Grey–green urban drainage performance and resilience under rainfall variability: An optimal coverage framework for Quito (Ecuador)

Climate variability combined with rapid urbanization and population growth is increasing pressure on urban drainage systems, amplifying flood risks and associated impacts on public health and the urban environment. To support evidence-based decision making in this context, this study presents an optimal coverage assessment framework to evaluate the performance and resilience of grey–green and hybrid urban drainage systems under spatio-temporal rainfall variability. The proposed framework integrates reliability, resilience, and sustainability concepts to jointly assess system exposure to rainfall stressors and its hydraulic response. Natural threats are represented through design storms associated with multiple return periods, extreme rainfall events derived from continuous time series, and plausible rainfall extreme events, while system response is quantified using key hydraulic indicators obtained from hydrodynamic simulations, including runoff generation, flood volumes, and network surcharging behavior. A multi-objective optimization approach is applied to identify the optimal spatial extent of green stormwater management practices, explicitly accounting for performance gains and land-use constraints. The robustness of policy-relevant outcomes is ensured through a multi-criteria decision analysis that enables transparent comparison of alternative infrastructure configurations. The framework is demonstrated through its application to the urban drainage system of Quito (Ecuador), a representative case of rapidly growing cities in complex topographic and climatic settings. Results show that hybrid grey–green configurations consistently outperform conventional systems, achieving high resilience levels, particularly in flood mitigation, despite requiring a limited allocation of urban space to green infrastructure. Overall, the proposed framework offers a transferable and decision-oriented tool to guide urban planners and policymakers in prioritizing resilient and sustainable stormwater management strategies under climate uncertainty.