Breaching of the reservoir dams would pose great threats to the lives and property of people downstream. In China, more than 85% of failed dams are homogeneous cohesive earthen dams, meanwhile, over 50% of the failure caused by overtopping-induced breach. Therefore, it has great significance to deeply study the overtopping breach mechanism of the homogeneous cohesive earthen dam, so as to improve the prediction accuracy of the breach hydrograph, and to provide theoretical and technical support to the emergency rescue following dam breach. The mechanism of the breach evolution in the three-dimensional space for homogeneous cohesive dam due to overtopping failure was studied according to the large scale model tests, after which a numerical model for overtopping-induced dam breach is developed. The forming position of the headcut is determined by the dam configuration and characteristics of the overtopping flow, and the breach flow discharge is calculated using the broad-crested weir formula. Then a backward erosion formula which considers the physical and mechanical properties of the dam materials is adopted to simulate the migration of headcut, while the mechanical analysis is utilized to judge the collapse of the dam body upstream of the headcut. The erosion coefficient of dam material is introduced, and the erosion rate equation of dam material is established by analyzing the flow shear stress and the critical shear stress of dam material. The erosion rate equation is then used to simulate the breach development of dam crest and downstream slope. In addition, the limited equilibrium method is adopted to model the instability of the side slope of breach, and the sliding surface is assumed to be a plane. Moreover, the model can consider incomplete and base erosion, as well as one- and two- sided breach. Three large scale model tests for homogeneous cohesive dam due to overtopping failure with detailed measured data are chosen to validate the proposed model. The comparison of measured and calculated data indicated that the relative errors of peak breach flow, final breach average width, and time to peak are all within ±25%. In addition, the calculated breach hydrograph is in accordance with the measured one. In summary, the case studies verify the rationality of the proposed model.
