Experimental And Modeling Study On Stormwater Control Performances Of Green Roofs

Green roofs,as one of the common practices for the Sponge City construction,have been increasingly gaining popularity in urban stormwater management.Based on observations of rainfall-runoff processes of 30 rainfall events in 2017 of 12 green roofs located in Beijing with three types(local planting soil,engineered soil and light growing medium),two depths(10cm and 15cm)of substrates and three types of vegetation cover(Portulaca grandiflora,Sedum lineare,Festuca elata and bare substrate),the influence of rainfall condition,substrates and vegetation cover on Stormwater Control of green roofs were analyzed using runoff and peak discharge reduction rates and time-delays in runoff generation and peak discharge as the hydrological performance indices.The results showed:(1)For all the 12 green roofs,there is a significantly(P<0.001)negative correlation between runoff reduction rate and rainfall depth,The average runoff reduction rates were 98.88%,88.17%,58.97%and 51.11%in light rain(0.1～9.9mm),moderate rain(10～24.9mm),heavy rain(25～49.9mm)and rainstorm(50)99.9mm),respectively.However,none significant correlation between peak discharge reduction rate,time-delays in runoff generation or peak discharge and rainfall conditions.(2)The average runoff and peak discharge reduction rates of engineered soil,local planting soil,light growing medium green roofs were 88.39%,87.68%,82.85%and 65.32%,63.87%,54.02%,respectively.The average time-delays in runofff generation and peak discharge of engineered soil,local planting soil,light growing medium green roofs were 125.0min,136.6min,65.3min and 5.6 min,2.5 min,3.8 nin,respectively.The average runoff and peak discharge reduction rates,and average time-delay in peak discharge of the engineered soil green roofs were all higher than that of other green roofs.The average time-delay in runoff generation of the local planting soil was longest and the peak discharge reduction rate was lowest.Light growing medium were worst in runoff reduction,peak discharge reduction and runoff delay.(3)The average runoff and peak discharge reduction rates,time-delays in runoff generation and peak discharge of green roofs with 15cm depth were 87.40%and 65.41%,65.5min and 4.8 min,and these indices of green roofs with 10cm depth were 85.21%and 56.74%,68.1min and 3.1min.The hydrologic performances of green roofs with 15cm depth were higher than that of the green roofs with 10cm depth.(4)The average runoff and peak discharge reduetion rates of green roofs with Portulaca grandiflora,Sedum lineare,Festuca elata and bare substrate were 84.69%,86.92%,80.98%and 55.28%,59.32%,47.97%,respectively.Vegetation cover could enhance the runoff and peak discharge reduction rates of green roofs,but it was little effect on runoff and peak discharge delay.Vegetation mainly affected the hydrological perfornances of green roofs by vegetation interception,evapotranspiration and root changes in soil properties.Green roofs with Portulaca grandiflora have better stormwater management performances because of their stronger interception and faster evapotranspiration rate.(5)Based on the water balance and Peuman-Monteith method,the hydrological model can accurately calculate the runoff depths of green roofs,and simulating the change of retention capacity and rainfall-runoff process of different green roofs.A simulation results show that the runoff volume of green roof decreases exponentially with the increase of substrate thickness.