Experimental and numerical investigation on rainwater management of dual substrate layer green roofs using biochar-amended soil

Biochar is one of the potential amendment materials in green roofs. The previous studies concentrate on the single layer green roof using biochar-amended substrate. The influence of biochar in green roof amended with dual layer of substrate is rarely studied. This study is aimed to evaluate the rainwater management of dual substrate layer green roof using biochar amended soil. Four biochar-amended soil columns (0, 5, 10, and 15%) were established to investigate the hydraulic processes. Hydraulic properties (hydraulic conductivity and soil water retention curve) of biochar-amended soil were measured by laboratory tests. The hydrological processes of single layer green roofs amended with biochar were evaluated using numerical simulation. The result shows that bare soil has the least surface runoff and 5% BAS the highest water storage. Thus, bare soil and 5% BAS can be preferred as upper and lower layer, respectively. B15BI0 (i.e., 15 cm bare soil combined with 0 cm bichar-amended soil), B0BI15, B12BI3, B10BI5, B7.5BI7.5, B5BI10, and B3BI12 were explored using numerical simulation method. Real rainfall data recorded in the local city was input into the program. The substrate B5BI10 possessed the highest reduction of peak outflow (19.52%), and longer delay (4.2 min) than single layer substrate. The substrate B12BI3 and B3BI12 shows the longest peak delay (i.e., 5 min). B2BI12 also corresponds to the highest rainwater outflow delay (i.e., 24 min). It provided a reasonable design guidance of dual substrate layer green roof using biochar-amended soil in local area.