Unusual rainfall is the primary cause of the failure of the tailing dams, and overtopping is the most representative model of the tailing dam failure. The upstream tailing dam was selected as the research object to study the whole process of breach extension and the overtopping dam-failure mechanism under the full-scale rainfall condition. The results showed that the significant size grading phenomenon in the front, middle, and end of the tailing pond was obvious due to the flow separation effect, and its average particle diameter was D50. At different moments of rainfall, the height of the infiltration line at different positions of the dam body was different; at the rainfall of 3600 s, the height of the infiltration line lagged behind the height of the tailing pond, and this phenomenon from the tail of pond to the outside of the dam slope became more obvious. After the rainfall of 3600 s, the height of the infiltration line lagging behind the water level in the pond basically disappeared, and the rate of infiltration line rise kept pace with the rate of water level. The process of overtopping dam-failure experienced dam overtopping (gully erosion), formation of a multistepped small “scarp,” breach rapid expansion, formation of large “scarp,” and burst (fan-shaped formation). The width and depth of the breach showed a positive correlation, and the widening rate of the breach was 3 to 8 times of the deepening rate, especially in the middle of the dam break, widening behavior occupied the dominant factor. The shape of the dam body after failure was parabolic, and the dam body had obvious elevation changes. These results provide the theoretical guidance and engineering application value for improving the theory and early warning model of the upstream tailing dam.

In China, tailings reservoirs often experience failure events due to inadequate discharge capacity or flood discharge system failures. To ensure the safe and reliable operation of the flood discharge system in a tailings reservoir, this study investigated the discharge capacity of a tailings reservoir in Henan Province, China, through hydraulic model tests and numerical simulations conducted using FLOW-3D. The results indicated that when the water head was below 0.8 m, the flow was in a non-pressurized regime with low discharge capacity. As the water head increased to 0.8–1.2 m, air began to be entrained into the flow, leading to an alternating regime between open-channel flow and full-pipe flow. Once the water head exceeded 1.2 m, the flow expelled air and transitioned into a pressurized full-pipe regime. At a water head of 1.6 m, the flow was in a pressurized full-pipe regime. The experimental flow rate, flow velocity, and flow regime of flood discharge shaft No. 1 closely matched the simulation results. Additionally, the discharge capacity of the shaft increased as the height of the discharge window was raised. The discharge capacity and flow velocity of the discharge windows on the eighth layer were 30.1% and 29.2% higher, respectively, than those on the first layer, which indicated that the system’s drainage capacity increased as the shaft length increased. However, increasing the height of the discharge window increased the likelihood of an alternating regime between open-channel flow and full-pipe flow. During the operation of the tailings reservoir drainage system, it is essential to ensure that the upper-layer discharge windows are functioning while minimizing the occurrence of unfavorable flow regimes. The discharge water head should be controlled to remain below 0.8 m. These operations not only ensure that the discharge capacity meets the requirements but also enable the drainage tunnel to operate safely and effectively over the long term. The findings of this study can provide a scientific basis for the design and safe operation of tailings reservoir drainage systems.