A multi-objective optimization model for synergistic effect analysis of integrated green-gray-blue drainage system in urban inundation control

The risk of urban inundation has been increasing worldwide. The integrated green-gray-blue (IGGB) drainage system is considered to have great potential in urban areas, as it has not only the resilience and sustainability of green and blue infrastructure, but also the reliability of gray infrastructure on stormwater drainage. This study aims to identify whether there is synergistic effect in IGGB drainage system and how to optimize it. An automatic optimization tool is needed for scenario generation and optimal decision-making because of the diversity, complicacy and decentrality of IGGB drainage system. So far, existing optimization frameworks focus on green infrastructure which cannot be applied to explore synergistic effect of IGGB drainage system. This paper proposed an integrated framework to combine hydrological model and optimization algorithm. Based on this framework, seven optimization strategies including green, gray, blue, green-gray, green-blue, gray-blue and green-gray-blue, were established with the optimization objectives of flood risk reduction rate (FRRR), life cycle cost (LCC) and land occupied (LO). A case study was conducted in Dongying, Shandong province, China. Based on the Pareto fronts of the seven optimization strategies, the average FRRRs of green, gray, blue, green-gray, green-blue, gray-blue and green-gray-blue strategy under the 24-hour rainfall with 30-year return period are 56.0%, 7.7%, 14.5%, 66.1%, 78.3%, 18.4% and 85.5% respectively. The green-gray-blue strategy shows the best performance. Then the existence of synergistic effect in IGGB drainage system is confirmed. The synergistic effect of green-gray, green-blue and green-gray-blue optimization strategies was 5.7%, 9.7% and 10.6%, respectively. However, synergistic effect was not generated in the gray-blue optimization strategy in this study. The synergistic effect is influenced by the hydrological mechanism of synergistic effect and the law of diminishing marginal utility. The results provide more evidence for the outstanding performance of the green-gray-blue integrated urban drainage system and provide technical support and greater clarity to decision-makers on how to integrate the green, gray and blue infrastructure.