Research On The Extensive Green Roof Substrate Structure Based On The Rainfall Runoff Retention And Pollution Interception Benefits

As one of the low-impact development（LID）measures,green roofs（GRs）have a certain mitigation effect on the waterlogging and non-point source pollution that are urgently to be solved in the city.Extensive GRs are widely used in actual projects because of their simple structure and low load requirements.The substrate layer is one of the important structure layers in extensive GR.It is very necessary to systematically study the structure of the Extensive GR substrate layer based on the benefits of rainfall runoff retention and pollution interception.In this study,the types of nutrient substrate and adsorption substrate,substrate ratio and substrate configuration adopted by most researchers are taken as the research objects,and the rainfall runoff retention and pollution interception benefits of extensive GR substrate layer are systematically evaluated and analyzed.This study can provide reliable data support for the construction of the extensive GR substrate structure in actual projects and related research,and at the same time provide a basis for the development of related design and construction standards for GRs in the future.The main conclusions of this study are as follows:（1）Compared with the difference in rainfall runoff retention benefits caused by different substrate types,substrate ratios,and substrate configurations,rainfall depth is one of the more important determinants.The delayed outflow time and runoff reduction rate of each device in different depth rainfall events and the entire simulated rainfall experiment show a significant correlation.Devices with strong delaying outflow ability also show a higher runoff reduction rate,and vice versa.The study indicates that the pollutant leaching effect of different substrates in the initial stage of construction has a more significant impact on the water quality of the device’s outflow.（2）The loam has significant SS leaching during the runoff erosion process,and the loam is prone to compaction and overflow,which affects the building safety.Coconut chaff can more effectively display the rainfall runoff retention benefits.The average runoff reduction rate is 43.77%,but it is manifested SS,NH₄⁺-N,NO₃^--N,TN,TP pollution sources in the rainfall runoff,especially TP,have an average TP pollution rate of 418%.Peat soil can intercept part of the NH₄⁺-N and TP of rainfall runoff,and the leaching effects of NO₃^--N and TN in the substrate should be weaker than that of coconut chaff.Therefore,used as a nutrient substrate for extensive GRs,peat soil is more suitable for practical engineering applications and experiments than loam and coconut chaff.（3）Compared with other adsorption substrates,ceramsite has a better rainfall runoff retention effect,which can effectively delay the outflow and reduce the rainfall runoff.Ceramsite do not flow out in the 25.5 mm and 27.3 mm rainfall events,and the average runoff reduction rate is 47.15%.Maifan stone has significant SS leaching during rainfall process,but compared to other adsorption substrates,there is almost no rainfall runoff pollution effect of NO₃^--N.Perlite shows the worst rainfall runoff retention benefit and the rainfall runoff pollution effects of NO₃^--N and TN.The average NO₃^--N pollution rate is 41.88%,and the average TN pollution rate is 9.97%.Vermiculite has excellent reduction and interception capacity for NH₄⁺-N,TN and TP in rainfall runoff,and green zeolite can show a better reduction and interception effect on NH₄⁺-N,NO₃^--N and TN in rainfall runoff.Both vermiculite and green zeolite can be as an excellent adsorption substrate for extensive GRs.However,considering the flexibility and feasibility of the adsorption substrate application in actual projects,vermiculite is the most suitable choice as the adsorption substrate for extensive GRs.（4）A large proportion of vermiculite can better increase the rainfall runoff retention capacity in the initial stage of substrate layer construction,but for the rainfall runoff retention benefits of the devices with different ratios substrate during the entire experiment,PB20 and PB35 show more significant rainfall runoff retention effect,the average runoff reduction rates are 43.53%and 39.92%.The rainfall runoff interception benefit of the devices with different ratios substrate does not simply change with the change of the peat soil ratio.The low leaching risk of the PB10substrate layer is the best for SS and TP in the rainfall runoff.Granular structure in the PB20 substrate layer enhances the adsorption efficiency of vermiculite and controls nutrient leaching characteristics for NH₄⁺-N,NO₃^--N and TN rainfall runoff pollution interception benefit is the best.20%peat soil+80%vermiculite to construct an extensive GR substrate layer can effectively retain and intercept rainfall runoff,which is the best substrate ratio choice in practical engineering applications and follow-up research.（5）Regarding the influence of different substrate configurations on the rainfall runoff retention benefits of extensive GRs,in the entire experimental research process,the delayed outflow time range of EGR-N is 9.66 min-37 min,EGR-L is 14.42min-58.23 min,EGR-M is 18.58 min-61.72 min,and NR is 3 min-28.37 min.The runoff reduction rate of EGR-N is 14.05%-37.12%,EGR-L is 17.60%-58.26%,EGR-M is 15.28%-52.17%,and NR is 4.04%-15.98%.EGR-L has better reduction and interception effects on SS and NH₄⁺-N in rainfall runoff,because the lower layer vermiculite which configured separately has higher adsorption efficiency for NH₄⁺-N in rainfall runoff.Compared with EGR-N,EGR-M has a significant rainfall runoff retention capacity,and shows better retention of NO₃^--N,TN,and TP in rainfall runoff,which shows that the addition of biochar helps to improve the granular structure of peat soil and vermiculite for the water retention capacity and inhibits the nutrients leaching of the substrate layer.