Installation of blue-green solutions at large scale to mitigate pluvial floods

 The growth of urbanization and the intensification of extreme rainfall events, that has characterized the last century, are leading to an increase of pluvial floods, which are becoming a significant problem in many cities. Among the different solutions proposed and developed to mitigate flood risk in urban areas, green roofs and rainwater harvesting systems have been deeply investigated to reduce the runoff contribution generated from rooftops. These tools have been largely studied at small scale, analysing the flood reduction that can be achieved from one single building or in a small neighbourhood, without considering the large-scale effects. In this work, the potential impact of the installation of green-blue solutions on all the rooftops of a city is evaluated, assuming to place green roofs on flat roofs and rainwater harvesting systems on sloped ones. We investigated nine cities from 5 different countries (Canada, Haiti, United Kingdom, Italy and New Zealand), representing different climatological and geomorphological characteristics. The behaviour of the blue-green solution was estimated with the help of a conceptual lumped ecohydrological model and the mass conservation, using rainfall and temperature time series as climatological input to derive the discharge reduction for different scenarios. Due to the high percentage of sloped roofs in most of the investigated locations, the cost-efficiency analysis highlights that the large-scale installation of rainwater harvesting tanks enables to achieve higher mitigation capacity than green roofs at lower cost. Green roofs, however, present many additional benefits (such as biodiversity contribution, thermal insulation for buildings, pollution reduction and increase of aesthetic added value) that need to be evaluated by urban planners and policy makers. The best achievable performance is given by the coupled system of rainwater harvesting tanks and intensive green roofs: for extreme rainfall events this solution guarantees a discharge reduction up to 20% in most of the cities.