[Objective] The aim of this study is to clarify the mechanism of waterlogging under heavy rainfall in the Lianhua Bridge area，analyze the operational characteristics of the drainage system and the causes of water accumulation，and develop a hindcast simulation model providing a quantitative basis for addressing drainage issues in the bridge area and other similar sunken bridge areas. [Methods] A coupled drainage network-river model was constructed using the MIKE+ platform，including the combined sewer network，stormwater system，and downstream river channels. The model was validated against measured river discharge and water depth data. Key hydraulic indicators，such as pipe fullness，node overflow，and river water levels，were analyzed. The influence of river backwater and backflow on network operation was assessed by combining river outfall discharges with river water level hydrographs. The formation mechanism of waterlogging was examined using node overflow hydrographs. [Results] （1） Simulated river discharge and water depth matched the measured data，with key evaluation indicators within reasonable ranges. Pipe fullness in the combined sewer system generally exceeded 0.8 during peak rainfall，with some sections under full-pipe or surcharged conditions. （2） No node overflow occurred in the stormwater system； overflow was confined to manholes of the DN1800 combined main pipe. （3） Upstream interception facilities diverted large volumes of rainwater into the combined main pipe，increasing hydraulic load，while rising downstream river levels caused reverse flow at the outfalls. The superposition of these factors triggered manhole overflow. （4） Maximum intercepted discharge of the four outfall interception facilities reached 2.41 m³/s； river backflow through Outfall E reached 1.80 m³/s. （5） Total inflow during node overflow was 3.11×10⁴ m³，with a maximum overflow depth of 0.41 m； 91.37% of inflow came from upstream interception，and 8.63% from river backflow. [Conclusions] This study identifies a coupled hazard mechanism where high-load upstream interception and downstream river backwater jointly trigger manhole surcharge. By quantifying the contribution proportions of these key factors，this research explains the phenomenon where waterlogging occurs even when rainfall intensity remains below the design standards of the stormwater system. The findings reveal that water accumulation is primarily triggered by manhole surcharge on the combined main pipe rather than insufficient capacity of the stormwater system. This study provides a scientific basis for outfall regulation，interception pipe optimization，and drainage retrofitting in similar concave overpasses.