| Stimulated by the trend of economic globalization,the process of urbanization in our country has been accelerated.Urbanization has caused changes in the regional water cycle process,making the local environment conducive to the formation of rainfall but not conducive to the discharge of rainfall,and frequent floods occur.Our country develops sponge city construction on the basis of Low Impact Development（LID）.Rain garden and green roof are important LID measures that can effectively control rainwater runoff,but they have their own limitations.At the same time,the acceleration of urbanization has made the land resources used for LID facilities more precious.Therefore,it is necessary to study the LID form of the green roof-rain garden device,which can solve the limitations of a single device and can more effectively play the role of coordinated regulation of runoff.In recent years,domestic scholars have carried out research on combined LID devices,but they have been studied in the form of model simulation.There are few cases of actual construction of combined devices and lack of monitoring data for control effects.In view of the above problems,this paper selects the thickness of the substrate layer as the key influencing factor of the design of the green roof device,and the setting of the internal water storage（IWS）as the key influencing factor of the design of the rain garden.According to the rainfall characteristics in Nanchang and the concentration characteristics of runoff pollutants in the study area,the local rainfall events in 1,3,5 and 10 years were simulated to study the rainwater runoff regulation effect of the combined device in different rainfall frequency events,and the operation effect of the combined LID device was evaluated by comparing the runoff volume control and pollutant reduction of the single device.The main conclusions are as follows:（1）The optimization experiment of green roof structure shows that the control effect of green roof on runoff volume increases with the increase of substrate thickness,but the increase amplitude decreases with the increase of substrate thickness.There is an optimal thickness to control rainwater runoff.When the initial thickness of 5cm increases by 5cm,the flood peak clipping rate increases by 3.9%,6.4%and 3.6%respectively.the runoff production time was delayed by 80.0%,10.2%and 12.0%,respectively;the peak residence time was delayed by 72.1%,47.2%and 13.8%,respectively;The total runoff reduction rate increased by 61.3%,17.9%and 32.2%,respectively.Finally,the suitable substrate thickness was determined to be 10cm by comprehensive analysis.After that,the materials to be used for the structural layer of green roof are graded and calculated,and the safety analysis is carried out by the list accumulation method.Finally,the design load of green roof is calculated as 3KN/m2.（2）The optimization experiment of rain garden structure showed that the total runoff reduction effect was not good under the 4 extreme rainfall events in the experiment.In the case of IWS or not,the total runoff reduction rates were 13.6%～20.1%and 30.2%～39.9%,respectively.However,it can still be found that rainfall intensity is significantly negatively correlated with the total runoff reduction rate.The reduction rate of NH3-N,NO3-N,TN,TP,COD and SS in runoff decreased with the increase of rainfall intensity.Among them,the reduction effect of SS and TP was the best,followed by the reduction effect of nitrogen pollutants,and the reduction effect of COD was poor.（3）Setting IWS can effectively improve the regulation of runoff in rain garden.In terms of runoff volume control,the existence of water storage area can delay runoff generation time by 211.7%～326.3%,with an average delay of 266.5%;The total runoff reduction rate was increased by 98.5%～134.0%,with an average increase of 120.8%.The peak residence time and flood peak clipping rate are slightly improved.In terms of runoff pollutant reduction,the installation of IWS in rain garden can improve the reduction effect of the device on COD,NH3-N,NO3-N and TN in runoff,with an average increase of 44.2%,51.4%,50.5%and 31.8%.It has little effect on the reduction of TP and SS in runoff.（4）The experiment of green roof-rain garden combined LID device shows that the combined device has significant runoff regulation effect.In terms of runoff volume control,the average total runoff reduction rate was 58.9%;The average runoff generation is 50min36s.The average peak residence time was 40min6s;The average flood peak clipping rate is 92.6%.There is still a significant negative correlation between the total runoff reduction rate and rainfall intensity.The effect of rain garden on runoff control by combined LID device is larger than that of green roof,and the effect is more obvious with the increase of rainfall intensity.In terms of pollutant reduction effect,the load reduction rates of NH3-N,NO3-N,TN,TP,COD and SS in the combined device were 88.8%,69.7%,75.8%,95.8%,73.5%and99.0%,respectively.The pollutant load reduction effect of the device decreased with the increase of rainfall intensity.（5）Compared with a single device,the combined device can significantly delay the runoff generation time and peak discharge,and control the outflow concentration of runoff pollutants.Because of the special classification structure of the combined device,the green roof reduces the inflow load into the rain garden,so that the latter rain garden can more effectively play the runoff control effect.The flow generation time,peak residence time and average delay of the combined device were 258.4%and 314.6%,respectively,and the flood peak clipping rate was also increased by 32.4%.In addition,the synergistic effect of the combined mechanism within the combined device system makes its control effect on the outflow concentration of each pollutant more significant than that of a single device.In this experiment,the combined device effectively solves the problem of unstable outflow concentration of SS and TP on green roof,and also improves the problem of high outflow concentration of COD in rain garden. |
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