Multi-objective Optimization Study Of Urban Stormwater Drainage System With Grey And Green Infrastructure

Waterlogging in urban areas presents the characteristics of high frequency,wide range and severe losses,and a sustainable urban stormwater management system is of great significance for solving waterlogging problems.The scientificity and rationality of grey and green infrastructure directly affects the performance of drainage system.Predecessor’s research mainly focus on optimization of green or gray infrastructure,while they rarely focus on system with both grey and green infrastructure.In this study,NSGA-II algorithm is adopted to build a multi-objective optimization model.It’s researched that the changes law of grey and green infrastructure’s cost and performance and multi-objective optimization of system with both grey and green infrastructure under different rainfall condition to explore the performance characteristics of grey and green infrastructure which is set separately or couplingly.First of all,the method of building SWMM model based on ArcGIS is introduced.Then referring to D8 method,a calculation method is proposed to evaluate waterlogging risk level of subcatchment based on SWMM and ArcGIS.The main steps are as follows:Firstly,calculating the ponding volume(Q1、Q2、Q3)in different ponding depth(0.15m,0.30m,0.50m)of each subcatchment.Then,calculating the total node overflow in each subcatchment over time and comparing it with Q1、Q2、Q3 to determine the level of ponding depth.Finally,determine the waterlogging risk level of each subcatchment according to the ponding depth and duration.Then a multiobjective optimization model is built where the cost of grey and green instructure(E),runoff coefficient(φ)and the total volume of waterlogged water(Q)are taken as the objective function.Decision variables are green instructure’s area and storage facilities’ volume,the maximum area of different underlying surface in each subcatchment and the hydraulic conditions limited by the specification are taken as the constraint conditions,and the risk levels serve as penalty factors.Visual C++is used to write NSGA-II code and SWMM interface code.The multi-objective optimization of system with both grey and green infrastructure is realized through the automatic call of SWMM.Torch District of Zhongshan City is used as the study area,and stormwater model of the studied area is built.And then combined with the analysis of waterlogging,the green infrastructure combination schemes of different land types and the layout location of storage facilities are preliminarily selected,which is the basis of multioptimization.According to the remote sensing satellite images,there are nine types of land use in the study area.Green Infrastructure are selected from the following five typical LID(Low Impact Development)measures:Green Roof,Rain Barrel,Rain Garden,Vegetative Swale and Permeable Pavement.Undeveloped land can be set with two types of green infrastructure,road and transportation facilities land and green land and squares can be set with three types,public land can be set with five types.And four types of green infrastructure can be set for the rest.Finally,on the basis of stormwater model and multi-objective optimization model,it’s studied that the change law of grey and green infrastructure’s performance with the scale and multi-objective optimization of system with both grey and green infrastructure under different rainfall conditions.The research shows that the total node overflow and runoff coefficient reduced by the unit cost green infrastructure are constant,which increases with the increase of rainfall return period.There is a obvious turning point in the relationship curve between grey infrastructure scale and the total amount of overflow,and the designing scale should be less than that of the turing point.What’s more,the runoff reduction of unit cost grey infrastructure is much lager than that of green infrastructure.NSGA-II algorithm can converge stably in this study.There is a synergistic effect with grey and green infrastructure,so grey infrastructure and green infrastructure should be considered into one modle when studying system with both of them.An evaluation system is established to evaluating designing scheme,and the objective function serves as evaluating indicators.Entropy weight analysis is adopted to calculate the weight of evaluation index,then TOPSIS analysis is used to comprehensively evaluate the multi-objective optimization results.Under rainfall corresponding to criterion of 80%annual runoff control rate,the weight of runoff coefficient is the largest.Under P=3a rainfall conditon,the weight of total cost of grey and green infrastructure is the largest,while under P=20A rainfall conditon,the weight of overflow volume is the largest.The study shows that the results of multi objective optimization and comprehensive evaluation is obviously relevant to rainfall condition,so in practical projects,optimization should be carried out under rainfall condition where grey and green infrastructure are effective,and other conditions should be checked under the final optimization scheme.