Impacts of slope and length on the hydrological performance of green roof drainage mats

Abstract

This study evaluated the impact of the roof slope and length on the retention and detention performance of the drainage mats of green roofs. Artificial rainfall events were applied to a test bed at two different lengths (1 m and 4.5 m), three slopes (2 %, 5 %, and 20 %), two drainage mats: retention mat (RM) and hard plastic egg-shape mat (ES), and a standard black roof as a reference. Retention and detention indicators were determined for each rainfall-runoff experiment. In addition, the study developed and calibrated a reservoir routing model to predict the outflows of the drainage mats at different slope and length values. The slope was found to influence both the retention and detention performance of drainage mats for low and high-intensity rainfall events while the length was found to affect the detention performance for low-intensity rainfall events. In addition, the RM and ES mats were found to retain up to 8 and 6 mm, respectively, of rainwater and to delay drainage mat runoff by 25 mins and 15 mins, respectively, demonstrating the significant role of drainage mats in the performance of green roofs. The reservoir model accurately simulated the outflow from the drainage mats (Kling-Gupta Efficiency > 0.75), while the parameters of the model were found to be influenced primarily by the slope of the roof.