Nationwide evaluation of changes in fluvial and pluvial flood damage and the effectiveness of adaptation measures in Japan under population decline

To implement strategic countermeasures against flood damage, assessing flood damage and its countermeasures is necessary. However, in Japan, there is a lack of scientific knowledge on how to implement strategic measures considering climate and demographic changes. In this study, we aimed to evaluate the impacts of climate and demographic changes on fluvial and pluvial flood damage and the effectiveness of adaptation measures across Japan. The main novelty of our methodology is the incorporation of Coupled Model Intercomparison Project Phase 6-based climate scenario data and the Japanese version of shared socioeconomic pathways into damage cost calculations based on inundation analysis to consider both climate and demographic changes. Our results showed that although fluvial and pluvial flood damage could increase in almost all prefectures due to climate change, damage could decrease in approximately 70% or more of prefectures by the end of the 21st century after considering demographic changes. In contrast, flood countermeasures are necessary, as the per capita fluvial and pluvial flood damage costs are estimated to increase. Furthermore, our assessment of adaptation measures suggested that demographic change causes a reduction in the effectiveness of adaptation measures. Therefore, we emphasise the need to consider not only climate change but also demographic change in adaptation planning.