Redefining Urban Flood Resilience: A Systematic Framework for the Synergistic Integration of Hybrid Nature-based Solutions

As climate extremes intensify, Nature-based Solutions (NbS) are increasingly integrated with traditional infrastructure to enhance urban flood resilience. However, current design paradigms often treat NbS and grey infrastructure as separate, additive components, failing to capture the complex hydraulic interactions required to withstand unprecedented flood events. Based on a systematic review of literature from 2015 to 2025, this study critically analyzes the engineering limits of hybrid systems and proposes a conceptual framework to operationalize true resilience. The review reveals a critical gap: while NbS is widely praised for its sustainability, its capacity to prevent the brittle failure of conventional systems remains under-quantified. Existing studies predominantly focus on volume reduction, overlooking how NbS can modulate hydraulic loading rates and provide functional redundancy during extreme events. We argue that urban flood resilience is not merely about increasing total retention capacity but about optimizing the synergistic coupling between the saturation characteristics of NbS and the discharge limits of grey infrastructure. To address this, we introduce an integrated resilience assessment framework that moves beyond static capacity analysis. This approach quantifies how NbS acts as a "resilience buffer," delaying system failure and extending the operational range of drainage networks. By shifting the focus from additive performance to synergistic interaction, this study provides a robust pathway for designing hybrid NbS that remains functional under deep uncertainty, offering a strategic guide for future urban flood management.

Acknowledgement
This work was supported by National Research Foundation of Korea(NRF) grant funded by the Ministry of Science and Technology (RS-2024-00356786) and Korea Environmental Industry & Technology Institute grant funded by the Ministry of Environment (RS-2023-00218973).