An experimental and numerical investigation of the mechanism of improving the rainwater retention of green roofs with layered soil

Improving the rainwater retention capacity (RRC) of green roofs has been proposed as an important component of urban stormwater management. In this study, a two-layered soil green roof model was established to enhance RRC compared to each single soil column model. The hydrological process of layered soil green roofs was simulated using the HYDRUS-1D program, with simulation results verified by measured results. The results showed that the RRC of the layered soil was 5% and 1% higher than that of each single substrate under a long-term dry-wet cycle and increased by 15% and 11% per event compared with the single substrates. In addition, higher peak drainage reduction and longer peak drainage delay were observed in the layered soil green roof compared to each single soil. The layered soil slowed the movement of the soil wetting front and increased the maximum water content of the upper soil. The water loss of the layered soil was reduced after rainfall and mainly occurred in the lower layer of the layered soil. These results suggest that the structures of green roofs with an upper layer with higher permeability and a lower layer with lower permeability have better hydrological performance.