## 4 Discussion and Conclusions  
The findings of this research have provided insights into the hydrodynamic characteristics of mudflow caused by the failure of a tailings dam considering the influence of grain-size. All presented findings are applicable to the prediction of mudflow behavior in terms of time and distance. From the study presented in this paper, the following conclusions may be drawn:  
* The hydrograph shape comprised of two peaks. The first hydrograph peak was composed of a water-dominant flow, mainly from the upper water-laid layer of the tailings pond, whereas the second contained a high-tailings load in a high viscosity flow.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Material Types**} & \multicolumn{4}{c}{**Front Velocity \(V\) (m\(\cdot\)s\({}^{-1}\))**} \\ \cline{2-5}  & \(S_{1}=1.5\) m & \(S_{2}=3.0\) m & \(S_{3}=4.5\) m & \(S_{4}=6.0\) m \\ \hline Tailings A (coarse) & 3.0 & 2.6 & 2.2 & 2 \\ Tailings B (medium) & 2.5 & 2.2 & 1.8 & 1.6 \\ Tailings C(fine) & 2.1 & 1.9 & 1.5 & 1.3 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Initial velocity at different sections for the three different particle sizes of the tailings.  
Figure 8: Velocity curve of the mudflow flood at locations \(S_{2}\), and \(S_{4}\) (3.0 m and 6.0 m downstream of the gate) for tests with \(H=30\) cm as the initial filling height. The curve comprises of two stages. Stage (A): rapid reduction stage; Stage (B): slow reduction stage. Note: The time t = 0 in Figure 4a,b are the starting time of tracker reaching station \(S_{2}\) and \(S_{4}\).  
* The mudflow flood hydrograph and pressure curve can be divided into three stages: rapid increase stage; rapid reduction stage; slow reduction stage. The velocity curve displayed two obvious primary stages, namely a rapid reduction stage and a slow reduction stage.
* Simultaneous to the increase in the particle-size of the tailings, the peak of dynamic pressure and velocity gradually increase in value. However, the peak of mudflow depth gradually decreases in value. Particle grain size has a significant impact on the hydrodynamic characteristics of tailings-mudflow.  
Due to the characteristics of large discharge, strong destructiveness, and sudden occurrence of dam-break mudflow, field observation, and test are difficult to achieve. Therefore, a model test is an important method to study the flow characteristics of mudflow from tailings dam-break at present. This paper researches the effect of grain size on the hydrodynamics of mudflow surge from a tailings dam-break by a self-designed Tailings Dam-Break Model Test System (TDBMTS), it achieves a good result. But there are also some shortcomings in the experiments, the actual downstream groove topography of tailings-dam is generally complex, but this test simplified the downstream groove, which only studied the flow characteristics of mudflow in the case of regular groove, but the change of groove width has a significant impact on the propagation characteristics of mudflow. Meanwhile, the accuracy of measuring instruments needs to be improved to obtain more accurate data. How to solve these problems are worth further exploration.  
Conceptualization, X.J.; Methodology, X.J.; Validation, Y.C., D.X., S.W., W.W. and T.Y.; Supervision, D.J.W.; Writing--Original Draft Preparation, X.J. and Y.C.; Writing--Review and Editing, X.J. and Y.C.  
The research reported in this paper has been funded by the National Natural Science Foundation of China (No.51404049; No.51804051); Natural Science Foundation project of Chongqing Science and Technology Commission (No.cstc2018 icyjAX0231; cstc2016 icyjA0319); China Scholarship Council (No.201608505157); The Post-Funded Projects of Chongqing University of Science and Technology (No.chqzz2008005), and grant Chongqing-0009-2018 AQ, Chongqing-0004-2017 AQ; the authors would like to thank the reviewers for their constructive comments that improved the paper.  
The authors declare no conflict of interest.