How decentralized blue-green-grey infrastructure reshapes urban drainage performance: A cost-reliability-resilience...

Urban drainage systems in high-density coastal cities are increasingly exposed to compound flood risks driven by non-stationary rainfall, land-use intensification, and infrastructure aging. Although blue-green-grey infrastructure (BGGI) has been widely advocated, the quantitative role of network decentralization in shaping cost, reliability, and resilience trade-offs remains insufficiently understood. This study develops an integrated GA-SWMM optimization framework to systematically assess how varying degrees of centralization (DCL = 0-90.9%) influence the performance of conventional grey infrastructure (GREI-only) and coupled blue-green-grey infrastructure (CBGGI) systems. The framework is applied to the Guiwan District (345.8 ha) in Shenzhen, China, under nine design storms with return periods of 10, 50, and 100 years and durations of 1, 6, and 24 h. Results indicate that decentralization significantly reduces system costs while enhancing operational robustness. As DCL decreases from 90.9% to 0, total life-cycle cost (LCC) declines by 30.6% for GREI-only systems and 32.9% for CBGGI systems, primarily due to reduced pipe depth, smaller mean diameters (up to 0.12 m), and lower operation and maintenance expenditures. Across all decentralization levels, CBGGI schemes achieve additional cost savings of 18-24 million USD compared with GREI-only layouts. Hydraulically, decentralization reduces overflow volumes by up to 23.5 × 10³ m³ during 100-year, 24 h storms and improves technological reliability by 0.3-2.7%. Under random pipe-failure scenarios, CBGGI systems maintain acceptable service (Oper_Rel ≥ 80%) up to 15.0-25.3% link failures, compared with 4.8-7.8% for GREI-only networks. Component-to-system resilience (Res_II) further increases by 5-8% in decentralized CBGGI configurations. The results demonstrate that resilience gains arise primarily from network reconfiguration rather than capacity expansion, highlighting decentralized blue-green-grey coupling as a cost-efficient and robust pathway for next-generation urban flood management.