Abstract: Overtopping failure of concrete-face rockfill dams involves a complex water-soil-structure coupling mechanism. Through flume model experiments and based on the theories and methods of water-sand dynamics, soil mechanics, and material mechanics, this study investigates the development process of the dam body's breach in both the longitudinal and slope surface directions during the overtopping failure of concrete-face rockfill dams. It also reveals the evolution mechanisms of breach morphology and breach hydrograph. In general, the overtopping failure of a concrete-face rockfill dam is a coupled process that includes continuous erosion of rockfill materials, intermittent instability of the breach slope, and multiple fractures of the suspended face slabs. In the longitudinal direction, under the action of overtopping flow, the breach development in the rockfill materials mainly undergoes tail cutting of the breach, head-cut retrogressive erosion, side slope collapse, full cross-section erosion, upstream erosion and downstream deposition, and river bed stabilization, accompanied by intermittent fractures of the face slabs. In the cross-sectional direction, the breach process is primarily characterized by retrogressive erosion, lateral erosion, and instability of the breach side slope, ultimately resulting in a splayed breach. The intermittent fractures of the suspended face slabs, caused by the continuous expansion of the breach size, lead to the multi-peak characteristics of the breach hydrograph, with the peak values serving as important indicators for assessing the primary and secondary degrees of failure.