Beyond Average Runoff Reduction: Event-Based Assessment of Climate Adaptation Benefits and Flood Vulnerability of Blue–Green Roofs

Climate change increases the need for urban stormwater management systems that can maintain hydrological performance under severe rainfall conditions. Although green and blue–green roofs are widely used as nature-based solutions for runoff management, their performance is commonly evaluated using average or cumulative runoff reduction, which may not capture differences that emerge during severe events. This study applied an event-based assessment to 31 rainfall events monitored on a conventional roof (Control), a Blue–Green Roof (BGR), and a Smart Blue–Green Roof (Smart BGR). Climate Adaptation Benefit (CAB) was calculated by integrating relative runoff reduction with rainfall severity, while the additional benefit of Smart BGR was evaluated using the difference in CAB (ΔCAB). The Severity-Weighted Runoff Vulnerability Index (FVI) and Severity-Weighted Runoff Vulnerability Reduction (FVR) were used to assess runoff-related vulnerability across four rainfall severity classes. Both systems provided substantial benefits under low rainfall severity, with relatively small differences. However, Smart BGR generally showed higher CAB and positive ΔCAB than BGR, while maintaining lower FVI and greater FVR under the observed higher-severity rainfall conditions. These findings suggest that the primary value of Smart BGR lies in maintaining greater climate adaptation benefits under severe rainfall conditions. The proposed event-based framework provides a complementary approach for evaluating rooftop nature-based solutions beyond average runoff reduction.