Quantitative vulnerability assessment for torrent processes

In risk assessment, risk is expressed as a function of the probability of occurrence of a hazardous process, the exposed elements at risk and their vulnerability. With respect to mountain hazards, the assessment of processes has been subject to extensive research during the last decades, in particular concerning torrent events. However, studies on the elements at risk, in particular concerning their economic value, evolved only recently. Thus, information on the vulnerability of elements at risk in dependence on process intensities is hardly available so far; moreover, the few existing approaches do not suggest a substantiated quantitative methodology. The objective of the proposed project is to close this gap for torrent processes by the development of quantitative vulnerability functions applicable in alpine countries. Vulnerability is defined as the expected degree of loss for an element at risk as a consequence of a certain event. Hence, vulnerability is described by the quotient between loss and individual value for each element at risk. The resulting value is dependent on the impacting process intensity and the susceptibility of the elements at risk. Consequently, vulnerability has to be assessed quantitatively by a combination of methods for the determination of process intensities evolving from (natural) sciences and economic approaches for the valuation of elements at risk and associated expected damage extents. To establish this link, data from test sites in Austria will be used to analyse and assess the vul-nerability of elements at risk in relationship to occurring process intensities. The process characteristics in the accumulation areas will be determined on the basis of process documentations, and supplemented by the analysis of data gathered from a re-calculation of the events. The elements at risk will be analysed for different object categories with respect to their spatial location, and will be monetarily assessed. The losses due to the studied events will be collected using information on the respective extent of damage. As a result, vulnerability functions will be developed, linking process intensities to object vulnerability values for defined object categories and - generalised - as an overall vulnerability function for use in risk assessment. Therefore, the results gained within the proposed project will be valuated by using additional results from international collaborators to obtain universally valid vulnerability functions for torrent events in mountain regions. Recommendations and guidelines for the application within the concept of risk assessment in European mountain areas will be developed. The results of the proposed project will allow for an ex-ante quantification of torrent risk on a local level. Consequently, future mitigation of torrent events can be implemented based on the precautionary principle, which will lead to a reduction of losses due to torrent events.

In the European Alps, the concept of risk has increasingly been applied in order to reduce the susceptibility of society to mountain hazards. Risk is defined as a function of the magnitude and frequency of a hazard process times consequences; the latter being quantified by the value of elements at risk exposed and their vulnerability. Vulnerability is defined by the degree of loss to a given element at risk resulting from the impact of a natural hazard, but has not been extensively quantified so far. To close this gap we developed vulnerability functions using data from well-documented torrent events in the Austrian Alps. These functions were applicable to different building categories exposed. The method applied followed a spatially explicit empirical approach within a GIS environment, and was based on process intensities, the spatial characteristics of elements at risk, and average reconstruction values on a local scale. Additionally, loss data were collected from responsible administrative bodies and analysed on an object level. The results suggest a modified Weibull distribution to fit best to the observed damage pattern if intensity is quantified in absolute values, and a modified Frechet distribution if intensity is quantified relatively in relation to the individual building height. Additionally, uncertainties resulting from such an empirical approach were studied; in relation to the data quality a 90 % confidence band was found to represent the data range appropriately. The results suggest that there is no need to distinguish between different sediment-laden torrent processes when assessing physical vulnerability of buildings toward torrent processes. Moreover, recent empirical studies suggested a dependency of the degree of loss on the hazard impact, and respective vulnerability (or damage-loss) functions. However, until now only little information was available on the spatial characteristics of vulnerability on a local scale; considerable ranges in the loss ratio for medium process intensities only provided a hint that there might be mutual reasons for lower or higher loss rates. We therefore focused on the spatial dimension of vulnerability by searching for clusters in the damage ratio of elements at risk exposed. Based on our results, the assumption that lower process intensities result in lower damage ratios, and therefore in lower vulnerability, and vice versa, has to be partly rejected. The spatial distribution of vulnerability is not only dependent on the process intensities but also on the overall land use pattern and the individual constructive characteristics of the buildings exposed. The finally derived vulnerability functions were further compared with validation data originating from other mountain regions of the world. The vulnerability relationships obtained allow for an improved quantification of torrent risk, but also for an inclusion in comprehensive vulnerability models including physical, social, economic, and institutional vulnerability. As a result, vulnerability to mountain hazards might decrease in the future.