How well does the 3–30–300 rule mitigate urban flooding?

The 3–30–300 rule is a new guideline for urban forestry and urban greening, which is rapidly gaining interest among city planners, international organizations and NGOs. However, the ecosystem service benefits of this new guideline have not been quantified and there has been no research to date on how implementing the 3–30–300 rule may mitigate urban flooding. In this study, we use a gridded implementation of the rational method, with flow attenuation included (ANaRM model), to assess the reduction in runoff that can be achieved by implementing urban land-use change to achieve 3–30–300 targets in three European cities of contrasting size and population: Aarhus Municipality (Denmark), Grad Velika Gorica (Croatia) and Paris Region (France). We find that the creation of new green spaces and tree cover can greatly reduce peak pluvial surface runoff rate at-site, and maintain peak flow reductions of several percent in sub-catchments of several square kilometres, including reductions of over 10 % in some sub-catchments of over 20 km 2 in Paris. The specific interventions required to meet aspects of the 3–30–300 rule vary between study areas, and the larger the interventions, the greater the peak runoff rate reduction that can be achieved. This study highlights the importance of linking research with policy in order to quantify the benefits of urban green infrastructure targets and show the real benefits of implementing nature-based solutions. • World-first study into how meeting the 3–30–300 rule could mitigate urban flooding. • New green space (grassland and trees) replaced built surfaces in 3 European cities. • 3–30–300 interventions can reduce localized runoff by over 70 %. • However, the number and size of interventions did not give large-scale mitigation.