Integrating flood-induced population movements into future fluvial flood damage estimates in Japan

Recent studies have highlighted that flooding can influence population dynamics. However, existing estimates of future fluvial flood damage primarily consider population changes driven by births, deaths, and migration unrelated to flooding. As a result, the potential impacts of flood-induced population movements (FIPMs) on future fluvial flood damage costs remain largely unexplored. This study evaluated the impacts of FIPMs on future fluvial flood damage costs in Japan, a country that faces flood risk and population decline. We developed a methodological framework that uses statistical causal inference to quantify FIPMs, integrates these estimates into future population and land-use projections, and evaluates future fluvial flood damage costs under scenarios of climate and land-use change. Empirical relationships between flood magnitude and FIPMs were estimated using grid-cell-level data from two flood events in 2019 and 2020 and municipality-level data from municipalities affected by flood disasters during 2015–2020, and then applied to future population projections using flood magnitude indicators derived from future fluvial flood inundation analyses. The results indicate that incorporating FIPMs leads to only modest changes in estimated fluvial flood damage costs at the national level (generally below 1 %), and similarly modest impacts at the prefectural level, except for a few prefectures with changes of approximately 2 %. However, greater variability is observed at the municipal level, with approximately 10 % of municipalities experiencing changes exceeding 1 % and some municipalities showing reductions in estimated fluvial flood damage costs exceeding 10 %. These findings highlight the importance of accounting for FIPMs in municipal-level fluvial flood risk management frameworks and policy evaluations.