Optimization of the integrated green–gray–blue system to deal with urban flood under multi-objective decision-making

The integrated green-gray-blue (IGGB) system is considered to be a new way of stormwater management, and a comprehensive evaluation of the green-gray-blue infrastructure layout mode under different return periods is the key to the implementation decision-making of stormwater management. In this study, a blue-green synergism evaluation model is established to optimize the layout of blue-green infrastructure. An evaluation framework combining the evaluation indicator system and the hydrology model is established. Stormwater storage, peak flow reduction, and life cycle cost are selected as evaluation indicators. On this basis, seven optimal scenarios, including green, blue, gray, green-blue, green-gray, blue-gray, and green-gray-blue, are established. The Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) method is used to analyze these seven scenarios under different return periods. The results indicate that (1) when the drainage infrastructures are arranged in combination, the peak flow reduction is significantly improved compared to that of a single drainage. (2) TOPSIS results show that green-gray and blue-gray perform better when the cost weight is 0-0.35, and green-gray-blue performs best when the cost weight is 0.35-1. (3) The integrated green-gray-blue system has obvious synergistic effects. This study can provide support for planning department workers for the urban stormwater management strategy.