The Elaborate Simulation Of Rainfall Runoff And Non-point Source Pollution In Nanning Based On SWMM Model

With the rapid development of urbanization,the percentage of impervious areas of cities were constantly growing,that resulted waterlogging and black odor frequently happened in urban river basin.As the permanent venue for ASEAN Expo,Nanning was an important hub for "The Belt and Road".It was also facing the central problem of increased runoff pollution.As one of the first batch of 16 national "sponge city" construction of the pilot,there were important significance to calculate the rainfall runoff and non-point source pollution in spatial distribution overall for Nanning.In this paper,150m grid was used as the basic unit to elaborately study the raifall runoff and non-point source pollution.And 3 scenarios of low impact development model were used to simulate the pilot area,the conclusions were as following:The mean error of elaborately simulation of raifall runoff in study area was only 4.73%by high precision modeling.The total runoff was 337 million cubic meters in 2015,which was increased significantly compared with 186 million cubic meters in 2009.The spatial distribution of runoff was shifting to the northeast of the city.The SS and COD increased in the non-point source pollution of research,while the opposite occured in TN and TP.The COD production increased from 1.588 thousand tons to 1.868 thousand tons,the SS increased from 3.880 thousand tons to 4.583 thousand tons.The TN decreased from 0.254 thousand tons to 0.228 thousand tons,the TP decreased from 0.073 thousand tons to 0.060 thousand tons.High pollution areas had varying degrees of transfer in spatial distribution.The study found that the main factors leading the runoff variation of the impervious surface changes,leading factors of COD and SS was the change of residential land and commercial land,the dominant factors of TN and TP changes in the industrial land,other land and green building.Green surface could effectively affect runoff and non-point source pollution analysised from the feature region.The peak value of runoff was delayed by 30 minutes under the designed precipitation,the same trend as peak values of pollutants.The low impact development scenarios model simulation in H region showed some conclusions.The reduction of runoff and SS was 21.04%and 66.37%,respectively,in mode 1 and 2.The other reduction was 13.74%and 46.71%under the optimal cost model(mode 3).The mode 3 is better than node 1,2 considered from the unit investment returns.The reduction rate of runoff and SS were 0.17(%/10 million yuan)and 0.89(%/10 million yuan)under unit investment returns.Based on the above analysis,to give some suggestions,the benefit brought by the changes of unit cost investment should taked into account when reconstructing the sponge city,which should depend on multi-angle planning.