Debris flow hazards due to land use change above source areas in torrent catchments. The case study of Les Arcs (Savoie, France)

For a few decades, land use has changed due to developments in mountainous regions and this has resulted in new pressures on the environment. Consequently, slope instabilities could be a physical expression of such imbalance. Since the 1960s, debris flows events have occurred with an increasing frequency on Les Arcs catchment as it never happened before. At the same time, a large skiing area has been expending on the upper slopes. The study of climatic series shows that rainfall events linked to debris flow triggering were not especially intense compared to the more extreme precipitations recorded in the area for the past middle century. This observation suggests that additional factors have taken part in the debris flow triggering for the last fifty years. Following this logic, the space-time study of land use has underlined the role of winter sport resort expansion on processes such as runoff and erosion affecting torrent banks and beds, directly at the origin of debris flow generation. A complementary analysis of effects on hydrology, supported by a hydrological modelling (PCRaster Software) has been carried out as well. According to modelling results, the land use conversion is responsible for a change of annual water balance resulting in a significant increase of torrent water flow. Particularly, these effects are emphasised by the localization of converted surfaces (roads, buildings, car parks, ski runs, sport facilities...) in the catchment recharge areas above steep slopes of torrent channels, where materials are liable to be mobilized. Indeed at this interface, during rainfall events the water flow and especially the peak flow are more intense than in the past, due to water rerouting and concentrating that may activate erosive processes above debris flow source areas. At the catchment scale, the increasing number of debris flow events seems to be the answer to this change.