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

Urban nature-based solutions (NBS) are considered cost-effective actions to address several social, economic and environmental challenges. 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 and adaptability of Mediterranean plants, and iii) water quality and storm water runoff control. The technological installation was carried out on a flat terrace in an industrial site of Livorno, using a light substrate of lapillus, pumice and compost and applying temperature sensors and a rainwater recovery system. Green roof was divided into two prototypes differentiated exclusively for the shrub Mediterranean species planted, with green or grey leaves. Temperature monitoring was performed using three data loggers to record both the external temperature and that under each vegetation type. The system was effective in thermal regulation, keeping the underlying environment cooler by about 3-4°C during the thermal peak and warmer by the same amount at the minimum external temperatures, with light variability due to substrate humidity and leaves colour (albedo). All species resulted suitable to the site and substrate, with low management (only one fertilization a year and emergency irrigations in summer). Among species, Helichrysum italicum and Rosmarinus officinalis showed the highest CO2 uptake capacity, while Thymus vulgaris maintained higher transpiration rates. Senecio cineraria and Helichrysum italicum showed the highest instantaneous water-use efficiency and electron transport rate, respectively. The analyses of rainfall, very scarce over the two years, revealed low levels of heavy metals before and after the substrate leaching. The results highlighted that not only this system allowed to reach environmental improvement, but also brought to a decrease of the loads bearing on the structure, suggesting to consider them in term of the sustainability for building engineering and architecture.