Retention capacity and thermal properties of a multilayer blue-green roof in Sardinia: two years of monitoring

Many different nature-based solutions have been proposed in the literature to contribute to the sustainable development of the urban environment. Among them, multilayer blue-green roofs are becoming more and more popular, thanks to their multiple benefits. As traditional green roofs, the multilayer ones guarantee high retention capacity during rainfall events, contributing to the pluvial flood mitigation. Thanks to the additional storage layer, not only the mitigation capacity is increased, but there is the possibility to store the collected water, and reused it for some urban purposes, such as garden irrigation. Moreover, these nature-based solutions ensure thermal insultation for the underneath buildings and they help lowering the air temperature, contributing to the mitigation of the urban heat island effects.  Finally, they improve the air quality, promote the biodiversity, and increase the aesthetic value of the overall city. In June 2019, a multilayer blue-green roof prototype has been installed at the university of Cagliari, and subsequently equipped with multiple sensors to monitor and evaluate the ecohydrological and thermal dynamics. The multilayer blue-green roof, with a surface of 16 m2, presents an 8 cm layer of soil, classified as sand, and a 10 cm additional storage layer. It is characterized by Cactaceae vegetation, which shows resistance to the high temperature and low water availability and does not require additional maintenance. The prototype has been equipped with a Smart Mill, that beside opening and closing of the valve to control the storage layer, enables to measure climatological variables, such as rainfall, air temperature and wind speed, and the water level in the additional layer. Four HOBO thermometers have been installed to measure the temperature in the soil, underneath the structure and on the lateral side. Two soil moisture sensors have been placed at opposite sides of the multilayer blue-green roof. Finally, a tank with a sensor to measure the water level have been collocated at the valve opening, to measure the outflow from the additional storage layer. The collected data have been used to model the ecohydrological and thermal dynamics, with the aim to quantify the potential benefits in terms of pluvial flood mitigation and thermal insulation. Results, collected during two full years of monitoring the prototype in Cagliari, are discussed, highlighting the potential benefits of a large-scale installation for the sustainable development of urban areas.