## 3 Results and Discussion  
### Mudflow Flood Hydrograph  
Parallel to the depth of the mudflow wave measurements, the evolution of the free surface profiles were captured using digital cameras capable of recording 300 frames per second (_fps_) at a resolution of \(512\times 384\) pixels. To obtain detailed information regarding the complex free surface flow, four digital cameras were used, and lateral views of the flow evolution were taken. The mudflow flood hydrograph at downstream monitoring points was analyzed based on the video images captured during randomly selected experimental runs. For different particle grain sizes, the mudflow flood hydrographs were plotted in Figure 4.  
Figure 4 shows the mudflow flood hydro-graph at locations \(S_{2}\) and \(S_{4}\) (3.0 m and 6.0 m downstream from the gate) relating to the tests where \(H\) = 30 cm was the initial filling height. The hydro-graph shape comprised of two peaks, with the first reaching 12.7 cm at a time of 1.05 s and the second peak reaching 10.8 cm at a time of 3.15 s (Figure 4a). The first hydrograph peak was composed of a water-dominant flow, mainly from the upper water-laid layer at the tailings pond, whereas the second contained a high tailing load in a high viscosity flow. The mudflow flood hydrograph is divided into three stages: Stage (A), rapid increase; Stage (B), rapid reduction; Stage (C), slow reduction.  
As shown in Figure 5, along with the decrease of tailings particle size, the peak of mudflow depth gradually increases in value. An analysis of the peak value of the location of \(S_{2}\) (3.0 m of downstream of the gate) shows that the first peak values of mudflow depth were 12.7 cm, 11.1 cm, and 10.0 cm, and the second peak values were 10.8 cm, 10.7 cm, and 9.1 cm for fine  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Index**} & \multicolumn{3}{c}{**Material**} \\ \cline{2-4}  & **Tailings A (Coarse)** & **Tailings B (Medium)** & **Tailings C (Fine)** \\ \hline Specific gravity (t/m\({}^{3}\)) & 2.8 & 2.8 & 2.8 \\ Moisture content (\%) & 12.5 & 12.1 & 12.8 \\ Porosity & 0.52 & 0.55 & 0.67 \\ Modulus of compression (MPa) & 17.6 & 15.1 & 13.8 \\ Coefficient of compressibility (MPa\({}^{-1}\)) & 0.078 & 0.086 & 0.135 \\ Permeability coefficient (\(\times 10^{-6}\) m\(\cdot\)s\({}^{-1}\)) & 9.86 & 6.67 & 1.46 \\ Cohesion (kPa) & 7.5 & 9.9 & 11.4 \\ Internal friction angle (\(\Phi\)/\({}^{\circ}\)) & 31.2 & 28.1 & 23.5 \\ \(d_{50}\) (mm) & 0.72 & 0.26 & 0.08 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Properties of the copper tailings sample.  
Figure 3: Size distributions of tested copper tailings (sorts A, B, C).  
tailings, medium tailings, and coarse tailings, respectively. Relative to the maximum depth of fine tailings-mudflow, the medium tailings-mudflow, and the coarse tailings-mudflow decreased by 12.6% and 9.9%, respectively. The grain-size affected the maximum depth of tailings-mudflow significantly.  
With a decrease in the size of the tailings particles, there is an increase in the number of particles in a constant volume. As a result of this, accordingly the number of interactions between particles increase, leading to an overall increase in viscosity (the resistance to force that causes flow) [25]; the mudflow height will increase in the flume, leading to an increase of the peak value.