Combined Effects of Substrate Depth and Vegetation of Green Roofs on Runoff and Phytoremediation under Heavy Rain

The quantity and quality of runoff from green roofs have so far been studied using an extensive vegetated roof (substrate depth > 150 mm). However, studies on various substrate depths and vegetation for runoff and phytoremediation in temperate and monsoon climates, where heavy rain is concentrated in a specific season, are limited. Therefore, the purpose of this study was to investigate combined effects of substrate depth and vegetation of an unfertilized green roof on runoff reduction and airborne pollutant purification based on rainfall intensity. A total of 21 plots were implemented on a roof top with three substrate depths (100, 200, and 400 mm), two vegetation s (vegetated or non-vegetated), and control (plot with standard frame only). The runoff reduction increased significantly (p < 0.05) with increasing substrate depth. Vegetated plots had a slightly higher runoff reduction than plots without vegetation. Compared to controls, turbidity and pH tended to increase regardless of vegetation or substrate depth, with the exception of electrical conductivity (EC). However, concentrations of heavy metals (Cu, Zn, Mn, and Cd) in the runoff of vegetated plots were all significantly (p < 0.05) lower than those of un-vegetated plots and controls. These results suggest that as the rainfall intensity increases, the depth of the substrate is more important than vegetation for runoff reduction. In addition, the vegetation can be an effective tool to neutralize acid rain to stabilize pH and effectively reduce EC and heavy metals in the runoff by remediating dissolved air pollutants from rainwater.