## 1 Introduction  
Debris flows are among the most recurrent and damaging geological phenomena in mountainous areas across the world and have consistently caused loss of life, agricultural land, infrastructure,and buildings. China is the country affected by the majority of debris flows events and it has the most serious impact to date [1]. According to [PERSON] et al. [2], 10927 debris flow disasters have been recorded in 23 provinces and cities between 2005 and 2015, with a direct economic loss of 14.2 billion Chinese Yuan. Debris flows can be divided into three types according to their formation process: frequent occurrence type, dam-break type and landslide type [3; 4]. Dam-break debris flows are caused by a large number of solid materials transported by landslides, generated by earthquakes or volcanic lava and these debris flows are typically generated by heavy rainfall and erosion of dam systems [5; 6]. The breaking of natural or constructed dams can cause disruptive floods that can pose a huge threat to the water conservancy facilities at the downstream section of the river as well as the safety of people's lives and properties along the river-bank. This type of disaster is very intense and fast because once the dam breaks, the consequent formation of the debris flow or flood typically causes damages downstream that also affect the restoration of the entire area for a long time frame [7]. Therefore, it is of great theoretical value and practical significance to study the dynamic characteristics of debris flow induced by dam-break in order to provide technical support for the prevention and control of debris flow disasters caused by dam-breaks. Previous studies on dam-break mainly focused on the prediction of peak discharge, evolution of the debris flows towards the downstream area and their propagation after a dam-break. From the insights made by [PERSON] equations in 1871 to [PERSON]'s first calculation formula of dam-breaking flow in 1892, [PERSON] extended [PERSON]'s formula investigating the debris flow propagation in the downstream area, and since then, several studies have been conducted to provide a better understanding of this phenomenon or to characterize single features typical of each region and environmental condition [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. To date, studies have also been carried out to experimentally characterize dam-break simulation experiments to simulate the process of dam-break and consequent flood evolution [19; 20]. Furthermore, field studies to observe this phenomenon and gather essential datasets have also been carried out [19; 20; 21; 22; 23]. Dam-break debris flows are characterized by complex formation mechanisms: they typically form due to the mixing of solid and liquid substances in the formation process, and are mainly affected by the type of terrain and geological conditions. Therefore, their properties depends on the particle size distribution, which consequently can generate uneven velocity distributions and large bulk densities. Based on this, the impact forces associated with debris flows induced by dam-break are much stronger and various, hence there is a significant need to expand our knowledge and further investigate these phenomena. To the authors' knowledge, research conducted to date on dam-break debris flow is limited, and only a few studies have obtained some preliminary results [6; 23]. Other studies have focused on the characteristics of partially and fully submerged range of the dams, identifying the correspondent impact force, speed of propagation, and accumulation depth to identify protective measures [24; 25]. However, to date, studies on the formation and the initiation process of debris flows induced by dam-break still remain limited to the quantitative description of the flows and sediments' conditions and the estimation of the magnitude of debris flows, while the formation dynamic mechanisms are not completely clear yet.