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Experimental and numerical investigation on runoff reduction and water stress of green roofs with varying soil depth and saturated water content under dry–wet cycles

Ma Ming, Wang Jun, Garg Ankit, Mei Guoxiong

Acta Geophysica

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2023

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Vol. 71, no. 2

893--903

EN

The soil layer is the most important structure for green roof runoff reduction and vegetation growth. The mechanisms of runoff reduction and water content of green roofs with varying soil depth and saturated water content (θs) under dry–wet cycles are not well understood. Field and numerical methodologies were adopted for investigation in this study. The green roof drainage and water content were observed for a given period (i.e., August 2020 to July 2021). A numerical model was calibrated and validated for the analysis of annual runoff reduction and water stress with different θs and soil depths. Based on climate in southern China, the green roof's annual runoff reduction rate (ARR) (100 mm soil) was 33%, and the annual water stress was 168 days. With an increase in θs by 0.1 mm3 /mm3 , the ARR of green roofs increased by an average of 5% while the water stress was reduced by an average of 32 days. With an increase in soil depth by 100 mm, the average ARR increased by 4%, whereas the average water stress was reduced by 6 days. It was shown that the runoff reduction is enhanced with an increasing θs and soil depth during a longer antecedent dry weather period, but it had no significant effect on runoff reduction during back-to-back rainfall events. Increasing soil depth had no significant improvement in runoff reduction and water stress beyond a certain point. Consequently, the optimal structural configuration of green roofs was considered as a soil depth of 200 mm (θs of 0.5 mm3 /mm3 ).

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Moisture management in biochar amended green roofs planted with Ophiopogon japonicus under different irrigation schemes: an integrated experimental and modeling approach

Liu Jiaqin, Garg Ankit, Wang Hao, Huang Shan, Mei Guoxiong

Acta Geophysica

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2022

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Vol. 70, no 1

373--384

EN

Green roof constantly suffers from the water stress that is developed during prolonged drought seasons. In general, periodical irrigation is required to ensure plant growth and hence serviceability of green roofs. Biochar, a carbon sink material, has been proposed as a substrate amendment in green roofs for enhancing water retention ability of soils. This study aims to conduct an assessment of the irrigation efficiency of green roofs with different biochar additions (0%, 5%, 10%, 15% and 20%; v/v) under sub-tropical climatic conditions. In order to achieve this objective, outdoor monitoring as well as numerical modeling using HYDRUS-1D was conducted. Soil columns mixed with different proportion of biochar were prepared. These columns were subjected to different irrigation schemes (three irrigation frequencies were assessed (i.e., per 3, 7 and 10 days after irrigation or rainfall); moreover, three irrigation amounts for the three irrigation frequencies were considered (i.e., to a fixed amount (FA10mm), to Field water holding capacity (FC) and to Saturated moisture content (SR))). As suggested from the results: (1) Biochar significantly improved water holding capacity and plant available water. 20% biochar delayed the onset of the significant plant wilting phenomenon by approximately 3 days and maintained the maximal transpiration rate of vegetation in the dry period. (2) As compared to irrigation scheme A (irrigation to FC per 7 days), the efficiency of scheme B (irrigation to SR per 10 days) was more vulnerable to the biochar amendment. Moreover, the total irrigation water and days of water stress decreased with an increase in the biochar addition. Furthermore, the combination of 20% biochar and irrigation scheme B could be the optimal choice for maintaining the health of the green roofs and water conservation. The present study helps to obtain desired outcomes in green roofs, e.g., stormwater management, cost reduction as well as providing greening.

3

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

Gan Lin, Garg Ankit, Wang Hao, Mei Guoxiong, Liu Jiaqin

Acta Geophysica

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2021

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Vol. 69, no. 6

2417--2426

EN

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 pro duction 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.

4

Utilization of coconut shell residual in green roof: hydraulic and thermal properties of expansive soil amended with biochar and fbre including theoretical model

Wang Hao, Garg Ankit, Zhang Xiaoyong, Xiao Yangyang, Mei Guoxiong

Acta Geophysica

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2020

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Vol. 68, no. 6

1803--1819

EN

The study intends to explore hydraulic and thermal properties of expansive soils treated with fbre, biochar and biochar–fbre mix. Both fbre and biochar are derived from coconut shell, which is highly common in coastal regions around the world. Besides, benefts, limitations and engineering feasibility of these geomaterials in green roofs are explored. Theoretical framework for thermal–hydraulic analysis is proposed based on mass conservation and the frst law of thermodynamics. Heat capacity, thermal conductivity, water retention curve, crack intensity factor (CIF) and saturated and unsaturated hydraulic conductivities of four kinds of soils are evaluated and compared. Characterizations of geomaterials are also investigated via thermal mass loss, micro-structure, surface area and functional groups identifcation. Both biochar and fbre admixtures contribute to improvement in soil heat capacity and saturated and unsaturated hydraulic conductivities. Biochar enhances saturated and residual water contents of expansive soil by 10% and 8%, respectively. Also, biochar decreases soil thermal conductivity and CIF by 31% and 5%, respectively, while fbre decreases soil-saturated and residual water contents by 15% and 29%, respectively, and reduces soil thermal conductivity and CIF by 21% and 50%, respectively. Soil–biochar–fbre composite is also recommended due to low air-entry value, acceptable water-holding capacity and limited crack propagation. The study flls the knowledge gap of how soil thermal–hydraulic properties are afected due to biochar and/or fbre admixture. It is recommended to pay more attention on production and utilization of biochar derived from coconut shell currently utilized for fbre extraction.

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Mechanism of compacted biochar amended expansive clay subjected to drying–wetting cycles: simultaneous investigation of hydraulic and mechanical properties

Wang Hao, Garg Ankit, Huang Shan, Mei Guoxiong

Acta Geophysica

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2020

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Vol. 68, no. 3

737--749

EN

Biochar has been extensively studied in the aspect of amendment of compacted sandy/clayed soils, whereas its application as amendment in expansive soil is rare. Hydraulic and mechanical properties of biochar-amended expansive soil especially impacts of drying–wetting cycles have been rarely investigated. Aiming at construction of sponge city, straw biochar-amended expansive soil and the control soil (i.e., without biochar) are subjected to drying–wetting cycles in this study. During drying–wetting cycles, energy-dispersive spectrometer and Fourier transform infrared (FTIR) spectroscopy analyses were conducted to investigate microchemical composition including. Pore size distribution and microstructure were measured using nitrogen gas-adsorption technique and scanning electron microscope, respectively. Further, changes in soil water retention curve, void ratio, crack intensity factor (CIF, i.e., ratio of cracked section area to the total soil area) and shear strength were also determined. It is found that there is no diference in water retention capacity between various soils for near-saturated samples. Under high suction, however, more water could be retained within mesopores of biochar-amended soil. FTIR analysis indicates that biochar-amended expansive soil shows stronger chemical bonding, irrespective of them being subjected to drying–wetting cycles. The weak alkalinity of straw biochar results from its main chemical composition (i.e., calcium carbonate). It is noteworthy that straw biochar improves soil water retention capacity, which further restrains desiccation cracks. Cohesion of biochar–soil composite is also improved due to chemical bonding. Aiming at green roofs, straw biochar could be promising option for expansive soil amendment technically and economically.