Study On Rainwater Retention Effect Of Extensive Green Roof In Chengdu

Increasing urbanization and extreme weather caused by global warming have made urban rainwater problems more serious and waterlogging more frequent.The hardening of underlying surface and the increase of impervious area are the main reasons for the increase of urban flood risk.As an effective means of compensating urban green space,roof greening plays a significant role in rainwater retention,among which extensive roof greening has attracted extensive attention in the field of environmental and engineering research because of its simple construction and good environmental and ecological benefits.In the context of Chengdu’s climatic conditions and roof greening construction,this research ranges from micro-individual research to macro-system effect research.Firstly,based on the rainwater retention of single extensive roof greening,the rainwater retention experiment was designed to explore the influence of matrix ratio and vegetation type on the rainwater retention effect of roof greening.Then,according to the actual monitoring data and structural parameters of the single roof greening device with the best retention effect in the experiment,Xipu Campus of Southwest Jiaotong University was selected as the research object to design three kinds of extensive roof greening universal application schemes with different greening norms(50% greening,75% greening and 100% greening).SWMM model was used to simulate the rainwater confluence process before and after the application of the three schemes.The rainwater runoff reduction of the whole campus roof greening system was obtained under six conditions of return period P= 2a,3a,5a,10 a,20a and 30 a,and the runoff reduction law of roof greening system based on system effect was analyzed.Specific research results are as follows:(1)The results of rainwater retention experiment showed that four commonly used extensive roof greenery plants,such as Sedum lineare Thunb,Sedum sarmentosum Bunge,Tall Festuca and Ophiopogon japonicus,could all survive and grow well under the climate conditions in Chengdu.Slope of the substrate thickness is 100 mm,platform for 3% of the cases,in terms of four plants the rain delay of storage capacity,Ophiopogon japonicus >Sedum lineare Thunb> Sedum sarmentosum Bunge > Tall Festuca,the matrix type was100% under the condition of lightweight soil,the average delay of dwarf lilyturf accumulation rate was 48.62% in 13 rainfall events lasting 1 hour.(2)The ratio of soil matrix will have an impact on the rainwater retention capacity of roof greening.In the case of unified planting of phyllophylla,the ratio of soil matrix type is50% peat soil + 25% rural soil + mixed matrix(perlite: ceramsite: Vermiculite =1:1:1)15%+10% biocarbon treatment group had the highest rainwater retention rate(53.47%),and the lowest(35.31%)when the substrate type was peat soil 50% + rural soil 25%+ ceramsite 25%in 13 rainfall events lasting 1 hour.Adding 10% biochar can significantly improve the water holding capacity of the matrix.(3)SWMM simulation research results show that compared with campus systems without roof greening,the popularization and application of extensive roof greening can significantly reduce the total amount of runoff,reduce the runoff coefficient,reduce the peak flow of runoff,and increase the control rate of total rainwater runoff.And as the return period increases,the roof greening system’s retention capacity for rainwater runoff gradually decreases.Comparing the three schemes,50% of all available roofs in the campus are extensively greened,which can reduce rainwater runoff by 7.88%,75% of green roofs can reduce11.83%,and 100% green roofs can reduce 15.78%,with the increase of the quota of greening,the rainwater reduction percentage of the roof greening system increases linearly.Based on the above research results,this article hopes to provide research ideas for the quantitative assessment of the rainwater retention effect of roof greening,and to provide references for the selection of vegetation and substrates for extensive roof greening in Chengdu,make more clear the contribution of roof greening in the urban rainwater management system,so that it can better play the role of rainwater regulation and storage.