Upgrading of Gray and Green Infrastructure optimized combination: Synergistic decision tools under resilience demand and investment constraints.

Urbanization and climate change exacerbate urban flooding risks, necessitating resilient drainage systems. This study developed a framework for flood resilience planning, including: (i) gray infrastructure optimization and performance evaluation, and (ii) multi-objective optimization of green infrastructure facilities. Results reveal that 15.49% of current pipelines in research site are insufficient for 1-year return period rainfall event. While new proposed gray infrastructure can address five flooding points (out of 9 total flooding area), it was insufficient to achieve comprehensive runoff control. Non-dominated Sorting Genetic Algorithm II (NSGA-II) combined with Storm Water Management Model (SWMM) simulated 300 scenarios of green infrastructure facilities construction, demonstrating that marginal costs increase significantly for improving volume capture ratio of annual rainfall (VCRAR) beyond 95%. Among scenarios which VCRAR ≥95%, Scenario S2 emerged as optimal, reducing low-, medium-, and high-risk flood hazard areas by 67.8%, 93.6%, and 100%, respectively, compared to baseline, while reducing green infrastructure facilities construction costs by 10.34% (¥14 million) relative to scenario S3. This research provides a validated framework for cost-effective flood resilience planning in water-stressed cities.