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 and how to invest in flood defense. Adaptation solutions should consider not only robustness but also adaptiveness in case the future unfolds other than as expected. 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 demonstrate that relying on a single-objective decision-making approach (focused only on robustness) can lead to biased outcomes. By examining the effectiveness period and flexibility of candidate solutions, we assessed their potential to 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) – emerged as one of the most robust and adaptive pathways, based on multi-objective trade-off analysis. This study highlights the importance of considering effectiveness period within predefined control targets and retaining flexibility to avoid path-dependency and minimize long-term regrets. The proposed framework is broadly applicable and can guide adaptive responses to future flood risks in other delta cities.