Research On Network Optimization And Low Impact Development Simulation Based On Improved NSGA-Ⅱ Algorithm

In recent years,with the gradual development of our social economy and accelerating process of urbanization,the laying of oil cyper road leads to the change of the underlying surface,which leads to heat island effect,leading to the increase of extreme precipitation events;The change of urban underlying surface hinders the infiltration of surface runoff and makes urban low-lying areas more prone to waterlogging,which brings a lot of inconvenience to people’s life.Secondly,the aging and single stormwater pipe network system has been unable to meet the needs of urban flood discharge and drainage,urban stormwater pipe network optimization and reconstruction and corresponding stormwater reduction measures have become urgent problems to be solved.Therefore,it is of great significance to explore the problems existing in the drainage system and optimize the design for the construction of sponge city and the maintenance of ecological stability.Taking A research area in A City of Yunnan Province as the research object,this paper simulated the surface runoff,pipeline bearing capacity,and overflow of inspection Wells of the drainage system in the region under different rainstorm recurrence periods,analyzed the filling degree of the main pipe and water logging law of the region,fully considered the actual drainage capacity of the pipe network,and identified the weak links in the operation of the pipe network.The intelligent evolution algorithm was used to establish a mathematical optimization model for the multi-objective optimization design of the pipe network,and the low-impact development measures were combined based on the sponge city theory.The same algorithm was used for the design,with the purpose of saving investment and greatly reducing surface runoff.So as to meet the city’s needs for drainage and waterlogging prevention.The main research work of this paper is as follows:1.According to the local survey data report,simplify the layout of the rainwater pipe network,organize and import the data,and establish the SWMM rainwater model.The study area was divided into multiple subcatchment areas,and the Chicago rain simulator was used to output rainfall process lines under different recurrence periods(P=1a,3a,5a,10 a and 20a),which were successively written into the model.Determine the corresponding parameters and determine the comprehensive runoff coefficient;The dynamic simulation and analysis of pipe network operation are carried out by using a variety of visual modules installed in SWMM,and the surface runoff continuity error and flow calculation continuity error are output,so as to prepare for the comparative analysis of the results before and after optimization of the rainwater pipe network2.Aiming at the problems of NSGA-Ⅱ algorithm in solving multi-objective problems,such as too many constraints,low computational efficiency,and easy to fall into local optimum,this paper proposes an improved NSGA-Ⅱ algorithm by introducing difference mechanism and elite strategy,which aims to enhance the global searching ability of individuals.The results show that the proposed algorithm improves the convergence speed and the uniformity of the optimal solution distribution by selecting three single peak functions and ZDT series test functions.In the optimization of pipe network,the pipe diameter and the elevation of the bottom of the inspection well were taken as decision variables,the objective function of water-saving capacity and economic indicators was set,and the pipe diameter,slope,flow rate and buried depth were taken as constraint conditions.Then,the improved NSGA-Ⅱ algorithm was used for optimization,and the Pareto solution set in line with the actual project was obtained.In order to test the optimal scheme,parameter N was set as 30,50,100 and 200,and optimization was conducted successively.The final optimization cost was 2.56 million yuan,which provided technical support for practical applications.3.In terms of urban rainstorm and flood management,based on the concept of sponge city,this paper chooses the combination design of five measures,including green roof,grass planting shallow ditch,rain garden,permeable pavement,and sunken green space,according to the topographic characteristics of the study area,analyzes the topographic characteristics and area suitable for construction of each measure in the study area,and takes the minimum cost of LID measures and the minimum total runoff as the objective function.Taking the laying area of five different LID measures in the subcatchment area as the decision variable,a multi-objective optimization model was established.The improved NSGA-Ⅱ algorithm was used to solve the model.The results show that LID measures have good effects on reducing surface runoff and improving rainwater infiltration rate.Therefore,three cost-effective schemes are selected for analysis,and the runoff reduction rates of the three schemes are respectively 44.3%,54.4% and 59%.Finally,the scheme evaluation system is constructed and the fuzzy evaluation method is used to evaluate the scheme.Four different weight combinations were constructed,and the analysis showed that scheme 3>Scheme 1> Scheme 2.