Mass movements in small alpine watersheds: a common but underestimated risk. The case of Savaranche Valley (Aosta Valley) on June 23, 2002

Very small watersheds (up to a few km2) joining major valley bottoms represent a significant source of risk during intense, even very short rainfall events. Landslides occurring in these basins mostly involve superficial deposits and can evolve into fast flows under favorable morphological conditions such as mass funneling and water contribution from the watershed. The meteorological event of Sunday, 23 June 2002 that struck the Aosta Valley was studied in detail to draw attention to these instability processes, to outline the most significant characteristics and to suggest actions for risk mitigation. The most striking characteristic of the event was the extreme spatial and temporal concentration of the instability phenomena. In less than one hour of very heavy rainfall and in a stretch of less than 7 km, the main road running along the Savarenche Valley bottom was interrupted by 13 flows that originated on both the right and left valley sides. Road use interruptions cut off more then 500 tourists in the upper part of the Savarenche Valley, and 3 cars were trapped between two subsequent flows.