The influence of structural factors on stormwater runoff retention of extensive green roofs: new evidence from scale-based models and real experiments

Green roofs, in contrast to conventional roofs, retain significant stormwater, and are considerably appealing for urban regions to help mitigate flooding events and addressing serious environmental impacts of excessive or uncontrolled runoff. In designing a green roof, a careful selection of substrate material/depths, vegetation or slopes, or its optimal combination, is imperative but this issue remains underexplored by practicing hydrologists. Scale-based models of extensive green roofs were purposely designed and simulated rainfall experiments were conducted to comprehensively quantify runoff retention performance of extensive green roofs, to precisely determine the contribution of structural factors to stormwater retention. The results indicated that green roofs can effectively retain stormwater and delay the start of the runoff. To aver with this finding, influential contribution of four structural factors are ordered as follows: substrate material > substrate depth > slope > vegetation. This finding denotes that substrate material and its depth are major contributors to a green roof's stormwater retention capability. Statistical tests conducted on runoff retention showed the substrate materials and vegetation types had statistically significant effects, no statistically significant difference in runoff retention abilities among the three substrate depth treatments and the slope gradients considered. Given the same conditions, the study ascertains that runoff retention of low intensity events is likely to be relatively higher, while that of the heavy events is lower than its counterparts. The antecedent moisture contents of the substrate have a significant negative influence on the runoff retention and the time to the runoff.