Improvement of mountain natural risks analysis: assessment of reach, seasonal expo- sure and presence probabilities

Mountain natural phenomena threaten people and infrastructures. Risk-informed decision making to select risk reduction measures always starts with risk analysis. Natural risks are assessed through a combination of hazard, exposure and vulnerability (equivalent to severity and probability in industrial technological contexts). In practice, characterizing the exposure is indeed not that easy since for a given magnitude, a phenomenon can have several possible trajectories, each of them corresponding to a sub-scenario with a given conditional probability. Seasonal mountain phenomena occurrence and human touristic occupation are highly variable inducing peaks in occupancy rates. This paper addresses the issue of operational assessment of assets exposure considering their seasonal reach and presence probability for different phenomenon sub-scenarios. Simplified and practical methodologies are proposed to first calculate risk based on seasonal phenomenon occurrence and exposure and secondly calculate the reach probabilities of their spatial extent. Simple examples are given for a first single phenomenon (torrential flood) and demonstrate the influence of seasonal occurrence and presence hypothesis on calculated risks. Methodologies can be extended to deal with multi-risk contexts.