Spatial optimisation for managed retreat and nature-based solutions in climate adaptation

Managed retreat and nature-based solutions (NBSs) are major pillars of climate change adaptation, offering alternatives or complements to conventional engineering-based solutions. When an NBS is installed in retreat areas, it not only mitigates local flood risks but may also reduce risks in other regions, generating co-benefits. In contrast, in inappropriate locations, NBSs may create spatial trade-offs due to unacceptable costs while providing limited risk reduction. Spatial optimisation models are powerful tools for sustainable land use planning, satisfying various urban needs. However, their application in integrating managed retreat with strategic NBSs remains underexplored. This study addresses two key research gaps: (1) how managed retreat should be implemented and (2) whether/where NBSs can be installed when managed retreat is implemented. To explore these questions, we employ a spatial optimisation model aiming to maximise urban net profit by considering revenues and costs related to flood resilience, city maintenance, and land conversion, and we conduct a cost–benefit analysis. This study focuses on discontinuous levees—one of Japan’s indigenous flood control technologies—as an NBS alternative to continuous levees, an engineering-based solution widely installed in modern society. These two types of levees influence flood spatial patterns differently, affecting optimal land use allocation in the model. A case study of Toyohashi City shows that certain discontinuous levee scenarios outperform continuous levees in cost-benefit performance while requiring less retreat. These findings highlight the advantages of integrating managed retreat with NBSs, bridging the gap between land use planning and flood infrastructure management to promote sustainable urban development. • A spatial optimisation model integrates managed retreat with NBS. • Japan’s indigenous discontinuous levee (Kasumi-tei) is explored as NBS. • NBS effectiveness depends on the feasibility of managed retreat at each site. • Some NBS scenarios outperform engineering-based solutions in cost-benefit terms.