Rooftops cover a large percentage of land area in urban areas, which can potentially be used for stormwater purposes. Seeking adaptation strategies, there is an increasing interest in utilising green roofs for stormwater management. However, the impact of extreme rainfall on the hydrological performance of green roofs and their design implications remain challenging to quantify. In this study, a method was developed to assess the detention performance of a detention-based green roof (underlaid with 100 mm of expanded clay) for current and future climate conditions under extreme precipitation using an artificial rainfall generator. The green roof runoff was found to be more sensitive to the initial water content than the hyetograph shape. The green roof outperformed the black roof in terms of all performance indicators (time of concentration, centroid delay, T50 or peak attenuation). While the time of concentration for the reference black roof was within 5 minutes independently of rainfall intensity, for the green roof was extrapolated between 30 and 90 minutes with intensity from 0.8 to 2.5 mm/min. Adding a layer of expanded clay under the green roof substrate provided a significant improvement to the detention performance under extreme precipitation in current and future climate conditions.
A method to assess the performance of a green roof under extreme events with a rainfall simulator was developed.
Underlying a thin extensive green roof with expanded clay provided significant improvement to the detention performance.
The study demonstrated that extensive green roofs with an added detention layer can have an effect also for extreme precipitation events including a climate factor.
New knowledge on the water distribution within the roof from antecedent precipitation and how it affects the future performance.
Demonstrating that the runoff can be predicted from the initial moisture, showing that for this roof the runoff exponentially increased with increasing initial moisture.