Experimental and numerical research on the hydrological characteristics of sunken green space with a new type of composite structure.

Based on the characteristics of concentrated rainwater runoff in the mountainous areas of southwestern China and the low rates of rainwater infiltration into low-permeability soils. We have built a new type of sunken green space structure with a combination of a "overflow port and rainwater storage layer" and carried out model tests of storage and drainage performance under heavy rain conditions. The hydrological response of the new composite structure parameters to the sunken green space was analyzed using the HYDRUS-2D program. The results show that the new composite structure has a significant impact on runoff reduction, drainage, and rainwater storage. For the 100a return period, compared with RSL-0 (0 cm rainwater storage layer), the initial and peak drainage times of RSL-25 were delayed by 30 min and 38 min, respectively, and the rainwater storage rate increased by 13.5%. Compared with no overflow port, the peak drainage increased by 78%, the initial drainage time advanced by 73 min, and the cumulative drainage volume increased by 186%. In addition, as the height of the overflow increased, the surface rainwater absorbed by the sunken green space gradually decreased. The sunken green space with OPH-5 (overflow port height of 5 cm) could absorb more than 75% of the rainwater in the rainwater overflow layer, while the absorption capacities of OPH-7.5 and OPH-10 (overflow port height of 7.5 cm and 10 cm) were basically below 75%. In this case, the OPH-5 and the depth of the storage layer not being less than 250 cm provide the best setting for the new combined structure of the sunken green space. In conclusion, the new composite structure designed in this experiment effectively increased the hydrological performance of the layered sunken green space.

A simplified model for analyzing rainwater retention performance and irrigation management of green roofs with an inclusion of water storage layer.

Rainwater retention and water content in green roofs are primarily influenced by structural configurations (i.e., soil layer, vegetation layer, and water storage layer) and climatic factors (i.e., rainfall and evapotranspiration (ET)). Based on the principle of water balance, this study proposes a conceptual model for simulating water flow in green roofs with water storage layers. Three green roof model experiments were conducted from August 1st, 2020 to July 31st, 2021 for calibrating and verifying the conceptual model. The proposed model was solved iteratively using a newly developed program in Visual Basic. The results showed that the conceptual model can capture the dynamic variations in the rainwater retention and water content of green roofs well. The average Nash-Sutcliffe efficiency coefficient is 0.65 and the average error is 6%. The annual rainwater retention capacity (RRC) of green roofs in the perennial rainy climate model was on average 28% higher than that in the seasonal rainy climate model. At the expense of water stress, high ET plants significantly increased the annual RRC of green roofs at a low level. As the water storage layer depth increased from zero to 150 mm, the annual RRC of green roofs increased by 41%, and the water stress decreased by 49%. Compared with an increase in water holding capacity and soil depth, the response of the annual RRC and water stress of green roofs for increasing water storage layer depth is much greater. As per climate of Southern China region, the water storage layer depth of 100 mm is found to obtain optimal rainwater retention and irrigation management in green roof with similar soil thickness (100 mm).

Experimental and numerical investigation on hydrological characteristics of extensive green roofs under the influence of rainstorms.

Green roof rainwater retention, peak runoff reduction, and runoff time delay are considered important hydrological performance indicators for assessing management of urban stormwater. In this study, simulated rainfall experiments were conducted on three green roof models with different water storage layer depths. The numerical model was established using Hydrus-1D program, and the sensitivity of main parameters, the hydrological response of green roofs with a water storage layer, and water storage on the soil surface were analyzed. In addition to the saturated water content of the soil, the depth of the green roof water storage layer is the most sensitive parameter to rainwater retention and initial drainage time. During the simulated rainfall experiment, the 25-mm-deep water storage layer (WSL-25) increased the rainwater retention capacity (RRC) by 46%. For a 20-year return period corresponding to South China region, the RRC of green roofs with WSL-25 increased by 31% compared with that without a water storage layer. The initial drainage time was delayed by 50 min, and the peak drainage rate was reduced by 89%. In this case, a 100-mm soil layer, a 50-mm water storage layer, and a 50 mm maximum surface water storage depth were considered the optimal structural configurations of green roofs. This shows that water storage on the soil surface and bottom water storage layer were equally important for improving RRC, reducing peak drainage and delaying drainage time of green roofs.

An experimental and numerical investigation of the mechanism of improving the rainwater retention of green roofs with layered soil.

Improving the rainwater retention capacity (RRC) of green roofs has been proposed as an important component of urban stormwater management. In this study, a two-layered soil green roof model was established to enhance RRC compared to each single soil column model. The hydrological process of layered soil green roofs was simulated using the HYDRUS-1D program, with simulation results verified by measured results. The results showed that the RRC of the layered soil was 5% and 1% higher than that of each single substrate under a long-term dry-wet cycle and increased by 15% and 11% per event compared with the single substrates. In addition, higher peak drainage reduction and longer peak drainage delay were observed in the layered soil green roof compared to each single soil. The layered soil slowed the movement of the soil wetting front and increased the maximum water content of the upper soil. The water loss of the layered soil was reduced after rainfall and mainly occurred in the lower layer of the layered soil. These results suggest that the structures of green roofs with an upper layer with higher permeability and a lower layer with lower permeability have better hydrological performance.

The rainwater retention mechanisms in extensive green roofs with ten different structural configurations.

Rainfall infiltration, rainwater retention, runoff and evapotranspiration (ET) are important components of the water balance in green roofs. These components are expected to be influenced by variations in the structural configurations (i.e., substrate layers) of green roofs. This study explores the influence of layered soil and green roof configurations on the rainwater retention capacity (RRC) of the roofs as compared to conventional improvements (i.e., soil conditioning and enhanced substrate depth). Ten different extensive green roof modules were designed by varying the substrate materials, substrate depths, storage/drainage layers and vegetation layers. For all modules, the RRCs ranged from 34 to 59%. The RRCs of layered soil were 1-4% higher than that for single-layer soil. The RRC increased by 13% in the presence of a water storage module. It can be concluded that highest RRC corresponds to a combination of high-permeability soil in the upper layer along with a relatively large water holding capacity in the deep layer. Water storage layer and layered soil could significantly delay the water stress in vegetation. The importance of wick irrigation, vegetation types, back-to-back rain events and the ET rate on the RRC were also discussed.

Erodibility assessment of compacted biochar amended soil for geo-environmental applications.

Biochar amended soil (BAS) has been explored as a cover material for geo-environmental applications such as landfill cover due to its vegetation potential. Soil erosion in these infrastructures can progressively lead to failure and hamper the workability of the system. BAS is compacted for geo-environmental applications, unlike agricultural soil, which are loose in nature. Furthermore, the love-hate relationship of biochar with water can potentially affect the functioning of compacted cover system. Thus, the performance of compacted BAS in the context of erosion potential is not well understood. The major objective of this technical note was to explore the erosion potential of compacted BAS sourced from four distinct biochars. Biochar were produced in-house and mixed with soil at 5% and 10% by weight. In total, 81 pinhole erosion tests were performed to gauge the erosion rate of bare soil and BAS at three different compaction states at same compaction energy. It was revealed that the erosion rate decreased with gradual increment in water content for BAS, which was mainly attributed to the change of particle orientation from flocculated to dispersed along the compaction curve. Addition of biochar to soil resulted in decrease of erosion along the dry state whereas the opposite was observed for wet state. This was attributed to the surface functional groups as well as particle gradation of biochar. Erodibility coefficient and critical shear stress plot of soil and BAS revealed that addition of biochar had minimal effect on erosion of compacted silty sand.

Investigation of cracking and water availability of soil-biochar composite synthesized from invasive weed water hyacinth.

Water hyacinth (WH), is one of the world's most intractable and invasive weed species. Recent studies explored the efficacy of this species as a biochar (BC) in improving soil fertility and metal adsorption. However, the soil water retention (SWR) property and crack potential of soil-WH biochar composite has still not been studied. The major objective of this study is to investigate the SWR property and corresponding crack intensity factor (CIF) for compacted soil-WH BC composites. Soil-WH BC composites at five percentages (0, 2, 5, 10 and 15) was compacted and soil parameters such as suction (ψ), water content and CIF were simultaneously monitored for 63 days (including 9 drying-wetting cycles). Results showed that soil-WH BC composite at all percentages retains more water (max. 19% and min. 6.53%) than bare soil at both saturated and drought conditions. Gradual inclusion of WH BC to soil decreases the CIF potential from 7% to 2.8%.