Stormwater management of biochar-amended green roofs: peak flow and hydraulic parameters using combined experimental and numerical investigation

Green roofs have been suggested as one solution to manage stormwater in cities. Soil amendments such as biochar appear to be very promising owing to its water retention capacity. Furthermore, biochar is more stable with time (expected half-life over 100 years) than other biomass (fibers), which is likely to degrade at higher rate upon microbial action. Rational utilization of biochar-amended soil in substrates is the critical factor for improving ability of stormwater management (peak-flow reduction and delay). The objective of this study is to evaluate the hydraulic performance of green roofs amended with biochar. Laboratory experiments combined with numerical approach were adopted to achieve the objective. A dual-layer substrate mode with biochar-amended soil was proposed for optimizing the ability of stormwater management. Four experimental columns were built to observe the hydraulic processes under artificial rainfall. Unsaturated hydraulic parameters of substrates were estimated by the inverse solution method. Numerical simulations have been conducted to explore the stormwater management of dual-layer substrates with biochar. Both biochar and vegetation are found to enhance saturated water content. However, their effects on hydraulic properties of soil were opposite, with vegetation having positive influence. Different biochar application modes showed varied performance in peak-outflow delay and reduction. In the selected rainstorm case, the substrate with 3 cm vegetated natural soil in upper layer and 12 cm biochar-amended soil showed a longer delay, higher reduction in peak outflow, and rainwater retention. The dual-layer substrate mode is a flexible utilization of biochar in green roofs. It provided a more reasonable design with the consideration of the local rainfall statistical data.