Enhancing resilience of green-grey infrastructure by integrating two redundancy strategies into a multi-objective optimization and service period assessment framework

Green-grey infrastructure is increasingly recognized for its role in mitigating flood risks and promoting sustainable stormwater management. Despite extensive research on optimization, enhancing system resilience through improved drainage network redundancy remains under-explored. This study aims to address this gap by integrating two redundancy strategies, i.e., parallel pipelines and redundant paths, into a multi-objective optimization framework to enhance the system resilience from the cross-sectional and top-down dimensions, respectively. Iterative layouts were designed using a graph-theoretic algorithm coupled with green infrastructure optimization. The predominant objectives were to reduce life cycle costs and enhance resilience against urban flood pressures caused by extreme storms and infrastructure failures. A service period assessment method was employed to evaluate the sustainability gains for the optimized layouts provided by redundancy strategies. The findings indicated that both parallel pipeline and redundant path strategies significantly improve sustainability, with pronounced effectiveness in decentralized and centralized layouts, respectively. Additionally, these strategies were more effective in addressing internal pressure of infrastructure failures than external pressure of extreme storm. Consequently, three decentralized green-grey infrastructure layouts incorporating both redundancy strategies are recommended for decision-makers. This framework offers novel insights into the application of redundancy strategies for the design and planning of sustainable urban drainage systems.