Numerical Simulation Research On The Control Of Regional Runoff And Water Quality Based On Sponge City Concept

The construction of sponge city can effectively alleviate urban water logging and reduce runoff pollution.However,it is subjective to address the complex problems of controlling water quantity and quality in a region in traditional decision-making methods.And numerical simulation is superior to the traditional method in terms of its objectivity and scientificity.In that case,rainfall runoff models of sponge facilities and rainwater pipe networks as well as hydrodynamic water quality models of rivers were constructed using SWMM and MIKE 11.Also,the overall water quantity and quality control was studied focusing on Dingjiazhuang area in Nanjing.Under different repetition periods of rainfall,the pipe network discharge runoff and pollution load data obtained from SWMM model were input into MIKE 11 model as boundary conditions,so as to combine SWMM with MIKE 11 models.Based on this,the control effects of water quantity and water quality in the study area were evaluated together,and a new sponge facility construction scheme for the existing problems were put forward.The main achievements were made as follows:As can be found in field investigations,permeable pavement,sunken lawn and ecological dry stream were built in the Dingjiazhuang district.Rain and sewage diversion adopted in the drainage system can avoid the influences of overflow and mixed flow on the river water.By monitoring the quality of river water sample and analyzing single factor pollution index for two months,it can be found that water qualities of Dingjiazhuanggou,the east branch of Beishilichanggou and Liutanggou reach the standard ofⅤ-class surface water under the significant impact on rainfall.Moreover,sponge city projects of various blocks in the study area were designed individually as per the water quantity and quality control requirements of different service areas in the lack of comprehensively evaluating the overall water quantity and quality control effect of the area.The SWMM simulation indicated that the construction of sponge facility can significantly lower the risk of waterlogging.When the repetition period of rainfall is set to 3 a,5 a,10 a,and50 a,reduction rates of regional runoff coefficient were 44.2%,43.1%,42.6%,and 42.3%,respectively,while reduction rates of the maximum discharge peak flow were 48.2%,44.1%,32.9%,and 7.4%,respectively.Also,the effect of sponge facilities in regulating regional water volume was gradually weakened along with the increase in the return period.Evidently,the current sponge construction failed to reach the annual runoff control rate of 75%as required.The MIKE 11 simulation showed that the construction of sponge facility can effectively reduce the impact of runoff pollution on the water quality of rivers.Change rules of COD,NH₄⁺-N,TN and TP of various rivers under different repetition periods of rainfall were consistent,of which,average reduction rates of peak concentrations were 31.5%,36.3%,33.1%and 27.2%,respectively.Also,the time to peak was delayed for about 10 minutes.The time to peak of pollutant concentration was 50～70 minutes after the beginning of rainfall with an insignificant change in peak concentration,indicating that runoff pollution had an obvious first flush effect.Further,concentrations of various pollutants in the river were decreased dramatically in 70～120 minutes after the beginning of the rainfall.To reach the annual total runoff control rate of 75%as required,new constructions of permeable paving and sunken lawn（scheme 1）,green roofs and rainwater tanks（scheme 2）and reservoirs（scheme 3）in the optimization of constructing sponge city were compared based on the existing sponge construction.Specifically,based on cost-benefit analysis,the costs per unit of runoff reduction in scheme 1 and scheme 2 were roughly 4457 yuan/m³ and 9913 yuan/m³,respectively,whereas scheme 3 costs only 800～1200 yuan/m³.It shows that the construction scheme for sponge city should be formulated through integrating green facilities and gray facilities as well as infiltration for emission reduction,and collection for utilization of rainwater on the premise of satisfying the total runoff control and runoff pollution control.