Vertical green systems (VGSs) and on-site storage for stormwater management

The escalating challenges of urban densification, climate change, and aging drainage infrastructure necessitate innovative and sustainable stormwater management solutions. This study explores the potential of vertical green systems (VGSs) combined with on-site storage to reduce stormwater discharge from high-density urban environments. It aims to answer how effective are these systems for stormwater management? and What are the wider implications for the urban water cycle? An urban water mass balance model was developed and applied to quantify the daily performance of the proposed system, over a 51-year period (from 1 January 1972 to 29 June 2023) across four Australian cities -Brisbane, Sydney, Melbourne, and Perth- representing diverse climatic conditions. A total of 440 scenarios (10 wall areas × 11 storage sizes × 4 locations) were simulated, including with and without implementing the proposed system. Key findings indicated that VGSs and on-site storage systems can achieve 60-100 % annual stormwater reduction, depending on the climate and the sizing of the system. The system was particularly effective in managing frequent, low-intensity rainfall events, achieving reductions above 80 % for 1.5-year ARI events across all four locations. Despite these benefits, the system introduced a trade-off between stormwater reduction and increased potable water demand for irrigation during dry periods. Results suggest that this trade-off can be minimised with appropriate sizing of the system. For instance, a wall-to-roof ratio of 0.82 m²/m² and a storage size-to-roof ratio of 0.3 m³/m² increased water demand by only 3 % of the total building demand during extreme droughts, such as those observed during the Millennium Drought in Perth. This additional water demand could also be met by re-using greywater, or similar strategies. Results presented in this paper demonstrate, for the first time, that evapotranspiration from vertical surfaces can serve as a primary hydraulic mechanism for stormwater management, effectively creating stormwater "sinks" close to the source. This finding has significant implications for restoring natural hydrology in urban environments and promoting sustainable urbanisation.