A HYDRUS-1D Based Evaluation of the Effects of Green Roof Designs on Runoff Control

Green roofs are increasingly being used as a source control measure for urban storm water management as they have considerable potential to delay and reduce the storm runoff at source. Simplified infiltration equations (e.g. Horton's, Green Ampt equation) have been widely used in simulation of green roof runoff. However, these equations cannot represent the movement of water in unsaturated soils and result in inaccuracy in the simulation, particularly in the long-term and continuous simulations. In this paper, the hydrological performance of green roofs was simulated based on a HYDRUS-1D model, which can numerically solve the Richards infiltration equation for one-dimensional saturated-unsaturated water flow. The HYDRUS-1D model was calibrated and then applied to evaluate the effects of green roof designs on runoff control under different rainfall conditions. The effects are measured by the rainfall retention amount during a storm event compared to the conventional roof design. A range of storm events with different rainfall amounts, durations and intervals are considered for holistic assessment of green roof. The results obtained indicate that a green roof has varied rainfall retention effects under different rainfall conditions: the ratio of rainfall retention to rainfall amount decreases significantly as the rainfall amount increases; rainwater retention effect of a green roof has little change as the rainfall duration increases; and a green roof can perform better in two continuous storms with interval greater than 48h in this study. The results provide better understanding of the effects of green roofs on runoff control under different rainfall conditions.