Restoring pre-development conditions in a urban environment using green roofs

As development progresses in a urban environment, the impervious areas that rapidly contribute runoff to thereceiving water bodies increase while the pervious areas that store and deliver subsurface flow over periods ofhours, days or weeks diminish, with the direct consequence of higher runoff rates and volumes and shorter timesof concentration. The construction of impervious surfaces modifies the surrounding soils through engineeredcompaction and eliminates superficial soil and its role as a significant pervious storage interface between thesubsurface and the atmosphere.There are documented case studies that conclusively link urbanization and increased watershed impervious-ness to hydrologic impacts on streams. The Effective Impervious Area (EIA) in a watershed is the imperviousarea directly connected to the storm drainage system that contributes to increased storm water volumes and runoffrates. It is shown in the literature that a reduction of EIA could compensate the adverse impact of possible globalwarming scenarios on urban hydrology and in particular on the efficiency of a combined sewer system.In this paper, the implementation of green roofs is analyzed as a technique able to reduce the amount ofEIA in order to mitigate the impact of urbanization on the hydrologic response of the urban catchment ofColle Ometti in the town of Genoa (Italy). Although no green roof installations are now present in the area, thisstudy modelled – using extensive green roof details – the hydrologic effects of hypothetical roof greening scenarios.The modelling of green roof systems was undertaken using the EPA SWMM and was calibrated and vali-dated on a small size green roof test bed completed in September 2007 in the laboratory of the Department ofCivil, Environmental and Architectural Engineering (DICAT – University of Genova).