Debris flows in Val Nure and Val Trebbia (Northern Apennines) during the September 2015 alluvial event in Piacenza Province (Italy)

During September 2015, some municipalities of Val Nure and Val Trebbia areas (Piacenza province) have been affected by a severe rainstorm event which has caused flooding along rivers and debris flows and landslides on slopes. A toll of 3 casualties and serious damages to infrastructures and roads was paid. In the mostly affected areas, rainfall in 6 hours reached 298 mm with an hourly precipitation peak of 107,6 mm/h. In an area of approximately 500 km2, 103 debris flow have occurred. The distribution of debris flows seems to be controlled by the distribution of the intensity of the rainstorm. Debris flows seems to have in many cases caused by slope failures in the upper part of the sub-basins. These phenomena remobilized slope debris that, subsequently, was transported by channelized debris flows. Major damages were suffered at locations where roads are crossing the creeks and streams along which the debris flow occurred. A preliminary analysis of affected sub-basins in one of the areas mostly affected by debris flows, indicates that a straightforward discrimination of the susceptibility of sub-basins based on simple morphometric and lithologic factors is not possible.