| As a typical form of green infrastructure,green roofs have made outstanding contributions to reducing urban stormwater runoff and alleviating urban non-point source pollution.Since China have the same limited urban land resources as many other countries,green roofs are becoming more and more popular as a sustainable development method that does not require additional land occupation.This study conducts an interactive study around two main lines:how does the structural layer material of green roofs and the rainfall characteristics of the Beijing area affecting the green roof water quantity reduction and water quality control.This paper comprehensively evaluates the capacity of extensive green roofs with different structural layer materials to control runoff water volume under the characteristics of single rainfall events and long-time rainfall scenarios in Beijing,and provides a theory for the impact of green roofs on runoff quantity and quality.This paper also provides the basis for the construction of China's“Sponge City”and the development of green infrastructure in many other northern cities in China.Mainly got the following conclusions:(1)The scale of green roofs,the type and thickness of the substrate and the characteristics of the rainfall all affect the retention capacity of the green roof.As the scale of the green roof module and the thickness of the substrate increase,the runoff retention capacity of the green roof increased,but the retention capacity of green roof is negatively correlated with rainfall depth.In the case of multiple rainfall events,it is more reasonable to use 6 hours as the minimum inter-event dry period for dividing the rainfall event.The monitoring period had a total rainfall of 51 fields(from July 2017 to October 2018)with a total rainfall of 628.6 mm.The runoff retention rate of the extensive green roof(green roof scale from 0.25 to 2.25 m~2 and substrate thickness from 100 to 200 mm)was 64.6%-89.9%.The retention of green roofs using improved soil as the substrate material was significantly better than that of ultra-lightweight substrate.(2)As the scale of the green roof modules and the thickness of the substrate increase,the peak flow reduction rate of the green roof increases.However,as the thickness of the substrate increases,the improvement of the peak flow reduction effect of the green roof was not obvious.When the porosity of the substrate layer material was increased or the water permeability of the drainage layer material was improved,the peak reduction capability of the green roof was greatly reduced.TID100-1.5 has the best peak reduction capability.The average peak reduction rate of TID100-1.5 is 95.9%under 51 rainfall events,and followed with TID100-1.0(90.8%).Although the substrate thickness of TID200-0.5 was twice that of TID100-1.0 and TID100-1.5,the peak reduction rate was only 81.8%.(3)The green roof had a good removal effect on the EMC of the Suspend solids but increased the event mean concentration(EMC)of COD,ammonia nitrogen,nitrate nitrogen,total nitrogen and total phosphorus in the runoff.The scale of the green roof modules had little effect on the average EMC reduction rate of suspended solids.However,as the thickness of the substrate increased,the average EMC reduction rate of suspended solids of the green roof increased.Seven different types of green roofs had significantly increased the EMC of COD,but in the runoff of green roofs using ultra-lightweight substrate as substrate materials,the EMC level of COD was lower than that of green roofs with improved soil.The EMC trends of total nitrogen,ammonia nitrogen and nitrate nitrogen were almost the same in this study,and as the thickness of the substrate increased,the nitrogen concentration of the green roof modules increased.The levels of total phosphorus in the runoff of the seven types of green roof modules were similar,and the total phosphorus content showed a significant downward trend with the accumulation of total rainfall depth.(4)The green roofs in this study is the sink of ammonia nitrogen,nitrate nitrogen,total nitrogen and total phosphorus,but the source of COD.The loads of suspended solids,ammonia nitrogen,nitrate nitrogen,total nitrogen and total phosphorus in three natural rainfalls and six simulated rainfall events were lower than those of normal roof.TID100-1.5 demonstrated the best load reduction capability for these six contaminants.By statistically reporting the pollutant contribution rates of green roof runoff and stormwater,it was found that the total proportion of each pollutant in the green roof runoff was similar to that of rainwater,which was COD>suspended solids>nitrate nitrogen>ammonia nitrogen>total phosphorus.But the content of suspended solids was significantly higher than the other five pollutants in the runoff of normal roof,followed by the COD content.(5)The substrate type significantly affected the runoff water quality control capacity of green roofs.By analyzing the nitrogen and phosphorus contents in the green roof substrate from August to December 2018,it was found that the ammonia nitrogen content in the ultra-lightweight substrate was similar to that of the improved soil,the concentration of ammonia nitrogen of both two substrates were lower than 5 mg/kg,and the nitrate nitrogen content of ultra-lightweight substrate was almost twice that of the improved soil.However,the total nitrogen content of the improved soil was much higher than that of the ultra-lightweight substrate,indicating that the improved soil was rich in organic matter,which also explained that the COD level in the discharge of green roofs with improved soil was higher than that of the ultra-lightweight substrate and normal roof.The total phosphorus content of the improved soil was also significantly higher than that of the ultra-lightweight substrate. |
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