Research On The Reconstruction Of The Old Urban Area Under The Low Impact Development

In recent years,the process of rapid urbanization has driven the rapid economic development of our country at the same time such as urban waterlogging and a series of water safety problems are also one after another,these problems are particularly prominent in the original inadequate design and aging infrastructure in the old city,which has seriously affected the daily life of urban residents,how to solve these problems has become imminent.The sponge city concept was proposed in China in 2012,and it provides a new type of urban stormwater management to improve efficiency of rainfall utilization and urban flood control with low impact development（LID）technology as the main core.Based on this,guided by the concept of low impact development,this paper takes a typical old urban area in Maiji District,Tianshui as the research object to establish SWMM model,and combines the regional characteristics of the study area to select appropriate renovation schemes for LID reconstruction,and simulates and analyzes the stormwater control effect of the study area before and after reconstruction under different recurrence periods of rainfall.The main conclusions of this paper are as follows:（1）The Chicago hyetograph method was chosen to generate the relevant return period rainfall process for this simulation,rainfall with a return period of 1 year,2 years,5 years,10years,20 years,30 years,50 years and 100 years was simulated for 120min with a time interval of 1min.Rain peak position coefficient r=0.3684.The design rainfall of different return periods is 16.12mm,23.27mm,32.71mm,39.86mm,47.01mm,51.19mm,56.46mm and 63.60mm.（2）According to the investigation of the actual topography of the study area and the distribution of inspection Wells,combined with the underlying surface and water flow direction,molecular catchment areas were divided and nodes were drawn as well as pipe network generalization.The SWMM model of the study area consists of 31 subcatchments,27 nodes,27 links and 1 outlets.（3）The usage of the runoff coefficient method to calibrate the accuracy of the parameters.The comprehensive runoff coefficient is 0.598.The simulated runoff coefficient is 0.590.SWMM simulation was used to calibrate the rationality of the model when the return period P=1a and P=3a,both of which met the value range of the runoff coefficient,indicating that these parameters met the requirements.The final calibration results of parameters are:N-Imperv is0.011,N-Perv is 0.26,Dstore-Imperv is 1.8mm,Dstore-Perv is 5.5mm,Max.Infil.Rate is76mm/h^-1,Min.Infil.Rate is 4.7mm/h^-1,Decay Constant is 10.4/h^-1.（4）SWMM was used to simulate the drainage status of the study area under different return periods.The results showed that with the increase of return period,the total runoff,peak runoff and runoff coefficient all increased,the peak time gradually advanced,and the pipe network drainage system gradually exceeded the load with the increase of return period.The main reasons are as follows:the development degree of the study area is relatively high,the density of buildings is high,the hardening degree of the road square is high,and the design return period of the drainage network is too low,which makes it difficult for the rainfall to penetrate and not discharge effectively,thus causing more serious water accumulation.（5）The sponge reconstruction policy of"storage retention,net use combination,appropriate infiltration,and safe discharge"was adopted.After the reconstruction,the annual runoff total control rate of Tianshui was set at 85%,and the SS reduction rate of non-point source pollution was set at more than 60%.Based on the planning and construction,topography and geology,rainfall and other regional characteristics of the study area,the reconstruction measures are permeable pavement,bio-retention cell and green roofs.The reconstructed permeable pavement is about 0.151km²,the bio-retention cell is about 0.151km²,and the green roofs is about 0.169km².After the reconstruction,the runoff control rate increased significantly in different return periods.When the designed rainfall is 18.8mm（the return period is 1.3 years）,the total runoff control rate is 86.39%（>85%）,which met the requirements of annual runoff control rate in Tianshui.When the return period is 1 year,the peak runoff reduction and peak lag time reached the maximum value of 59.06%and 12min.When the return period is 1,1.3,2,5 and 10 years,there is no overload of node pipe network,and the overload of node pipe network is significantly reduced in other recurrence periods.The average discharge,maximum discharge and total pollutant TSS decreased to some extent under different recurrence periods.When the recurrence period was 1.3 years,the pollutant reduction rate reached 83.56%,which was much higher than the target 60%.The growth rate of infiltration reaches the maximum of48.83%when the return period is 2 years.The growth rate of storage gradually increases to73.3%and stabilizes at about 73%.The drainage after the reconstruction is at least 25%lower than that before the reconstruction in each return period.and the amount of accumulation reduces up to 71.4%when the return period is 1 year.It shows that the LID reconstruction can effectively reduce the total runoff,reduce the peak value of runoff and lag the peak time,and effectively reduce pollutants.LID measures can effectively increase infiltration and have a good regulating and storing effect,and can effectively intercept and utilize rainwater through reasonable"infiltration,stagnation and storage"technology,so as to reduce pavement water.