Multi-hazard analysis in natural risk assessments

Analysis of natural risks in mountainous regions includes several typical natural processes such as snow avalanches, floods, earthquakes, and different types of landslides. Separate investigations of single processes only might lead to a misjudgement of the general natural risks for these areas. To avoid this trap, natural risk assessments should not focus on a singular process but on multiple processes. Within this study a general methodology is developed to analyse natural risk for multiple processes. The method is applied in Bildudalur, NW-Iceland. In particular snow avalanches, rock falls and debris flows pose a hazard to the village of 300 inhabitants. The natural risk calculation is a function based on the input parameters hazard, vulnerability, probability of the spatial impact, probability of the temporal impact, probability of the seasonal occurrence and damage potential. First, the risk posed by each process is calculated. Results are presented as individual risk and object risk to life, and as economic risk for each process. Finally, single process risk maps are combined into multi-hazard risk maps. In the study area the highest risks throughout all of the analyses (individual risk to life, object risk to life and economic risk) are caused by debris flows, followed by snow avalanches and rock falls. It is demonstrated that risk varies heavily depending on the process considered. The total risk to life caused by snow avalanches, debris flows, rock falls and multi-hazards is 0.19, 0.63, 0.009 and 0.83 deaths per year, respectively. Multi-hazard approaches are not only valuable to get an overview on the overall risk but have also a high significance for planning effective countermeasures. It can be concluded that the newly developed method is applicable to other natural processes as well as to further catchments in Iceland as well as in other countries with different environmental settings.