Mitigation actions against natural risks in mountains are based both on non-structural measures such as land-use control and on structural measures such as civil engineering protection works. To prevent or, at least, limit risks, protection works play an essential part in reducing both the causes and effects of phenomena. The analysis of their effectiveness is an essential stage of the risk management process. Decision support tools are needed to take into account their structural state and functional effects on phenomena (stopping, braking, guiding, diverting etc.) but also their economic efficiency, comparing their construction and maintenance costs to direct and indirect economic losses reduction. The concepts of dependability analysis, functional analysis and failure modes and effects analysis (FMEA), already used in civil engineering especially for hydraulic dams, are extended to assess and evaluate the structural and functional effectiveness of protection works. The methodology elicits the expert reasoning process and evaluates the danger level of the protection works. It uses of both dependability analysis, an emerging technique for natural hazards domain and uncertainty theories such as fuzzy logics, possibility theory to take into account the imperfection of information. Fuzzy intervals and possibility distributions are used to assess the indicators that describe the possible failures and the performance level of disposals. Indicators are defined as information formalised so as to make its use repeatable and reproducible. They provide indications on a function performance. Their implementation is based first on a system description and on the proposal of indicators related to structural and functional failure modes. The concept of risk, the conditions and limits of cost-benefit analysis are discussed with regard to imperfect of knowledge about frequency, physical effects on phenomena, vulnerability curves and economic valuations of losses.