| Green roof is one of the important measures in the Construction of Sponge City(CSC),which shows many advantages such as improving urban ecological environment,alleviating urban inundation and promoting sustainable development.As a"sponge material"for urban hydrologic regulation,green roof can effectively retain rainwater and delay the runoff.In the green roof structure,the substrate layer has the greatest retention capacity for rainwater.This study aims to take full use of the hydrologic advantage of biochar(i.e.,light weight,high water retention),the biochar-amended green roofs was proposed.Lab-scale tests were conducted on four pre-fabricated soil columns.The influence of rainfall conditions,vegetation,and the structure of substrate on the runoff interception were investigated by using combined experimental and numerical investigation.The main work contents and conclusions are as follows:(1)A new type of artificial rainfall simulator was designed in this study.This rainfall simulator performed the artificial rainfall with controlled intensity,and the rain disc with a bit of glue needle can process a uniformity rainfall.The results show that the optimized artificial simulated rainfall device can effectively improve the accuracy and uniformity of simulated rainfall intensity,which provided the raindrop simulation to the subsequent model tests as per rain event in the practice.(2)Two groups of single-layer soil column(natural soil N,modified soil B)and two groups of dual-layered soil column(top 50%N+50%lower,upper 20%N+B below 80%B)were built for model tests to simulate the green roof substrates.Different rainfall intensity(10 mm/h,30 mm/h,50 mm/h))and rainfall duration(360 min,120 min,70 min)conditions(each group of simulated rainfall events is repeated twice)were conduct on the lab-scale soil columns.The water content at different depth(3 cm,6 cm,9 cm and 12 cm)in the substrate,surface runoff and bottom leakage from the soil columns were monitoring.The results showed that the infiltration capacity of the matrix soil decreased and the saturated moisture content increased under the condition of repeated dry and wet cycles.Biochar-amended soil can effectively improve the retention capacity of substrate,which is about 30%higher than that of natural soil(i.e.,column N).The layered soil design improve recovery of the available rainwater retention of the substrate before the secondary rainfall,thus significantly improving the rainwater interception capacity of the green roof.The average rainfall-interception capacity of the dual-layer substrate(20%N+80%B soil column)is 3.91%higher than that of the substrate with single-layered biochar-amended soil(column B)and 14.41%higher than that of the substrate only with natural soil(column N).(3)Two groups of single-layer soil columns and two groups of layered soil columns were planted with grass under 28 days of growth and cultivation.Artificial rainfall tests were conducted to further explore the influence of vegetation cover on rainfall infiltration and rainwater interception.The results showed that vegetation cover group significantly improved the water infiltration capacity and rainwater retention performance of green roofs because of the preferential flow path along the plant roots and root-formed space in the soil.(4)The commercial numerical software HYDRUS-1D was used to simulate and analyze the four soil columns in the test.The hydraulic parameters are estimated based on the indoor artificial rainfall data.The accuracy of selected parameters is verified by comparing the simulation and outdoor rainfall interception results.Based on the local meteorological data collected by the weather station in the experiment site,four typical rainfall events within a year were selected as the boundary conditions in the numerical simulation model.The influence of the structure of green roof substrates on the runoff delay and peak-flow reduction were discussed.Under the condition of short duration light rain intensity,the green roof showed better runoff interception ability.The addition of biochar-amended soil has a significant effect on the delay effect of runoff and the reduction of peak flow.Because of difference in the infiltration and storage capacity of different layered structures,the biochar-amended green roofs with different substrate structures show different peak-flow delay and reduction effects during the rain event.(5)The hydrological response of four soil columns under different initial water contents(0.10 m~3/m~3,0.15 m~3/m~3,0.20 m~3/m~3 and 0.25 m~3/m~3)were investigated by means of numerical simulation.The results showed that under the same rainfall conditions,the initial water content has a significant effect on the interception capacity of rainwater.The delay and reduction effect of peak-flow decreased with the increase of the initial water content.Results showed that under the lower initial water content,the peak-flow delay and reduction effect of dual-layered substrate was better than single-layered substrate.While with the increase in the initial water content,the single-layer substrate with biochar-amended soil shows the best delay and reduction effect among the four test columns.Using combined experimental and numerical investigation,this paper discussed the application of biochar on the green roof substrate.The peak-flow delay and reduction effect of substrates with different layer structures were compared and discussed.Even though the biochar can improve the retention capacity of substrate,results indicated that an optimization design in substrate structure based on the local rainfall conditions was need.Green roofs with a dual-layer substrate is more reasonable in practice,because it can efficiently make full use of the biochar-amended soil.This paper provides a more reasonable and efficient design for the green roofs with the application of biochar. |
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