Study On Control Effects And Comprehensive Benefits Of Low-impact Development Facilities

At present,many cities in China have serious urban waterlogging and non-point source pollution disasters.The effects of low impact development(LID)facilities in different scenarios were simulated and evaluated.It has important theoretical and practical value to solve the environmental problems of flood and water pollution in Chinese cities.In this study,multiple evaluation indexes of LID facilities in environmental,economic and social benefits were selected to establish an evaluation index system for layout optimization.Taking Fengxi New City in Xixian New Area as the research area,the effects of water quality and water quantity regulation under different rainfall conditions and different layout schemes were simulated by combining the actual monitoring situation and model simulation method in the research area.Based on the established evaluation system,the monetization calculation method was used for the comprehensive benefit analysis and the optimal layout plan was sought.The main conclusions are as follows:(1)A total of 14 indicators reflecting the comprehensive effect of sponge cities were determined by the method of comprehensive analysis and Delphi method.The indicators include "environmental benefits,social benefits" and "economic benefits".It included"environmental benefit","social benefit" and "economic benefit".According to the Analytic Hierarchy Process(AHP),the target layer,criterion layer and index layer of the sponge city evaluation index system were determined.By constructing judgment matrix and checking the consistency,the weight of each index was clarified.The relative weight of comprehensive benefits of LID facilities was "environmental benefits">"economic benefits">"social benefits".Among them,the weight value of total runoff reduction rate,number of overflow points reduction,overflow flow reduction and utilization rate of rainwater resources accounted for a large proportion.On the basis of the construction of the benefit evaluation index system,a method of monetizing the construction benefit of sponge city was constructed by using the methods of market value,shadow price and alternative market.(2)Through monitoring and sampling analysis of multiple rainfall events,the pollution intensity and characteristics of rainfall runoff in different functional areas were analyzed.The range of SS,TN,TP,COD and other pollutant indexes was relatively large,and the variation coefficient of SS was 0.89,which was the largest difference of measured values among these indexes.The average concentrations of COD and TN pollutants were higher than the standard for Class V water,and the average concentrations of TP and NH3-N meets the standard for Class ? water.Based on the analysis of different functional areas,the water quality of the drainage outlet of the general monitoring point(the drainage outlet of Y23)was the worst,followed by the road and commercial area,and the residential area and cultural parish were the best.The results of the analysis on the pollution degree of different functional areas of each water quality index were as follows:the concentration of TN and COD:Y23 total outlet>traffic area>commercial area>cultural education area>residential area;the concentration of TP:Y23 total outlet>traffic area>commercial area>residential area>cultural education area;the concentration of NH3-N:Y23 total outlet>traffic area>commercial area>residential area>cultural education area;the concentration of SS:traffic area>Y23 total area>business district>residential district>cultural education area;the concentration of NO3-N:commercial area>traffic area>residential district>cultural education>Y23 total outlet(3)The effects of non-point source pollution and waterlogging reduction in different sponge city facilities were studied by model simulation.In different rainfall recurrence periods,the low-impact development mode(12%LID)reduced the number of overflow nodes,overflow flow,overloaded pipe sections,total runoff and pollution load compared with the traditional development mode.However,with the increase of the recurrence period,the effect of cutting is gradually weakened.Under the area ratio of 2%,4%,6%,8%,10%and 12%,the reduction effect of each indicator is improved with the increase of area ratio.The reduction rate of the number of overflow points ranged from 33.14%to 90.99%,the reduction rate of the overflow flow from 39.47%to 95.12%,the reduction rate of the total amount of runoff from 31.82%to 88.74%,the reduction rate of the overloaded pipe segments from 5.56%to 36.15%,and the load reduction rates of SS,COD,TN and TP were 28.54%to 85.42%,27.31%to 85.13%,28.76%to 85.26%and 29.87%to 87.23%,respectively(4)Through the method of monetization,it was shown that with the increase of area ratio,the benefit value of each index was improved.The annual benefit values(unit:ten thousand yuan)of total runoff reduction rate,number of overflow points reduction rate,overflow flow reduction rate,SS reduction rate,COD reduction rate,TN reduction rate and TP reduction rate were 120.56-336.22,125.56-344.73,149.52-360.37,21.048-136.95,49.82-149.11,10.07-31.39,1.24-3.68 and 0.027-0.080,respectively.In the economic benefit,the construction cost was from 21243.6-1274616 thousand yuan,and the maintenance cost was from 118448-7094688 thousand yuan.The annual benefits of rainwater regeneration rate and pipe network leakage control rate were 55,388-332,226 yuan and 436,250-2,617,500 yuan,respectively.The comprehensive benefit value was obtained by the weighted sum of the benefit value after the monetization of the environmental,economic and social indicators.The results showed that the variation range of the comprehensive benefit value gradually decreases with the increase of the area ratio of LID installation.When the area ratio was 10%,the curve of comprehensive benefit appeared inflection point.Therefore,the 10%area ratio was the optimal proportion of LID installation in this area.