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<p>In response to the intensification of water-related disasters driven by climate change, there is a growing need to promote comprehensive river basin flood control. In Japan, a new flood risk reduction policy, “River Basin Disaster Resilience and Sustainability by All” (RBDRSA), was recently launched. Traditionally, in Japanese flood control projects, exposure reduction measures (e.g., preventive relocation) have been implemented primarily as alternatives to hard hazard control measures (e.g., dike improvements); thus, no scheme existed for their parallel implementation. Consequently, a methodology for quantitatively evaluating their combined project-level effectiveness has been lacking. To address this gap, this study develops a housing-unit-level analysis to evaluate the economic rationality of the parallel implementation of hazard control and exposure reduction measures from the perspective of minimizing total social cost, defined as the sum of direct physical flood damage costs and project expenditures. Applied to the Rokkaku River basin, we found that even when conventional hazard control measures are implemented over a 20-year period, combining them with the infill-type preventive relocation of 123 highly vulnerable housing units is consistently more cost-effective. Specifically, total social cost is minimized by targeting housing units projected to still suffer above-floor inundation (depth ≥ 0.5&#xa0;m) during a 1/30 annual exceedance probability (AEP) rainfall event. Furthermore, sensitivity analyses reveal that the economic superiority of this combined strategy remains relatively insensitive to assumptions about the discount rate. These findings provide quantitative evidence that project effectiveness under the RBDRSA framework can be significantly enhanced by appropriately integrating hazard control and exposure reduction measures.</p>

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Exposure reduction measures in the new flood control policy in Japan: is it efficient to implement exposure reduction measures in parallel with conventional hazard control measures?

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年7月3日

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  • Yoshitaka Nagamatsu,
  • Genki Shimokubo,
  • Taiga Higuma,
  • Hironao Tsuru,
  • Yuki Akiyama,
  • Tatsuro Sato

摘要

In response to the intensification of water-related disasters driven by climate change, there is a growing need to promote comprehensive river basin flood control. In Japan, a new flood risk reduction policy, “River Basin Disaster Resilience and Sustainability by All” (RBDRSA), was recently launched. Traditionally, in Japanese flood control projects, exposure reduction measures (e.g., preventive relocation) have been implemented primarily as alternatives to hard hazard control measures (e.g., dike improvements); thus, no scheme existed for their parallel implementation. Consequently, a methodology for quantitatively evaluating their combined project-level effectiveness has been lacking. To address this gap, this study develops a housing-unit-level analysis to evaluate the economic rationality of the parallel implementation of hazard control and exposure reduction measures from the perspective of minimizing total social cost, defined as the sum of direct physical flood damage costs and project expenditures. Applied to the Rokkaku River basin, we found that even when conventional hazard control measures are implemented over a 20-year period, combining them with the infill-type preventive relocation of 123 highly vulnerable housing units is consistently more cost-effective. Specifically, total social cost is minimized by targeting housing units projected to still suffer above-floor inundation (depth ≥ 0.5 m) during a 1/30 annual exceedance probability (AEP) rainfall event. Furthermore, sensitivity analyses reveal that the economic superiority of this combined strategy remains relatively insensitive to assumptions about the discount rate. These findings provide quantitative evidence that project effectiveness under the RBDRSA framework can be significantly enhanced by appropriately integrating hazard control and exposure reduction measures.