Comparative Effectiveness and Future Viability of Flood Control Adaptation Measures Under a Changing Climate and a Declining Population

ABSTRACT Adaptation measures play a crucial role in mitigating the increasing severity of flood disasters driven by climate change. While various strategies are implemented globally, their geographical characteristics and long‐term effectiveness remain uncertain. This study addresses this gap by evaluating and comparing the regional effectiveness levels of six major flood adaptation measures in Japan, thereby considering the interplay of climate change and demographic decline. Given the inherent uncertainties in long‐term climate and socioeconomic projections, this study focuses on a relative evaluation of damage reduction rates rather than providing absolute damage predictions. Our findings reveal that while stilt houses constitute the most effective measure at the national level, their effectiveness varies significantly at the prefectural level, highlighting that a one‐size‐fits‐all national strategy is suboptimal and that tailored, region‐specific adaptation portfolios based on local geographical and socioeconomic conditions are essential. In the near future, the impacts of climate change are projected to overwhelm the benefits of most individual adaptation measures, leading to increased economic damage. However, toward the end of the century, the decline in the population of Japan may significantly reduce flood exposure, potentially outweighing climate change impacts and creating a risk of maladaptive overinvestment in infrastructure if not planned dynamically. This complex dynamic necessitates the development of flexible, adaptive pathways for flood risk management rather than static, long‐term plans. Furthermore, we find that measures with notable co‐benefits, such as those contributing to climate change mitigation, hold strategic importance despite their lower direct flood‐reduction effectiveness. These findings provide crucial insights for policy‐makers to design sustainable and efficient flood risk management strategies that are spatially explicit, dynamically adjusted over time, and integrate both adaptation and mitigation goals.