Monetary evaluation of the psychological impacts of floods: Lessons from the application of a French methodology for cost-benefit analysis to the Or catchment area (France)

The implementation of hydraulic development projects in flood risk prevention policies is based, in France as in many other countries, on a cost-benefit analysis to justify the efficiency of the projects. The methodology of this type of analysis, following the recommendations of the French government, currently only includes the monetary evaluation of material damage. However, numerous post-flood field surveys show that inhabitants can be strongly impacted psychologically even when they are flooded at a low level. It is important to take into account these impacts, such as post-traumatic stress disorder, depression, etc., as they can result in significant costs such as medical expenses, inability to work or pursue domestic or leisure activities, loss of well-being, etc. However, their assessment is difficult as there is currently little empirical data on the psychological damage of flood-affected populations. In order to improve the scope of the indicators of the cost-benefit analysis, various experts on post-flood health impacts, on monetary valuation of health effects and on socio-economic evaluation of flood management projects have been mobilised by the French State. Based on an analysis of the international literature and the expertise of the people called upon, a methodology for the monetary estimation of psychological impacts was produced. It was designed to be operational, i.e. usable by any project developer in France. This methodology only takes into account post-traumatic stress disorder (PTSD), which is the main psychological damage in the event of flooding. The damage is calculated according to the level of exposure of the inhabitants during each flood scenario, with, for single-storey dwellings, a threshold corresponding to a water height of more or less 1 metre in the dwelling. Different levels of exposure are then assigned a probability of post-traumatic stress disorder as defined in the meta-analysis by Chen et al, 2015. Finally, an average cost of PTSD to the community was defined. This methodology was then applied to five contiguous case studies in the Or basin (Hérault department, France), where hydraulic projects and cost-benefit analyses had been carried out in 2016. This application shows that taking into account psychological damages compared to other material damages has a very variable effect depending on the case. Moreover, the values of the calculated psychological damages are not directly anticipable in relation to housing damages. Indeed, a dwelling may be less damaged by the project without removing its inhabitants from the floodplain. Finally, the results of this study show the interest of integrating this method of psychological damage assessment in the cost-benefit analysis of projects. Thus, depending on the nature of the projects, taking into account the psychological impacts can significantly reinforce the interest of a project or, on the contrary, only add a very marginal benefit. We also show under what conditions it can be effectively applied (what data is needed), with a view to anticipating its possible use in non-French contexts.