Extreme events and aging of defence works challenge the definition of residual scenarios for debris-flow hazard assessment: the case of the Rotiano Creek (Province of Trento, Italy)

Between October 27th and 30th, 2018 the Eastern Alps were hit by a severe storm named Vaia. The storm brought strong wind and heavy rainfall, both culminated on Monday 29th. In Italy, the windstorm caused the loss of 8 million m3 of standing trees (Motta et al., 2018), and the rainfall triggered floods and related geomorphic processes, such as bank erosion, sediment/vegetation transport, and debris flows. In the Province of Trento (275 mm of rainfall on average in 3 days measured by 97 ground stations, corresponding to a median 48-h return period of 115 years), more than 300 events of such type were reported during those days, counting only the most important (Piccolroaz and Michelini, 2018). One of the most violent was the extreme debris flow of the Rotiano Creek, which caused the collapse of check dams and struck the village of Dimaro. The extraordinary magnitude of the debris flow occurred along the Rotiano Creek in October 2018 put the attention on how paroxysmal scenarios, due to concurrent severe meteo-hydrological factors and possible failure of existing hydraulic defence works, should be considered in debris-flow hazard evaluations. Due to the ongoing tendency towards more frequent extreme rainfall events and considering the progressive aging of the existing control works, converging towards an effective definition of residual hazard scenarios is getting more and more important and, at the same time, challenging.