Robust Adaptive Pathways for Long-Term Flood Control in Delta Cities: Addressing Pluvial Flood Risks under Future Deep Uncertainty

Abstract. Delta cities are increasingly vulnerable to flood risks due to the uncertainties surrounding climate change and socioeconomic development. Decision-makers face significant challenges in determining whether to invest in high-level flood defenses for long-term planning. Adaptation solutions should be given considerable attention not only to robustness but also to adaptiveness if the future unfolds not as expectation. To support decision-making and meet long-term multi-objective targets, we propose a synthesized framework that integrates robustness analysis, adaptiveness analysis, and pathway generation. This framework was applied to evaluate alternative solutions for managing pluvial flood risk in central Shanghai. The results show that using a single-objective decision-making approach (focused only on robustness) tends to yield biased options. By examining the valid period and flexibility of candidate solutions, we assessed whether alternative solutions could meet long-term flood control targets. The analysis reveals that a combined option—incorporating increased green areas, an improved drainage system, and a deep tunnel with a 30 % runoff absorption capacity (D+G+Tun30)—is the most robust and adaptive pathway, based on multi-objective trade-off analysis. This study highlights the importance of considering valid period within predefined control targets and retaining flexibility to avoid path-dependency and minimize long-term regrets. The proposed framework can be applied to other delta cities to guide adaptive responses to future flood risks.