Population decline in inherently high flood-risk areas as a counter to Disaster Risk Increasing Development: evidence from the Arakawa River, Tokyo

Urban development in floodplains generates economic gains but also increases hazard exposure. This study would like to name such contradictory development patterns as Disaster Risk Increasing Development (DRID), drawing on sociohydrological discourse. Flood control achieved through pre-disaster disaster risk reduction (DRR) investments in the process of DRID may lower risk perception (the levee effect), encourage settlement, and ultimately increase potential losses (the safe development paradox). Japan, with its DRID-with-DRR investment model, has promoted pre-disaster DRR investment globally; however, it continues to face challenges such as low economic growth, population aging, and climate change. In this context, how DRID can be suppressed despite the degradation of risk perception arising from DRR investment remains unclear. This study investigates whether heightened flood-risk perception associated with hazard map disclosure can coincide with population decline in inherently high-risk areas, suggesting a potential pathway to reduce long-term DRR burdens while setting aside the urgent short-term resolution of such conditions. The study focuses on the downstream area of the Arakawa River in Tokyo (11 wards), which has consistently received DRR investments over many years and has experienced fewer flood incidents in recent decades. Building on prior research that revealed statistically significant land-price declines in higher flood-hazard zones from 2005 to 2024, we evaluate development and exposure trends by overlaying a probable maximum rainfall hazard map (1-in-1000-year event) with multiple spatial datasets: 100-m mesh land-use subdivision data (1976–2021), 500-m mesh census population statistics (1970–2020), and zoning revision records (2011–2019). The results indicate that the population share in the highest hazard categories (rank ≥ 3) has declined since around 2000. Regression analysis further reveals statistically significant population decline in higher-risk areas during 2010–2020, although the total population of the Tokyo 23 wards had been consistently increasing since 1996. Further analysis of zoning changes and new residential zone development suggests that risk information, together with regulatory and planning interventions, can work in alignment to counter DRID by shifting residential exposure away from the highest flood-risk areas reversing the past levee effect.