Evaluation of Green Roof Performance in Mitigating the Impact of Extreme Storms

Green roofs are used in urban areas to mitigate the adverse effects of stormwater. Through numerical modeling, this study evaluates the impacts of design parameters on green roof hydrological performance under different rainfall characteristics. A calibrated model is run with long-term precipitation data series for three locations (Hong Kong, China; Beltsville, MD, USA; and Sidney, NY, USA). The results show that the amount of peak runoff reduction increases with the duration of the storm return period in Beltsville and Sidney; while the trend is opposite in Hong Kong. Percentage peak reduction generally shows a decreasing trend with the storm return period in three locations. For average runoff reduction, the amount of reduction increases with the storm return period, whereas the percentage reduction presents an opposite trend in all three locations. The actual values vary between the three locations due to differences in rainfall characteristics. Both peak and average runoff reduction increase with green roof thickness, but in practice, it is not cost effective or feasible to increase the thickness beyond a certain threshold. The hydraulic conductivity can then be optimized for peak runoff reduction and it is found to increase with the return period. However, hydraulic conductivity has a minimal effect on average runoff reduction. Overall, this paper studies green roof hydrological performance in response to different rainfall characteristics and provides recommendations on green roof designs related to soil thickness and hydraulic conductivity.