Mediterranean green buildings: vegetation cover and runoff water quality assessment in a green roof system

Urbanization not only supports social and economic development but also causes various problems, such as increase of population, traffic, pollution (water, air, noise), heat island effects and reduction of resources and biodiversity. Urban nature-based solutions (NBS) generally defined as solutions that are inspired or supported by nature, have been brought forward as ecosystem services delivered by urban green or blue infrastructure. Green roofs represent one of the most important NBS of the urban green spaces. Since 2018 we are studying technical aspect of a green roof system having focused on i) waste and recyclable materials ii) types of Mediterranean plants and their adaptability iii) green roof as a tool to improve water quality and to reduce storm water runoff. The technological installation of a green cover was on a flat terrace in an industrial site of Livorno, applying temperature sensors and a rainwater recovery system. A simple and light substrate, made up of lapillus, pumice and compost (20% -65% -15%) and green roof was divided into two prototypes that differ each other exclusively for the shrub species planted, suitable for the Mediterranean climate, but with green or grey leaves. Monitoring was performed using three data loggers for measuring the external temperature and that recorded under each type of substrate. A weather station allowed to check temperature, wind, humidity and rainy events in the site. The results obtained in the two years of experimentation show the effectiveness of the roof in containing temperatures, able to keep the underlying environment cooler by about 2°C during the thermal peak and warmer by the same amount at the minimum external temperatures, with variability due to substrate humidity and to colour of plant cover (albedo). The physiological performance of vegetation was detected by indexes as photochemical efficiency of photosystem II, electron transport rate and leaf gas-exchange parameters. Leaf gas exchanges were carried out to evaluate the contribution of the different species to carbon sequestration and to the cooling effect due to leaf water transpiration. All species resulted available to planting site and substrate, with low management (only one fertilization a year and survey irrigations in summer). The analyses of rainfall, very scarce over the two years, revealed low levels of pollutants before and after the substrate leaching. Moreover, the results highlight that not only this system allows to reach environmental improvement, but also brings to a decrease of the loads bearing on the structure, suggesting to consider them in term of the sustainability for building engineering and architecture.