## 1 Introduction  
The high-energy debris flow generated after tailings dam failure poses a major safety threat to downstream residents and facilities, and tailings dam failure will cause serious environmental pollution [1; 2; 3; 4; 5]. Among the main causes of tailings dam failure, overtopping dam break is difficult to prevent [6]. Extreme climate combined with the failure of spill-way and other drainage facilities will lead to the rapid rise of reservoir water levels and overtopping, and the corresponding risk assessment and emergency operation response are important links in the feasibility analysis of related projects [7; 8]. Under the action of overtopping flow, the tailings dam will be eroded and gradually develop into dam failure [9; 10; 11]. Due to the unpredictability of extreme climate and other influencing factors, it is necessary to analyze the impact of overtopping failure on newly built or expanded tailings reservoirs [12].  
In engineering practice, two main methods are usually used to analyze the influence of overtopping dam break on the downstream, which are physical model tests and numerical modeling calculation. The reduced-scale physical model test method is intuitive and can consider the complex spatial characteristics of the site, which has certain advantages in the project decision analysis. Good similarity between overtopping dam-break model and prototype is the premise of the physical model test, and reasonable selection of tailings dam model material is the key to ensure the similarity of the tailings dam-break process [13]. In practice, the dam overtopping model test often uses the same soil or even the same compaction state as the prototype. The previous studies of the cohesion of sediment particles shows that the selection of the model scale of overtopping dam failure may affect or change the movement form of sediment in the process of breaking, so it is recommended to use a large-scale ratio as far as possible to design the model [14; 15; 16]. In the normal gravity physical model test of cohesionless soil, the use of model sand with small internal friction angle can improve the similarity between the mechanical properties of the material and the prototype, but it is very difficult to achieve complete similarity [17].  
In the overtopping model test, the scouring process of the reduced-scale model must be similar to that of the prototype, so the similarity of the model sand is an important criterion for the model design [18]. The similarity of sediment incipient motion under the action of gravity flow is an important condition for determining the simulated sand, so the analysis of sediment incipient motion state is the key to the similarity analysis of the overtopping dam-break model. However, due to the complexity of the real environment of the riverbed, the stress of different tailings particles may be different because of the geometric shape, size, volume, weight and other factors. It is quite challenging to simulate the incipient state under the action of similar water flows in model tests. In the study by [PERSON] [18], recommendations were given for the selection of tailings dam materials in model tests based on the similarity analysis of the critical incipient state of the material. Furthermore, the optimization of the inception similarity was validated through chute inception tests. In order to better explore the model test technology of tailings dam break due to overtopping flow, this paper designs and carries out the numerical test of a prototype and model based on the principle of inception similarity optimization, and the results further show that inception similarity optimization can make the test technology of tailings dam break closer to prototype reality.