Multi-stage planning pathways and decentralized blue-green-grey networks for climate-resilient urban flood adaptation

High-density coastal cities face increasing pluvial flooding risk as extreme rainfall intensifies, sea level influences grow, and urban areas continue to densify. Nature-based solutions, including blue-green infrastructure, are widely promoted for stormwater management and the delivery of broader ecosystem services, yet most modeling studies still design these systems for a single, static land use state. As a result, the combined influence of planning sequence and drainage decentralization on the long-term performance and trade-offs of hybrid blue-green-grey infrastructure remains poorly quantified. This study develops an integrated modeling framework to evaluate multifunctional stormwater solutions in a rapidly urbanizing coastal district. Focusing on the Qianwan district in Shenzhen, China, we couple an SWMM-based hydrologic and hydraulic model with a genetic algorithm and multi-criteria decision analysis. Forward and backward multistage planning pathways are compared under several drainage decentralizations. For each pathway, hybrid layouts that combine pipes, permeable pavements, bioretention cells, and blue roofs are optimized and evaluated in terms of life cycle cost, technical and operational reliability, and resilience under extreme rainfall and pipe failure scenarios. Results show that planning direction is as influential as drainage decentralization in shaping long-term adaptation outcomes. Backward planning with decentralized layouts achieves the most robust balance among cost, reliability, and resilience, whereas forward planning provides greater adaptability in the early development stage by deploying more extensive blue-green infrastructure on a lighter grey backbone. Overall, increasing decentralization systematically shortens flow paths, reduces surcharge, and enhances recovery after shocks. The framework demonstrates how integrated modeling can quantify co-benefits and trade-offs of nature-based solutions across development stages and provides transferable decision support for climate-resilient sponge cities and urban adaptation strategies.