

Location

The La Valette mudslide is located on the South-facing slope of the Barcelonnette Basin (Southern French Alps), over the municipality of Saint-Pons, on the right side of the Ubaye valley. 

The landslide complex  of La Valette is one of the biggest landslides in France, with a total volume estimated at 4.106 m³. of displaced material between 1982 and 2000. The landslide originated in the upstream parts of the Serres and La Valette torrents. In this area, clayey and permeable morainic deposits are underlaid by clay-shales (Terres Noires) of Callovo-Oxfordian age which are highly impermeable. This condition favours slope instability as excess pore water pressures will be encountered during period of snow melt and prolonged rainfall. This problem is intensified by the leakage of various sources which spring from the discordance contact between Terres Noires and the flysch into the moraine.  

The upper main scarp of the landslide complex is situated at a height of 2000 m while the toe lies at 1250 m. The length of the mudslide is nearly 1400m, and the average slope gradient is 18°. Velocities lie in the range from 0.05 to 0.40 m.day^-1.

La Valette mudslide from the bottom in 2001 and schematic cross-section

Risk and major stakes

The level of risk is high, as several surges and the triggering of rapid mudflows have been observed  in 1988 and 1992. These mudflows were considered to be a serious threat to the community of Saint-Pons located downstream, and the local stakeholder RTM (of Department Alpes-de-Haute-Provence)  is in charge of developping and maintaining remedial measures to prevent further movement, and an early-warning system. 

Since 2001, an important retrogression of the main scarp and the crown is observed. The failed material from the main scarp is progressively loading the upper part of the accumulation zone, which is a prohibited area since 2006.

3-D View of the La Valette mudslide with Google Earth (2008)

Photograph of the La Valette mudslide in 2001 from the opposite slope of the Ubaye valley

Research 

Since the 1988, the mudslide is monitored by the local stakeholder RTM. Mitigation works have been carried out and an eraly-warning system has been developped. 

The La Valette mudslide has been the subject of many research studies and several PhD or post-doc projects.

PhD Thesis completed

• Casson, B. 2004. Apports de l'imagerie optique haute résolution pour l'étude 3D des glissements de terrain. Thèse de Doctorat, Université Claude Bernard, Lyon, 281 p.  [pdf]
• Le Mignon, G. 2004. Analyse de scénarios de mouvements de versants de type glissements-coulées. Application à la région de Barcelonnette (Alpes-de-Haute-Provence, France). Thèse de Doctorat, Ecole Nationale des Ponts et Chaussées, 261 p. [pdf]
• Squarzoni, C. 2003. Mesure des champs de déplacement de surface et modélisation numérique des glissements de terrain. Thèse de doctorat, ENS Lyon.

Phd Thesis in progress

• Travelletti Julien 

Post-doc projects

• Spickermann Anke

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