Enhancing vulnerability, reliability, and resilience of integrated green-grey networks via multi-objective optimization and multi-scale assessment of redundancy strategies.

Legacy drainage infrastructure increasingly fails to accommodate escalating runoff volumes induced by climate change. While Integrated Green-Grey Network (IGGN) have emerged as sustainable alternatives, existing research lacks a comprehensive assessment of redundancy strategies within these systems. To address this gap, this study proposes a multi-objective optimization framework using NSGA-II to balance Life Cycle Cost (LCC), Life Cycle Assessment (LCA), and rationality of system (Rat_Sys). The optimized IGGN are subsequently reinforced with redundant paths and parallel conduits, followed by a holistic evaluation of their incremental burdens and multi-level performance enhancements in vulnerability, reliability, and resilience. Optimization results reveal intricate trade-offs, with TOPSIS-selected schemes exhibiting LCC of $46.39-59.39 million and emissions of 2.88-3.29 million kg CO2 eq. Following redundancy integration, fully centralized networks demonstrated the highest potential for retrofitting. Crucially, while redundant paths incurred no incremental economic or environmental burdens, parallel conduits escalated costs and emissions by 25.6% and 13.7%, respectively. Component-level indicators under extreme rainfall show that parallel conduits handle acute events more effectively, while redundant path excel in failure or long-term usage scenarios. Additionally, system-level analysis demonstrates that redundancy can even enable centralized networks to outperform decentralized ones. These findings underscore the critical role of redundancy in bolstering IGGN performance and highlight the necessity of multi-scale assessments, providing valuable theoretical insights for the design of sustainable urban drainage systems.