Influence of biochar amendment on stormwater management in green roofs: experiment with numerical investigation

Green roof is known to minimize urban waterlogging owing to its water retention capacity due to the presence of substrate soil layer. Biochar, which is a carbon-negative material, appears to be an essential soil amendment in green roof due to its water-holding capacity and stability. Recently, incentives are provided in developed countries to enhance commercial production of biochar for usage in green infrastructure, with an aim to meet carbon reduction goals of 2030. Further, biochar has a longer half-life (over 100 years), compared to other materials that are easier to degrade. In this study, the influence of different biochar contents on hydrological performance of green roof is evaluated using a combination of experiment and numerical simulation. Four soil columns with different biochar contents (0, 5, 10 and 15%) were subjected to artificial rainfall. Hydraulic parameters were obtained using inverse solution from the collected rainfall data. Numerical simulations were used to explore the impact of different biochar contents on green roof rainwater management performance during real rainfall process. Biochar is found to enhance saturated water content and, however, tends to reduce saturated hydraulic conductivity. The green roof with 10% BAS (10% biochar content) has better ability of comprehensive rainwater management, with the highest peak outflow reduction and the longest rainwater outflow delay. Green roof with 5% BAS has highest runoff reduction and longest peak outflow delay. These results provide a suitable selection of biochar content for urban areas with different rainwater management requirements.