3.1.2. Flow Rate of Leaked Tailings

As shown in Figure 5, the maximum velocities of tailings flow are all located downstream of initial dam, and the maximum evolution velocities of tailings flow increase with the elevating terrain, which are 10 m/s, 10.9 m/s and 13.5 m/s, respectively. In the original terrain scenario, the terrain and altitude difference decrease sharply within the areas downstream of initial dam and upstream of Village III after dam failure, thus enabling fast evolution of the leaked tailings flow. At the downstream section of Village III, however, the altitude difference changes little and the evolution distance is long. Therefore, a gradual decline of flow rate is noted with the increasing evolution distance. According to Figure 5, after elevating terrain by 5% and 10%, the altitude difference changes rather greatly for a scope from the initial dam to the 1000 m downstream section, and the flow rate of leaked tailings continues to heighten. The velocity tends to stabilize when the downstream altitude difference narrows down.

**Figure 5.** The maximum velocity distribution of leaked tailings flow under different slope conditions. (**a**) Original terrain slope; (**b**) elevation by 5%; (**c**) elevation by 10%.

#### 3.1.3. Impact Force of Leaked Tailings Flow

This study investigated the effect of the impact during the evolution process of leaked tailings flow on its downstream constructs, and the exerting range of impact force is the region that the discharged sediments flow through. The impact force of leaked tailings flow on the downstream constructs is calculated using Equation (7) through Equation (9) [27,37–40]. Multiple factors, including the impact angle, flow rate and density of tailings flow, can affect the computational results by varying degrees [38–40].

$$P\_i = k\rho\_f v^2 \tag{7}$$

$$k = 1.261 \epsilon^{\mathbb{C}\_w} \tag{8}$$

$$F = P\_l h \tag{9}$$

where *Pi* denotes the impact pressure imposed by leaked tailings flow on other objects within the flow region; ρ*<sup>f</sup>* refers to the fluid density of leaked tailings flow; *h* denotes the maximum depth of leaked tailings flow; *k* is the impact coefficient, and its value range from 1.261 to 3.427 [26,27]; υ is the flow rate of leaked tailings at arbitrary time; *Cw* represents the weight concentration; *e* is a constant; and *F* is the impact force per unit width.

According to Equation (9), increases in the flow rate and depth lead to enhanced impact force per unit area.

From Figures 3 and 6, it can be seen that the locations of maximum impact forces in three different conditions are all adjacent to the maximum evolution velocity location. In the original terrain condition, the maximum impact force is located downstream from the initial dam with the value of 706 kN/m. With the elevation of slope, the maximum impact force increases first and then decreases, which is 883 kN/m at an elevation by 5% and is 495 kN/m at an elevation by 10%, showing a value approximately 70% of the original terrain scenario.

**Figure 6.** The maximum impact force of leaked tailings flow downstream under different slope conditions. (**a**) Original terrain slope; (**b**) elevation by 5%; (**c**) elevation by 10%.

## *3.2. E*ff*ects of Debris Blocking Dam on Tailings Flow*

Debris blocking dams are set up at three characteristic locations downstream of initial dam in this study (Dam 1: valley mouth upstream of Village III, 1000 m away from the initial dam; Dam 2: valley mouth upstream of Village II, 600 m away from the initial dam; and Dam 3: valley mouth downstream of Village I, 400 m away from the initial dam). The height of these dams is 10 m.

According to the data collected in situ and the information provided by mining companies, there are villages downstream of these three locations, where the constructions of the debris blocking dams at these characteristic locations downstream are more conducive to reducing losses. Additionally, these three places are located at the top of the valley, the terrain on both sides is higher and the cost of dam constructions is relatively low.
