## 1 Introduction  
In recent years, mine security accidents have occurred frequently, and the situation of the safe production of a mine has been in the severe condition [1; 2; 3]. In China, currently, there are more than 12,000 tailings impoundments nation-wide, most of which pose a potential hazard [4]. A characteristic common to the majority of tailings dam failures in the mining sector is the liquification of tailings, which results in a high-energy mudflow. When swept downstream, this creates the potential for extensive damage to property [5]. To assess the prospective damage in the case of such a failure, it is necessary to be able to predict the characteristics of the flow and the possible extent of flood movement. However, due to the differences in mineral processing techniques, the particle size and the distribution of tailings utilized during the construction of tailings impoundments are unequal; accordingly, there is an ostensible difference in the particle size of the resultant mudflow after the occurrence of a tailings dam failure. The mechanical properties of mine tailings are heavily influenced by the particles' grain size [6]. Over the past few decades, the mechanical properties of fine and coarse tailing materials have been studied based on a large number of physical and mechanical experiments [7; 8; 9]. In a study conducted by [PERSON] [10], two individual tailings impoundments were studied (based on a series of model tests), the former of which using coarse tailings and the latter using fine tailings, to examine the effect of particle grain size on seepage laws. Moreover, [PERSON] [11] and [PERSON] [12] examined fine particle content of tailings dams and their effect on the liquefaction and engineering properties of tailings. Furthermore, many researchers have studied rheological behaviors relative to the effect of tailings particle grain size over the past few years. [PERSON] [13] conducted a series of flume experiments to predict flow characteristics and the possible extent of flood movement, consequently analyzing these findings based on flow movements observed in several case studies where failures have occurred. [PERSON] [14] analyzed factors affecting rheological behaviors, particularly with regard to the grain size of iron ore tailings. [PERSON] [15] conducted a study on the flow and beaching behavior of sub-aerially deposited, polymer-flocculated oil sand tailings. [PERSON] [16] elucidated the influence of the type of viscous model--pure cohesive versus pure frictional--with the numerical reproduction of two different real flowslides that occurred in 1966: the Aberfan flowslide and the Gypsum tailings impoundment flowslide. The hydrodynamic characteristics of debris flow are influenced not only by the roughness of the downstream gully [17; 18; 19; 20] but also by the grain size, sediment concentration [21; 22; 23], although the behavior and influence of grain size on mudflow resulting from a tailings dam failure have not been examined sufficiently. To be able to assess the potential for damage in the event of tailings dam failure accurately, it is necessary to characterize the hydrodynamic properties of different particle sizes of the resulting mudflow. This paper presents the factors affecting the hydrodynamic behaviors of mudflow surging from a tailings dam-break, particularly relative to the effect of grain size on these behaviors. This has been achieved via the utilization of a self-designed Tailings Dam-Break Model Test System (TDBMTS) which consists of glass flumes, a mudflow-height observation system, and a measurement system for the velocity and impact force of mudflow [24]. A series of flume experiments were conducted to observe the hydrodynamic behaviors of mudflow using three grain-particles of different sizes (\(d_{50}=0.72\) mm; \(d_{50}=0.26\) mm; \(d_{50}=0.08\) mm, respectively).