Classification Index System for Use of Floodplains and Polders in Middle and Lower Reaches of Yangtze River

[Objective] Floodplains and polders in the middle and lower reaches of the Yangtze River are an important part of the Yangtze River flood control system, which plays a key role in expanding the flood storage capacity of the river and ensuring the flood control safety in key areas. However, difficulties in decision-making on activation and timing of operation have become prominent issues in their operational management, making them a weak link in the basin’s flood control system. This study aims to establish a scientific and systematic classification index system for the use of floodplains and polders, in order to refine scheduling and coordinate the overall basin flood control safety with socio-economic development within floodplains and polder areas. [Methods] This study took the floodplains and polders in the middle and lower reaches of the Yangtze River as the research object, and innovatively constructed a hierarchical classification index system consisting of three dimensions: flood control capacity, planning attributes, and socio-economic impact. The index system included eight specific indicators, covering aspects such as levee height deficiency, relationship with urban master planning, relationship with floodplain operation planning, permanent population, key protected areas, cultivated land area, and GDP. Based on the principle of sectional classification, the middle and lower reaches of the Yangtze River were divided into six river sections: Jingjiang, Chenglingji, Wuhan, Hukou, Hukou-Datong, and downstream of Datong. Differentiated weights were assigned to the upstream and downstream sections of Datong. By constructing a decision matrix and combining standardization of qualitative indicators based on classification with linear normalization of quantitative indicators, the comprehensive scores of floodplains and polders were obtained and divided into four categories (from Class I to Class IV), clarifying their order of activation. [Results] A total of 243 floodplains and polders (excluding those that had been completely decommissioned) along the main stream of the middle and lower reaches of the Yangtze River were evaluated. The classification results showed that there were seven Class I floodplains and polders with a total flood storage capacity of 20 million m3 and relatively low population and GDP; Class II had the largest number, with 154 floodplains and polders providing a total flood storage capacity of 3.81 billion m3; Class III included 65 floodplains and polders, featuring the largest flood storage capacity (4.87 billion m3) and the highest population (about 450 000 people); and Class IV comprised 17 floodplains and polders, with a total GDP of 10.83 billion CNY. Correlation analysis indicated that “levee height deficiency” had the greatest impact on classification (correlation coefficient =-0.7), followed by “permanent population” and “GDP” (both=-0.6), while “cultivated land area” had the least influence. Taking the Chenglingji section as an example, the classification results were highly consistent with the actual flood control requirements and the socio-economic characteristics of the area. Further verification through a 1954-type flood regulation simulation showed that, compared with the baseline scheme, the classified regulation scheme reduced excess flood volume by approximately 0.55 billion m3 in the vicinity of Chenglingji, and significantly improved the overall flood control effectiveness. [Conclusion] The classification index system for the use of floodplains and polders proposed in this study has strong scientific and practical value. Its innovation lies in the systematic integration of planning attributes, socio-economic impacts, and flood control capacity, achieving a conceptual shift from a “single flood-control orientation” to a “flood control-development coordination” approach. The proposed index system provides a scientific basis for the “sectional control and hierarchical scheduling” of floodplains and polders in the middle and lower reaches of the Yangtze River. It supports the optimization of flood regulation schemes and real-time decision-making during flood control consultations, offering important practical significance for improving the overall flood regulation and scheduling capacity of the Yangtze River Basin.