Is blue-green infrastructure effective in reducing urban flood depth and area?

Cities are prone to pluvial flooding because they are dominated by impervious surfaces. Urban pluvial flooding can cause substantial damages to properties and life. Upgrading existing grey stormwater drainage network is a costly solution. Cities are increasingly turning to blue-green infrastructure to manage stormwater because it provides multiple socio-ecological benefits to cities such as cooling and habitat provision. The volume and peak flow rate of stormwater run-off are commonly used metrics to assess the flood reduction benefits of blue-green infrastructure. However, they do not indicate the severity and extent of flooding. Instead, flood depth and flood area are direct indicators of the severity and extent of flooding. This study aimed to review studies that assessed the effectiveness of blue-green infrastructure in reducing flood depth and flood area on the catchment scale. Five types of blue-green infrastructure were included: stormwater harvesting systems, bioretention systems, urban trees, green roofs, and urban parks. We identified 14 catchment-scale modelling studies that reported the impacts of one of these five types of blue-green infrastructure on flood depth or flood area. Overall, our review found that the median reduction in flood depth across all five types of blue-green infrastructure was 13% (n=11) with urban trees being the least effective (1%) and stormwater harvesting systems the most effective (15%). The median reduction in total flood area was 8%  (n=10) with urban trees being the least effective (0%) and green roofs the most effective (38%). We also found that blue-green infrastructure cannot substantially reduce flood depth or area in large rainfall events. However, there is emerging evidence that long-term economic benefits lie in reducing flood in small and medium rainfall events because they occur far more frequently than large ones. Future studies should prioritise assessing the long-term economic benefits of blue-green infrastructure rather than focusing solely on its effectiveness in flood mitigation in discrete rainfall events.