On the sensitivity of risk calculation and evaluation of mitigation measures against natural hazards

The Swiss population has had to cope with natural hazards for a long time and will have to do so in future as well. Since the rearrangement of the financial compensation, the effectiveness and efficiency of mitigation measures against natural hazards have to be assessed. Therefore, tools have been developed to calculate the risk of the area under analysis and to support the evaluation of planned mitigation measures (e.g. EconoMe). The parameters that influence the risk calculation are associated with uncertainties which in turn can have a direct impact on the planning of mitigation measures. Therefore, a sensitivity analysis of the risk calculation and the evaluation of measures was performed on the basis of case studies. The analysis was carried out with regard to the factors intensity, spatial probability of the process, mortality rate and vulnerability of objects. The outcome of this analysis showed that every parameter has an impact. However, the intensity is the factor with the highest influence, followed by vulnerability, spatial probability and mortality rate. Regarding the Swiss praxis, these results connote that a good quality of intensity maps is essential. Furthermore the existing uncertainties should be communicated by operating with the full range of estimated risks instead of absolute single values. Dealing with uncertainties, however, requires that priorisation of mitigation measures must be revised based on a cost-benefit analysis.