Green roof benefits for reducing flood risk at the catchment scale

Among the European countries, Italy is one of the most susceptible to hydrogeological instability events related to the risk of flooding in intensely populated basins, especially inside the Po Valley. A clear example is the Seveso river basin, whose territories are regularly stressed several times per year by flooding events, which in recent years have increased in frequency and intensity. However, it is now impossible to intervene with structural projects aimed at the lamination of flood events, even in modest return periods because of excessive urbanization and continuous residential and industrial expansion. Using extensive and detailed green roof parameters, this study modelled the hydrologic effect of three hypothetical roof greening scenarios at the catchment scale (conversion of 5, 30, and 100 % impervious to green roofs). The modelling of the green roof performances was performed using the Environmental Protection Agency (EPA) Storm Water Management Model (SWMM) and was calibrated over five years of hourly discharge measurements at the closure section of the basin. Hydrologic modelling demonstrated that widespread green roof implementation significantly reduces peak runoff rates and runoff volumes by up to 30 and 35 %, respectively, in the case of 100 % conversion.