Risk mitigation strategies for flood control in a semi-enclosed bay under large-scale human activities

Large-scale land reclamation and hydraulic engineering have substantially modified hydrodynamic processes in many semi-enclosed bays, increasing flood and drainage risks under extreme weather conditions. Yueqing Bay, a typical semi-enclosed bay in southeastern China, has experienced pronounced geomorphic alteration and hydrodynamic restructuring due to the Xuanmen reclamation project. Under the combined forcing of frequent typhoons, heavy rainfall, astronomical tides, and storm surges, prolonged high water levels and urban waterlogging have become critical challenges. In this study, a hydrodynamic numerical model is developed to investigate the long-term hydrodynamic response of Yueqing Bay to the staged implementation of the Xuanmen Project. Four representative periods (1975, 1993, 2006, and 2017) are simulated to quantify the progressive evolution of flow structures, tidal propagation, and seawater exchange capacity induced by successive enclosure. In addition, multiple gate-operation schemes are designed based on the existing gate system, and their emergency drainage and flood-control performance under extreme weather scenarios is systematically evaluated. The results indicate that reclamation-induced channel constriction significantly alters bay-scale flow patterns and leads to a marked reduction in seawater exchange. Gate regulation plays a critical role in mitigating flood risk, with the fully open gate scheme producing the largest short-term water level reduction and the highest drainage efficiency. Based on spatial variations in hydrodynamic response and risk exposure, a zonal and scenario-oriented gate operation framework is proposed. This study provides quantitative insights into the hydrodynamic impacts of reclamation projects and offers practical guidance for optimizing gate-based emergency drainage management in semi-enclosed bays.