Quantitative vulnerability assessment of buildings to debris-flows in Fella River Basin using run-out modeling and damage data from the 29th of August 2003 event.

Extreme rainfall events trigger floods and debris flows with considerable consequences in mountain regions worldwide. The reduction of possible human and material losses is dependent on the assessment of risks as well as design and implementation of effective reduction strategies. These in turn rely not only on the analysis of the magnitude and frequency of the harmful events but also on the comprehensive evaluation of exposed elements and their vulnerability. Vulnerability is defined as the degree of loss to a given element or set of elements at risk resulting from the occurrence of a hazard of a given magnitude in a given area, expressed as a percentage of loss (between 0: no damage, to 1: total damage) (Varnes, 1984). Although, conceptual frameworks for quantitative vulnerability estimations have been developed (Papathoma-Köhle et al., 2011), including more recent studies on uncertainty analysis (Eidsvig et al., 2014, Totschnig and Fuchs, 2013, Kaynia et al., 2008), there is still a great need for advancing methodologies in areas where they are essential for risk mitigation investments and improved decision-making. This study aims to develop a set of vulnerability curves for buildings impacted by debris flows in the Fella River Basin (Eastern Italian Alps). The area experienced a major rainfall event in August 2003 which triggered more than a thousand debris flows resulting in significant economic losses and two casualties. Two methodological approaches are applied for the estimation of process intensities: one based on dynamic run-out modeling (regional scale), and another based on interpretation of aerial and photographic documentation (local scale). The results are compared with damage values estimated shortly after the event, as well as vulnerability curves from the literature. Asset characteristics are collected through field work and desktop mapping, and population distribution and value for each building is calculated based on census and real estate data. The obtained vulnerability curves together with population distribution and building monetary value are subsequently used in an exposure and risk analysis at regional scale in Fella River Basin.