Debris flow and debris flood hazard assessment in mountain catchments

Debris flow and debris flood phenomena threat the life of thousands of citizens living in mountain areas and endanger buildings and infrastructure worldwide. The assessment of the probable magnitude of these events is a key process in hazard mapping and the research community has been improving their comprehension of this topic in the last decades and consequently the capacity to predict the impacts of such events. The study analyses the current state of the art for hazard mapping and it pays particular attention to the concept of residual hazard. Through an extensive analysis of events that caused major damages, processes, factors and basin conditions, which are strictly related to debris flow and debris flood events, are investigated in order to improve the hazard-mapping reliability. Drawing from a thorough analysis of the literature and four complex events directly investigated, the study proposes a procedural framework to develop more reliable sets of possible scenarios for hazard mapping. The developed methodology proposes to include in the hazard assessment of mountain catchments (i) exogenous forces (climatic forces, natural and anthropic disturbances), (ii) alteration of the system condition (countermeasures malfunctions/failure and bed/banks erosion) and (iii) flow type variation (spatial and temporal variation of the flow and change in transport typology). The result is a perspective hazard map that takes in account all these factors and processes together with an estimation of their mid-long term evolution, accounting for climate change conditions. Here, future catchment responses are incorporated in a global catchment view, which allows the prediction of seemingly infrequent processes that are sometimes not rare for certain mountain basins. The proposed framework aims to assist practitioners and civil authorities in better defining the hazard classes for a given area thereby reducing uncertainty related to possible debris flow and debris flood events.