Hydrological effectiveness of an extensive green roof in Mediterranean climate

In urban water management, green roofs provide a sustainable solution for flood risk mitigation. Numerous studies have investigated green roof hydrologic effectiveness and the parameters that influence their operation; many have been conducted on the pilot scale, whereas only some of these have been executed on full-scale rooftop installations. Several models have been developed, but only a few have investigated the influence of green roof physical parameters on performance. From this broader context, this paper presents the results of a monitoring analysis of an extensive green roof located at the University of Calabria, Italy, in the Mediterranean climate region. To obtain this goal, the subsurface runoff coefficient, peak flow reduction, peak flow lag-time, and time to the start of runoff were evaluated at an event scale by considering a set of data collected between October 2015 and September 2016 consisting of 62 storm events. The mean value of subsurface runoff was 32.0% when considering the whole dataset, and 50.4% for 35 rainfall events (principally major than 8.0 mm); these results indicate the good hydraulic performance of this specific green roof in a Mediterranean climate, which is in agreement with other studies. A modeling approach was used to evaluate the influence of the substrate depth on green roof retention. The soil hydraulics features were first measured using a simplified evaporation method, and then modeled using HYDRUS-1D software (PC-Progress s.r.o., Prague, Czech Republic) by considering different values of soil depth (6 cm, 9 cm, 12 cm, and 15 cm) for six months under Mediterranean climate conditions. The results showed how the specific soil substrate was able to achieve a runoff volume reduction ranging from 22% to 24% by increasing the soil depth.