A two-dimensional (2D) hydrodynamic model is developed to investigate the dike-break flood at Tuanzhou township alongside the Dongting Lake on July 5, 2024. Integrating both lake and township into a single model, we comprehensively reconstruct the dike-break flood, capturing macroscopic flood processes and detailed flow structures. The model also assesses the flood risk associated with a potential failure of the Qianlianghu-Tuanzhou dike. The model accurately determines when the water levels inside and outside the breach reach equilibrium. It provides precise historical data on breach discharge, water levels inside and outside the breach, flood storage volume, and inundated areas in Tuanzhou township over time. Additionally, the model replicates the water surface scarp around the breach, explaining why water levels near the breach are lower than those in the downstream lake area. The water level and flow velocity distributions around the breach are plotted and analyzed. Moreover, the characteristics of concave water surface outside the breach and the convex water surface inside the breach, together with the water-level gradients are also quantitatively examined. Comparison with field data reveals that the model’s water level error is generally below 10 cm during the dike-break flood (except for the initial breach stage), and drops to less than 5 cm once equilibrium is reached. The discharge error is typically under 5%, with peak discharge error at only 2.5%. The model’s water conservation error is 0.6%, and the discrepancy in maximum flood volume between the model and hydrological department’s results obtained from flood volume versus water level curve is 6.8%. Based on the dike-break flood reconstruction, we design three breaches along the Qianlianghu-Tuanzhou dike and simulate dike-break floods with the existing breach at Tuanzhou township under both blocked and unblocked scenarios. We further quantitatively assess flood risks related to potential dike failures by analyzing the flow field, discharge processes, flood storage, and inundated areas. The findings offer technical support for flood risk assessment and levee protection. The systematic method for simulating real dike-break floods in this study includes integrated modeling of rivers/lakes and townships, detailed township modeling, and iterative calculations to determine breach topography over time. These methods enable accurate simulations of dike-break floods and can serve as a reference for similar studies on dike-break floods.