A catchment scale analysis of green roofs retrofit potential to mitigate hydrological risk in a Mediterranean environment

The increase of impervious surfaces in an urban basin raises a number of environmental issues. Increasing impervious areas radically changes the hydrological response of a catchment to rainfall events by reducing infiltration and increasing surface runoff which result in more severe and frequent flooding and associated deterioration of the urban water environment. In order to face this criticality, adaptation measures are foreseen for conventional drainage systems with limited capacity and flexibility, in order to improve their resilience against the adverse impact of land use changes in urban environments. Among adaptations approaches, techniques of Sustainable Urban Drainage Systems (SUDS) for stormwater management such as green roofs (GRs) represent a very attractive approach because of the variety of benefits associated to the vegetated roof covers. The presented work aims at illustrating the potential of GRs retrofitting to mitigate hydrological risk in a Mediterranean catchment, the Sarno peri-urban basin (Southern Italy), selected as case study since it represents a hydrogeological hazard prone system where flooding and landslides events occurred rather frequently, especially during the last decade. The evolution of urban cover has been mapped by the use and the elaboration of remote sensing images (COSMO-SkyMed sensor), showing that from 1995 to 2016 the catchment built up index has doubled. The investigation has concerned a greening scenario with 20% of impervious surfaces converted into GRs and distributed according to the current land use in the catchment and a baseline scenario without green roof infrastructures. The hydrological simulations have been performed using the Storm Water Management Model (SWMM) of the US Environmental Protection Agency (USEPA) and the hydrological impact of GRs roofing scenario has been evaluated in terms of changes in runoff flooding volume and flooded urban drainage nodes. The results prove indeed only a moderate hydrological effectiveness of the eco-roofs at the basin scale