Optimization of green and grey infrastructure for performance enhancement of urban drainage system under future conditions.

Flood control has received significant attention in recent years due to the extensive and devastating damage of flooding to communities and economies. Stormwater management and flood control are closely related, both aiming to manage and mitigate the impact of excess water in urban and natural environments. In this study, low impact developments (LIDs) and pipe replacements were chosen as strategies to manage surface runoff and flooding using a multi-objective optimization (MOO) approach. Six scenarios with varying rainfall intensities were created to explore trends and solutions for minimizing surface runoff, flood areas, and construction costs at a study site, with rainfall adjustments made to reflect future rainfall nonstationary conditions. The findings revealed nonlinear increases in both runoff and flood area reductions as investment increased, with distinct trends between runoff volume reduction and flood area reduction across the six scenarios. Considering flood area reduction, a 25-year-24-h storm scenario resulted in the highest benefit-cost ratio and was recommended to use for future design and planning in terms of stormwater management and flood control. Moreover, spatial analysis revealed the effectiveness of clustering LIDs in subcatchments with largely impervious areas, and integrating LIDs with pipe replacement was crucial to control flooding efficiently. These findings could help city planners in drainage system design and urban planning with the consideration of limited budgets and flood control under future conditions.