Large-scale impacts of multilayer blue-green roofs: pluvial flood mitigation capacity and water storage capacity

The continuous urbanization growth, combined with climate changes, that are leading to more frequent intense rainfall events, is increasing the pluvial flood risk in urban areas. With the aim to mitigate urban floods, several blue-green solutions have been proposed in the literature. Among them, multilayer blue-green roofs have shown high potential, ensuring a high retention and storage capacity and providing many additional benefits to support a sustainable development of smart and resilient cities. Stored water can be reused for different domestic non-drinkable purposes, such as the irrigation of private gardens or cleaning the streets. Moreover, multilayer blue-green roofs facilitate the building thermal insulation, reducing the energy consumption, and help contrasting the urban heat islands. These nature-based solutions, however, have been mainly investigated only at building scale. In this work, we aim to evaluate the potential impacts of a large-scale installation of multilayer blue-green roofs over a large neighborhood or an entire city. Two multilayer blue-green roofs, installed in Cagliari and Perugia (Italy) as part of the EU Climate-KIC Polderroof field lab project, are used to calibrate an ecohydrological model to simulate the potential retention and storage capacities of these nature based solutions. The impact of a large-scale installation of multilayer blue-green roofs over the cities of Cagliari and Perugia is estimated, after having identified all the potential suitable roofs of these urban areas. The potential discharge reduction and water storage capacity are discussed also in a context of climate changes, simulating the impact of future climate scenarios, derived from the IPCC projections (RCP8.5), on the runoff mitigation capacity of multilayer blue-green roofs.