A tailored framework for debris flow and debris flood hazard assessment in mountain catchments

In mountain areas, debris floods and debris flows threaten the lives of people and endanger buildings and infrastructures worldwide. The hazard assessment of such phenomena is a crucial process for hazard mapping and, eventually, design mitigation structures. The classical approach consists in the evaluation of the return period of a certain rainfall intensity that can mobilize a given amount of sediment and generate a debris floods/flows phenomenon consequently. Researchers have made progress in understanding these phenomena over the past decades, enhancing the ability to predict their impacts. Through an extensive literature review, we have fine-tuned an innovative procedural framework that takes into consideration most of the predisposing factors and processes that lead to the increase of destructive potential of debris flow and debris flood phenomena. The investigated aspects are: (i) exogenous forces (climatic, natural and anthropic disturbance related aspects); (ii) alterations of the catchment and channel conditions (countermeasures malfunctions/failures, bed/banks/slopes disposal to erosion, Large Wood presence); (iii) flow type variations (changes in transport behaviour and typology). The outcome of the study is a perspective hazard map that accounts for all of these factors and processes together with an estimation of the probable long-term evolution of the catchment response, accounting for climate change too. The study was supported by the analysis of four different catastrophic debris flow and debris flood events for which factors and processes increasing the destructive potential have been analysed.
The study highlights that joined processes and basin conditions, which are not necessarily related to rain events of high return period, should also be considered in the hazard evaluation of mountain catchments. The related hazard assessment should move toward a global and tailored assessment of the potential catchment responses, and possibly accounting for the residual hazard component. The proposed framework aims to outline guidelines to assist practitioners and civil authorities in better defining the hazard classes and consequently reducing the uncertainty associated with probable future debris flow and debris flood events.