Integrating nature-based solutions for compound flood risk mitigation in China: A case study of Shanghai

Coastal cities are increasingly vulnerable to escalating flood risks due to climate change and socio-economic development, highlighting the urgent need for effective risk mitigation strategies. This study develops a comprehensive modeling framework to assess the effectiveness of hybrid green and grey strategies in reducing future flood risks in Shanghai, a coastal mega-city in China. The framework comprehensively integrates flood risk analysis, adaptation scenarios design, and cost-benefit analysis. The results indicate that without additional adaptation, the Expected Annual Damage (EAD) could reach US$ 0.5 billion/year, representing 0.09 % of Shanghai's 2020 GDP. Under the RCP2.6 scenario, the EAD is projected to increase by US$ 0.62-0.77 billion/year from 2030 to 2100 without timely interventions. The analysis demonstrates that integrating grey and green infrastructure is both effective and economically viable, yielding a positive benefit-cost ratio. The most promising strategy integrates storage tanks, drainage enhancement, deep tunnels, and dry floodproofing, yielding greater economic benefits than individual measures. This study provides valuable insights for long-term flood adaptation planning, particularly in deltaic regions facing climate uncertainty.