Evaluating the urban flood risk mitigation and cost-effectiveness of green roofs: A case study of Kunming, China

As a key sponge city infrastructure, green roofs serve as an effective source control measure for mitigating increasingly frequent urban flooding. Nevertheless, studies remain scarce regarding post-implementation flood risk variations and cost-benefit analysis of green roof retrofits. This study adopted a combination of monitoring and simulation methods to calibrate the parameters of green roofs. We further employed evaluation metrics and scenario simulations to analyze their impact on urban flood risk, supplemented by a life cycle cost-benefit assessment focusing on runoff regulation and flood damage reduction. Results indicate that: (1) The implemented green roofs effectively reduced runoff from rainfall events below 21 mm, with the calibrated model showing high validity (Nash-Sutcliffe efficiency coefficient = 0.829). (2) Green roofs reduce overall runoff and surface water accumulation, thereby lowering flood risks. But its effectiveness wanes as rainfall increases. Under moderate, heavy, and storm rainfall conditions, the reduction in runoff in high-risk areas was 58.94 %, 8.99 %, and 2.86 %. (3) Over a 30-year lifecycle, the cost of implementing 0.97 km² of green roofs is about 149.45 million dollars, while the benefits from water regulation and flood loss reduction amount to approximately 511.14 million dollars, yielding a cost-effectiveness ratio of 0.2924.