Exposure and social vulnerability of French cities to flood risk: a spatiotemporal analysis at a fine scale (1999-2017)

More than half of the municipalities in France are affected by flood risk. In this context, our study aims to measure both the physical and social vulnerability of urban populations to flooding over the last twenty years (1999-2017). Physical vulnerability is defined here by the exposure of populations to the flood hazard, and social vulnerability by the propensity of populations to suffer the negative consequences of a major flood over the long term. The measure of vulnerability is based on a very fine-scale disaggregation of census data. The results reveal a phenomenon of accumulation of exposure and social vulnerability at the national scale: social vulnerability indicators are systematically higher in flood-prone areas than outside flood-prone areas. However, this accumulation does not seem to worsen over the study period. For the indicators of social vulnerability, the gap between flood-prone and non-flood-prone areas does not increase, while for exposure, the population growth in flood-prone areas remains lower than overall demographic growth. However, these general observations conceal very different situations depending on the city, and our analysis highlights the importance of a geographical approach to vulnerability.