Environmental benefits of retrofitting green roofs to a city block

Although they are widely in use and in many cases are mandated in Europe and North America, the installation of green roofs has been relatively limited in Melbourne and Australia as a whole. A case study is presented that investigates the implementation of green roofs across the Melbourne central business district (CBD) in Australia. The study aims to give an immediately possible and best-case scenario for the benefits achievable depending on the proportion of Melbourne's roof area covered with green roofs. The following environmental benefits (EBs) of the retrofit are assessed for both scenarios for a city block: (EB No. 1) reduction in urban heat island (UHI) effect (EBUHI), (EB No. 2) reduction in runoff volume (EBR), (EB No. 3) improved water quality (EBL), (EB No. 4) reduction in storm water runoff peak flow (EBPF), and (EB No. 5) greenhouse gas (GHG) emission reduction (EBGHG). These benefits are then scaled up, assuming that the retrofit of green roofs is expanded to cover the entire CBD roof area in the same proportions. This demonstrates the range and scale of benefits that could be achieved under widespread implementation, such as temperature reductions, significantly reducing the UHI effect, and estimated reductions of storm water runoff peak flow, which may mitigate or reduce the frequency and severity of flash flooding.