Torrential floods in the upper Soana Valley (NW Italian Alps): Geomorphological processes and risk-reduction strategies

This paper investigates two severe floods that occurred in the upper Soana Valley (NW Italian Alps) in September 1993 and October 2000. In both events, intense rainfall triggered violent inundations along the Soana stream and its lateral creeks, producing extensive damage to several small urban areas along the riverbeds. Comparing these two very similar flood events, which occurred 7 years apart, we tried to understand the reasons why the new remediation implemented along the riverbanks after the first flood of 1993 failed to protect houses and infrastructure during the flood of 2000. Numerous field surveys have been carried out since the flood of October 2000 to gather information about the instability on the slopes and along the streams as well as to assess damage to the built environment. Physiography, geomorphology and land-use analyses were also performed using multitemporal aerial photographs and old maps. Additional information was collected using historical archives. The results highlight that land-use decisions, in particular urban planning and management that occurred in the 1960s and 1980s, along with design flaws in the flood defence infrastructure, in particular the rip-rap and reins built after the 1993 event, were responsible for the many collapses and damage suffered during the flood of 2000. An issue with such remediation projects is the wrongly held belief that these structures are perfectly adaptable to the typology and location in any geomorphodynamic context, regardless of the characteristics of the basin and watercourse.