Increasing flood hazards threaten metro system resilience under climate and demographic changes

Intensification of short-duration rainfall poses an increasing threat to urban metro systems, necessitating a thorough understanding of existing infrastructure's resistance in a warmer climate. This study systematically examines how future rainfall extremes and societal changes jointly affect metro system performance, using Hong Kong as a case study. We simulated city-scale flood inundation under the current climate baseline and three future representative emission scenarios using a grid-based flood hydrodynamic model. The study quantifies and characterizes flood hazard and risk, as well as metro performance curves at both station and line scales through integrating the simulated flood dynamics with demographic projections and infrastructure characteristics. Results show a substantial increase in flood hazard in low-lying coastal urban areas from the near to the far future under the highest emission scenario, while risk profiles remain comparatively stable. Short-duration rainfall intensity dominates the deterioration rates of metro functionality, while residual functionality and recovery capacity are primarily influenced by long-duration cumulative volume. Among performance-based resilience metrics, robustness emerges as the decisive factor influencing other components. High emissions in the far future present the most challenging scenario for metro system resilience, while other emission scenarios show more manageable impacts. Compared to demographic changes, climate-induced rainfall intensification exerts more significant influence on metro system resilience, particularly through cumulative rainfall volume. This research contributes a transferable framework for assessing infrastructure resilience under combined climate and societal stressors. By comparing their impacts, the study yields generalizable insights to guide adaptation of critical urban infrastructure, supporting robust planning for a complex future.