Large scale installation of multilayer blue-green roofs as solution for a sustainable urban water management

The modern society is facing multiple challenges, that are reshaping urban areas: the fast population growth, with a consequent high urbanization, combined with an increase of the average temperature and an intensification of extreme rainfall events, facilitates the pluvial flood risk in cities. Several solutions have been proposed in the literature to mitigate the runoff generation from rooftops and to contribute to a sustainable water management. In this context, multilayer blue-green roofs incorporate the high retention capacity of traditional green roofs with the storage capacity that characterizes rainwater harvesting systems. Moreover, these innovative nature-based solutions present countless benefits for the creation of smart, resilient and sustainable cities, e.g., they contrast the urban heat island, reducing the surrounding air temperature, they contribute to the building thermal insulation, limiting the energy consumption, they attract multiple species of insects and small animals, increasing the biodiversity, etc. 

The potential impacts of multilayer and traditional blue-green roofs and rainwater harvesting systems on the runoff generation reduction have been investigated mostly at local scale, analysing the impact of the installation of these tools on single buildings. However, in order to estimate and to evaluate the potential benefits and limitations for a sustainable urban development, it is fundamental to simulate the potential implications of a large-scale installation of these tools on large neighbourhoods or entire cities. For these reasons, in this work we simulate the installation of multilayer blue-green roofs on all the suitable roofs of the cities of Cagliari and Perugia (Italy). Thanks to the two multilayer blue-green roofs, installed in Cagliari and Perugia as part of the EU Climate-KIC Polderroof field lab project, it was possible to calibrate an ecohydrological model to simulate the potential retention and storage capacities of these nature based solutions. The potential discharge reduction and water storage capacity at large urban scale are discussed using as input for the model long historical time series of local rainfall and temperature.