Flood responses of sub-regional river networks to basin-scale flood regulation: a case study of the Hangjiahu Plain, China

Introduction Basin-scale flood regulation can alter external hydraulic boundaries, thereby affecting flood propagation and drainage safety in connected sub-regions. Previous studies have mainly focused on basin-scale water levels and regulation efficiency, whereas the mechanisms by which rule changes reshape sub-regional flood responses via hydraulic boundaries remain insufficiently quantified. Methods To address this gap, this study employs a one-dimensional hydrodynamic model to investigate the effects of the 2011 and 2023 basin-scale flood regulation rules on flood responses in the Hangjiahu Plain, a typical river-network region in the southern Taihu Basin, China. Northward drainage of this plain is hydraulically constrained by Taihu Lake and the Taipu River, and the 2023 revision further tightens boundary drainage pressures by raising the external operating water levels. Three representative Meiyu and three typhoon rainfall events are considered to compare flood responses under different combinations of rainfall and regulation rules. Results The results show that the 2023 rules increase peak water levels by up to 0.12 m and extend the duration exceeding safe water levels by approximately 25%, with more pronounced impacts during Meiyu rainfall events. These effects are primarily caused by changes in hydraulic boundary conditions of the plain, which reduce northward drainage capacity, induce backflow, and ultimately redistribute flood pressure among drainage pathways. Discussion For mitigation, the lakeside pumping scheme effectively reduces local flood levels but may exacerbate backflow effects. In contrast, full closure of the Taipu River openings can improve northward drainage capacity, though its implementation faces cross-basin coordination challenges. Polder pre-drainage also offers mitigation potential but depends strongly on forecasting accuracy. These findings highlight the importance of coordinating basin-scale regulation with sub-regional flood management and provide practical insights for sustainable flood control in complex river network systems.