Implementation of an integrated management strategy to deal with landslide triggered debris flows: the Valloire case study (Savoie, France)

Abstract

The Rieu Benoît is a debris-flow-prone catchment located in Valloire (Savoie, France). In 2011, a lateral landslide was detected about 600 m upstream of the fan apex where houses are present. This landslide has evolved slowly since 2011 but is likely, in case of rapid collapse, to provide up to 150,000 to 200,000 m3 of material to the channel and generate intense debris flows thus threatening human settlements on the fan and in the Valloire ski resort. This paper presents a contribution to the definition of a protection strategy based on the principle that a catastrophic evolution of the landslide can be detected sufficiently in advance to set up an effective alert procedure. Such early warning system can be designed provided (i) the landslide is instrumented to properly detect its evolution and characterize the volumes likely to mobilize into debris flows, this is carried out using photogrammetric, seismic, and electrical techniques; (ii) the interaction between the landslide and the channel is observed and sufficiently understood, this is carried out using a time-lapse camera taking a picture every two hours and at higher frequency once a flow is detected by a geophone; (iii) subsequent debris flows are observed and characterized in terms of flow thickness and velocity, this is carried out at a monitoring station located at the fan apex and equipped with a radar flow stage sensor and three geophones; (iv) consequences on urbanized areas are evaluated a priori on the basis of scenarios, this is carried out by simulating the spreading of debris flows for different volumes and material properties. The final step consists in building alert and evacuation procedures in collaboration with local authorities.