Based upon large-scale model tests conducted at Nanjing Hydraulic Research Institute, China, the surface erosion and intermittent mass failure along a dam’s axis and the formation of headcut and its migration in the longitudinal section were determined as the key breaching mechanisms for a cohesive dam due to overtopping. In this work, based on the breach mechanism, a numerical model has been developed to simulate the overtopping process of cohesive dams. A comparison among the performances of three large-scale model tests shows that the proposed model exhibits the best overall performance among the three selected physically based dam breach models. The sensitivity studies show that the three models are all sensitive to soil erodibility, although overall the proposed model and Windows Dam Analysis Modules (WinDAM) B are more sensitive than the National Weather Service (NWS) BREACH model. In addition, it is demonstrated that the proposed model performs better and provides more detailed results than the three selected parametric models.