| Climate change is changing the rainfall characteristics of cities,with gradual extremes in the frequency and magnitude of rainfall.This causes significant changes in the hydrological and water quality characteristics of urban rainfall and runoff.Urban stormwater management system has also been seriously affected,leading to waterlogging and non-point source pollution.In urban stormwater management systems,grey and green stormwater facilities in sponge cities have complementary functions and can effectively mitigate urban flooding and non-point source pollution.However,the response of urban stormwater management systems to climate change will be more complex under the uncertainty of future climate change,and it is unknown whether the grey-green stormwater facilities can be resilient enough to cope with the future flood risk and non-point source pollution.Therefore,it is important to select adaptive grey-green stormwater facility under future scenarios to improve the resilience of urban stormwater systems.This study takes a typical area in the Xiaozhai district,Xi’an,Shaanxi Province(total area is 20.15 km2)as the research area.The research was conducted through field monitoring,model simulation,environmental economics methods,optimization algorithms and adaptive decisionmaking.The historical and future urban rainfall change patterns were analyzed.A decisionmaking index system for grey-green stormwater facility layout was established for historical and future scenarios.A decision model for optimizing the layout of grey-green stormwater facilities under historical and future scenarios was developed.This study provides a reference for solving the problem of the multi-objective layout of grey-green stormwater facilities in sponge cities under uncertainty conditions.The main research results were as follows:(1)Through the downscaling analysis and bias correction of the rainfall data from different scenarios in the Coupled Model Intercomparison Project Phase Sixth(CMIP6),historical and future rainfall data and their rainfall characteristics were obtained and analyzed for the study area.The simulation accuracy of CMIP6 for historical rainfall was high(R2>0.57),which can accurately reflect the historical rainfall evolution characteristics of the study area,and was applicable to the analysis of future rainfall data for scenarios SSP1-2.6,SSP2-4.5,and SSP5-8.5.The rainfall concentrations of future rainfall were mainly distributed between 10-20,which had obvious seasonal variation during the year and was more evenly distributed than historical rainfall.Compared to historical annual cumulative rainfall,the annual cumulative rainfall in the near future(2023-2050)is increased 13.60 mm,the annual cumulative rainfall in the medium future(2051-2080)is increased 20.20 mm,and annual cumulative rainfall in the far future(2081-2100)is increased 37.66 mm.From the extreme rainfall indicators,it can be seen that the future annual cumulative rainfall,total rainstorm,rainstorm days,and daily maximum rainfall had increased significantly.The increase in heavy rainfall days was not obvious,while the rate of heavy rainfall decreased significantly.(2)Considering water ecology,water safety,social and cost aspects,a hierarchical analysis was used to establish a decision-making index system for grey-green stormwater facility layout.The 1D-2D MIKE model of urban stormwater and non-point source pollution driven by design rainfall duration curves was constructed.And the characteristics of urban stormwater and nonpoint source pollution based on traditional development mode were analyzed.Under the traditional strategy,with the increase of rainfall return period,the historical surface runoff shows an increasing trend,which increases the amount of stormwater entering the pipe network.The 87.79-99.67%of the pipe sections were overloaded,which further leads to the 50-88.52%of the nodes overflow.The COD and SS pollution loads discharged by the pipe network were high.The location of flood risk in the study area was mainly concentrated in the lower central and western regions.The flood areas increase from 1.57 km2 to 4.83 km2 with the increase in the return period.Compared with the historical scenario,the urban catchment runoff under the climate change scenario increased by 10.70-20.70%,the overload pipe section increased by 1.58-5.85%,the overflow node increased by 24.61-46.60%,and the discharge emission flow increased by 16.5728.85%.The peak values of discharge SS and COD pollution load showed a fluctuating increase trend.With the increase in rainfall return periods,the medium and high flood risk areas in the future were increased.(3)Based on the decision-making index system for grey-green stormwater facility layout,a grey-green stormwater facility layout optimization model was established and the optimized layout of grey-green stormwater facilities was analyzed.Under the grey-green strategy,there was a synergistic relationship between the maximum total benefit and the maximum safety benefit objectives of the grey-green stormwater facilities in the historical and future.There was a clear competitive relationship between the maximum total benefit objective(or the maximum safety benefit objective)and the minimum cost objective.The more suitable layout ratios of grey-green stormwater facilities were obtained by weighing the relationship between benefits and cost objectives.The layout ratios of grey-green stormwater facilities were weighed against each other,making it better for overflow and pollutant reduction.Furthermore,the medium risk areas are reduced by 0.05 km2,0.39 km2,0.41 km2 and 0.38 km2 for the grey-green stormwater facility layout in the historical,SSP1-2.6,SSP2-4.5 and SSP5-8.5 scenarios respectively.The high-risk areas are reduced by 0.78 km2,1.36 km2,1.33 km2 and 1.4 km2 for the grey-green stormwater facility layout in the historical,SSP1-2.6,SSP2-4.5 and SSP5-8.5 scenarios respectively.The optimized grey-green stormwater facilities increase the resilience of the stormwater system by 54.27-57.80%,increasing the resilience of the stormwater system to 0.72-0.74 in the future scenarios.The grey-green stormwater facility strategy effectively controls rainfall runoff and floods and improves the resilience of stormwater systems to climate change.(4)Considering the uncertainty of future climate change,an adaptive optimization model for the layout of grey-green stormwater facilities was established,and an adaptive optimization layout of the grey-green stormwater facilities for future scenarios was developed.There were significant differences in the sensitivities of different objectives to the uncertain factors.The cost objective was not obviously sensitive to the change in rainfall.The benefit objective was not only affected by the change in rainfall but also by the coupled coordination mechanism of layout ratio of grey-green stormwater facilities.Adaptation scenarios obtained by considering climate change and uncertainty in the layout of grey-green stormwater facilities can show a high degree of adaptability to a wide range of climate changes and facility layouts.The difference in flood risk reduction between the adaptive and optimised solutions is not significant.The resilience of the adaptive solution increases by 54.27-57.79%,increasing the resilience to 0.72-0.74.The optimal layout ratio of grey-green stormwater facilities obtained through the adaptive optimization model can show strong adaptability to the uncertainty of future rainfall.And it can better reduce the risks of urban flooding and control non-point source pollution,which is of great significance to guide the actual layout of grey-green stormwater facilities. |
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