Evaluation of blue-green-grey drainage systems for urban flooding based on resilience components

This study proposes a practical evaluation framework to assess the pre-flood defense performance of urban drainage systems. This framework integrates the four components of resilience (rapidity, redundancy, resourcefulness, and robustness) with a Flood Defense Index (FDI), a metric designed to quantify the system's hydraulic capacity utilization. The proposed framework was applied to two urban watersheds in Seoul, South Korea, i.e., the Gunja and Banpocheon watersheds, under three blue-green-grey drainage system scenarios i.e., (1) sewer pipes only (SP), (2) SPs with detention basins (SP-DB), and (3) SP-DB with pumping stations (SP-DB-PS), using EPA SWMM 5.2 and Huff third-quartile design rainfall. The results demonstrated that the integrated SP-DB-PS infrastructure scenario provided observably higher flood defense performance and a more stable response under extreme conditions compared to the other scenarios. Short-duration, high-intensity rainfall noticeably reduced the FDI in the SP case, and systems with DBs and PSs provided stable flood defense performance. In addition, comparison between the design and real-world rainfall events revealed strong alignment, confirming the practical applicability of the proposed framework. The findings of this study demonstrate the effectiveness of combining structural capacity and adaptive operation within a 4-R-based evaluation framework. The proposed framework is expected to support strategic planning and proactive decision-making by enabling systematic comparisons of drainage strategies under various rainfall conditions. Its interpretable results can also be utilized for quasi-real-time-performance analysis to evaluate potential system responses to incoming storm patterns.