Modelling the hydraulic behaviour of green roofs through a semi-conceptual reservoir element model

Green roofs represent one of the most diffused and representative Low Impact Development techniques (LID). Green roofs consist of a number of layers, including a vegetation layer, a lightweight soil medium, a drainage layer and an impermeable membrane. Recent studies have confirmed the benefits of green roofs. Green roofs are able to retain water, reduce peak flow, improve biodiversity and mitigate heat island. Although the benefits are evident, the transition from traditional urban drainage systems to the sustainable ones is very slow. A limiting factor in spreading the use of such systems is the lack of proper modelling tools for design, especially for the infiltration processes in a growing medium. The infiltration process in the growing medium of the green roof is strongly unsaturated. In this study, a conceptual model is proposed for the hydraulic modelling of the green roof. The model is built on reservoir elements, which are included in most of the existing hydrologic-hydraulic software and which can be easily implemented in any programming code. The green roof is modelled as a cascade of reservoirs. Each layer of the green roof is modelled as a reservoir with a specific flow discharge curve. In this study, the proposed model is calibrated and validated against measured discharges of a green roof installed at the University of Calabria. Several statistical indices have been used in order to assess the accuracy of the model in correctly describe the hydraulic behavior of a green roof. Results confirm the suitability of the proposed model.