Understanding cross-regional mutual influence mechanisms driven by polder drainage capacity changes in plain river network areas

Demonstration Zone of Green and Integrated Ecological Development of the Yangtze River Delta, a plain river network area. Intensified polder construction and drainage capacity modifications have caused cross-regional flood risk transfer, with underexplored quantitative mechanisms. This study aims to quantify cross-regional flood interaction, develop a sensitivity assessment tool, and establish a coordinated flood management framework via an integrated hydrological-hydrodynamic model coupled with Drainage Response Index (defined as the water level gradient per unit drainage capacity change). Through scenario simulations of 8 drainage schemes and 3 pumping operations under a 100-year design rainstorm, the results reveal that: (1) Enhanced drainage stabilizes internal polder water levels (Jiashan reduced by 64.6 cm under S8) but elevates external river levels by 8.1~22.3 cm, transferring flood risks outward. Meanwhile, pump scheduling induced 24.3 cm internal fluctuations. (2) Flood propagation exhibits distance-decay characteristics: Wujiang’s drainage elevated downstream Jiashan’s peak by 14.2 cm and Qingpu’s by 8.1 cm, while Qingpu’s drainage impacts upstream Wujiang by only 2.7 cm. (3) DRI slopes were negative, Jiashan to Wujiang DRI dropped from 0.1336 to 0.0010, indicating that adjacent water level is most sensitive to the current drainage condition of a polder. Findings support optimized multi-jurisdictional flood coordination.