Green roofs hydrological performance and contribution to urban stormwater management

Rapid urbanization displaces green spaces with impermeable surfaces, harming the urban water cycle. Also, urban areas are challenged by climate change with more frequent and intense precipitation events that oppose with drought periods. Combination of these two factors, with the development of urban areas, makes it impossible for the traditional urban drainage systems to manage precipitation-generated surface water, straining the city’s pluvial drainage systems. Green Roofs are a biotechnological engineering option that contributes to minimize drainage systems stress, due to their retention and detention capacity. This study tested the hydrological efficiency of an extensive pilot green roof (GR), containing LECA® (lightweight expanded clay aggregates) on its composition, assessing system’s capacity to retain and detain rainwater, runoff coefficient, and downstream drainage system impact. The extensive GR reached a mean peak reduction of 96.8%, with 14 min of peak delay and a runoff delay between 3.70 and 6.21 min (with precipitation simulations of 15 min maximum duration). The water retention percentage ranged between 10.1% (5 min precipitation) and 39.8% (15 min precipitation), corresponding to 108.0 mm/h and 39.56 mm/h, respectively, over the whole period. The experimental work also showed that the LECA-based GR system allows lowest diameters for the drainage system downstream when compared to other roof types, leading to a potential decrease in the costs associated with the drainage network installation.