Research On The Optimization Design Method Of Separated Urban Drainage System Under Uncertainty

There are many important choices to be made in terms ofparameter uncertainties,such as scale,structure,layout,and technology,andinfluencing input uncertainties,including climate change and urban development,when designing an urban drainage system.Given these uncertainties,traditional design methods struggle to find optimal solutions that take into consideration things like economic cost,resource utilization,and environmental impact.Therefore,this study aims to design and optimize a sustainable separate urban drainage system that takes into consideration not only mid-andlong-term issues,like climate change,urbanization,and technological advances,but also short-term perturbation,such as rainfall.Taking both input and parameter uncertainties as its starting point,this study proposes a five-step method for separate urban drainage system design: problem identification;data collection and quantitative projection of input uncertainties;system design and multi-objective optimization on baseline scenario;system performance evaluation under uncertain conditions;and alternatives screening and optimal solution recommendation.Of the above procedures,the third and fourth steps are most crucial.The aim of the third step is to solve the problem caused by diverse alternatives with multiple objectives,so system design parameter optimization is executed on the basis of a comprehensive model that is developed by integrating random sampling,graph theory,and genetic algorithms.The calculations include generating the layouts and sizing the pipelines for both sewer and stormwater systems,designing runoff detention facilities in stormwater system,determining the locations and scales of the wastewater treatment and reclaim plants as well as their corresponding service regions in sewer system.In this step,the design input is a settled baseline,consisting of the served population,domestic water consumption,rainfall conditions,and other necessary information.The five optimization objectives are the economic index,represented by the system life cycle cost;the environmental index,represented by the annual emission load;the resource index,represented by the proportion ofreclaimed water supply and demand;the vulnerability index,represented by the floodingvolume under different rainfall;and the adaptation index,represented by the number and duration of flooded nodesunder different rainfall.At the end of this step,a set of optimized alternatives with different features and irreplaceable performances is provided.The aim of the fourth step is to evaluate the performance of the optimized alternatives in a changing environment,with consideration given to uncertainties such as changing precipitation and domestic water usage caused by climate change,changing population growth and land use caused by urbanization,and changing water conservation through the application of advances in technology.Scenario analysis and the multi-indicator evaluation method are used,with evaluations covering drainage hydraulics,emission load,and inundation characteristics.Ultimately,alternatives with the best performance in both baseline and uncertain scenarios are selected and recommended.Thisfive-step method and design model is applied to the northern district of Kunming city in southwest China as a case study.An optimal solution set,consisting of 24 alternatives for the sewer system and two alternatives for the stormwater system,is proposed for the area,showing the economic,environmental,resource,invulnerability,and adaptive advantages compared to the existing system.Furthermore,the “design input—structural configuration—system performance” relationship is analyzedbased on the results,and the laws for determining the optimum structure and functions of the urban drainage system under various uncertain conditions are identified as well.The feasibility and practicability of the proposed method and model are demonstratedthrough the case study.