Assessing the Scale Dynamics of the Rainfall Retention of Green Roofs in a Subtropical Climate

Abstract

Various studies have provided insights about how green roof rainfall retention capacity is affected by factors including typology, vegetation, growing media’s physical-chemical-biological properties, drainage system, climate conditions, and roof age. Despite its importance, little has been studied about the effect of drainage area on green roof retention capacity. In this work, three experimental extensive green roof units with different drainage areas (1 m2, 3 m2, and 12 m2) were assessed to evaluate how the rainfall characteristics and the scale dynamics of the green roofs impact the retention during rainfall events. The findings showed that the drainage area had a significant impact on rainfall retention and outflow volumes. The average retention capacity increased, and the outflow generation capacity decreased with increasing drainage area. The largest green roof area (12 m2) presented better performance with respect to rainfall retention, being 36% higher than the 1 m2 area, and 13% higher than the 3 m2 green roof area. In turn, the performance of the 3 m2 green roof was 21% higher than the 1 m2 area. Factors such as rainfall depth and maximum 1-h rainfall intensity also affected the retention and outflow behavior. However, the antecedent dry weather period had little effect on retention and outflow, possibly due to frequent rainfall caused by subtropical climate.