Abstract
Green roofs are recognized as a sustainable infrastructure to improve the environmental quality of cities. Among many benefits, green roofs reduce the rate and volume of runoff helping to improve rainwater management. This study investigated the runoff retention capacity of three pilot extensive green roof assemblies with different vegetation (grass, shrub and intercropping of the two plants). Rainwater runoff data were collected for 18 rainfall events that ranged from a minimum of 1.6 mm to a maximum of 157.9 mm. Average precipitation event retention efficiencies were 46.7, 59.7 and 61.6% for intercropped, shrub and grass green roofs, respectively, while the annual runoff retention rates were 43.8, 57.3 and 59.7%. The difference in retention rates for the green roofs with different vegetation was not statistically significant. The rainfall intensity influenced the retention rates, with the highest retentions for small events (<10.0 mm) followed by medium (10.0-24.9 mm). Retention was found to correspond significantly to rainfall depths. On the other hand, regression analysis failed to provide a relationship between retention and antecedent dry weather period (ADWP). The organic soil used as substrate appears to be the deciding factor for rainwater retention.
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