Abstract: The centrifugal model test system developed by the authors is employed to study the breaching process of a clay core dam due to overtopping. It is found that there are significant differences in failure mechanisms and the breaching processes between clay core dams and homogenous dams. With increasing erosion by overtopping flow downstream of the clay core dam, shear failure occurs and flood flow through the breach increases rapidly abruptly. Using the experimental result, a numerical model is developed to simulate the breaching process of clay core dams due to overtopping. The numerical solution of the model is discussed. In the model, an empirical formula derived from steep channel experiments with different soils is given. The formula can be used to calculate the erosion rate downstream of the clay core dam during the overtopping events. The mathematical relationship between the degree of erosion and shear damage is also obtained. The broad-crested weir formula is employed to calculate the dam-breaking flow. The proposed numerical model is validated by comparing the numerical simulation with the experimental data.