Experiment and numerical simulation of rainwater retention capacity of layered-soil green roofs

Abstract

[Objective] Improving the rainwater retention capacity(RRC) of green roofs has been recognized as an important part of urban stormwater management. As the substrate layer is a key structural element of green roofs, investigating the influence mechanism of layered soil substrates on the RRC of green roofs has a positive impact on green roof design. [Methods] Green roof models were established using layered soil and two single-soil configurations with different material mix ratios. Hydraulic parameters of the layered soil were obtained using the inverse solution module of the HYDRUS-1D software. Through four months of dry-wet cycles, the RRC of the three models was studied. Simulations were performed to examine the effects of initial water content, saturated water content, and other factors on the RRC of layered-soil green roofs, followed by the sensitivity analysis of these parameters. Additionally, simulations were conducted to examine the RRC of layered-soil green roofs under different rainstorm return periods. [Results] The result showed that the RRC of layered soil was 5% and 1% higher, respectively, than that of the single-soil configurations. The layered soil showed greater reduction in peak drainage and longer peak drainage delay. Initial water content, saturated water content, permeability, and depth ratio of upper to lower layers all have certain effects on the RRC of layered-soil green roofs, with saturated water content having the strongest effect. When the maximum surface water storage depth was 10 mm, peak surface runoff of the green roofs was reduced by 22%, 15%, 14%, and 12% under different rainstorm return periods, respectively. [Conclusion] The findings show that layered soil exhibits higher RRC than single-soil configurations, demonstrating better hydrological performance. To achieve better hydrological performance of layered-soil green roofs, the substrate layer should comprise a high-permeability upper soil layer and a low-permeability lower soil layer, with the depth ratio of the two layers maintained within a reasonable range. Allowing surface ponding infiltration during rainstorms can effectively reduce peak runoff and delay the onset of surface runoff.

Keywords

green roofs / layered soil / rainwater retention / HYDRUS-1D software / hydrological performance / runoff reduction / low impact development / rainstorm return period