Recent advances in vulnerability assessment for the built environment exposed to torrential hazards: challenges and the way forward

Abstract Irrespective of the magnitude and spatial extend, the consequences of a natural hazard are strongly connected to the vulnerability of elements at risk (e.g. people, buildings and infrastructure). It is, therefore, obvious that an analysis and quantification of vulnerability is required for successful disaster risk reduction. The need for vulnerability assessment is crucial due to the effects of climate change as well as the socio-economic dynamics that may directly impact vulnerabilities and coping capacities of affected communities. Vulnerability is multidimensional (physical, social, economic, etc.), however, the dimension which is mainly related to the direct costs and threat to human lives is the physical one. We focus here on the physical vulnerability of buildings subject to torrential hazards occurring in small and steep mountain catchments. These processes include fluvial sediment transport, debris floods, and debris flows. Furthermore, we included flash flood hazards if these are related to torrential catchments. Physical vulnerability to torrential hazards in mountain areas is a topic that has been under scientific investigation over the last 20 years. Several methods to assess physical vulnerability of buildings towards flash floods, debris flows and hyper-concentrated flows can be found in the literature. The plethora of methods and approaches may be classified under the following three categories: vulnerability matrices, vulnerability curves and vulnerability indices. We provide a short review of these methods which became available over the last decade and which dominate the scientific debate in mountain hazard risk management, giving an emphasis to vulnerability curves. The approaches presented herein are highlighted through case studies from the European Alps and beyond. Furthermore, challenges in vulnerability assessment including data requirements, need for improved event documentation, uncertainties and challenges related to future climate and socio-economic changes are outlined. Finally, a discussion on progress-driving factors such as new technologies (e.g. mobile apps, drones), citizen science and new innovative assessment methods is provided.