## 3 Experimental Results and Analysis
### Jet Properties  
Jet properties affect the mean or fluctuating pressure on the floor slab of a plunge pool because the break-up and air entrainment of the jet is influenced by surface tension and turbulence effects. These effects, in turn, determine the thickness of the slab. Nappe regimes at various sediment concentrations are shown in Figure 3. The air entrainment of the falling jet is obvious. The break-up of the jet in the air is best understood by vertically dividing it into three major flow regimes. The first region was characterized by the initial formation of waves on the surface of the jet. Surface tension resists the growth of these disturbances. The flow exiting the outlet was glassy and smooth, and the nappe section was similar to that of a sharp outlet. This glassy smooth region increased in length with increasing sediment concentrations. The second region was characterized by the growth of instabilities on the water surface. Small, regularly spaced waves formed and were amplified in the direction of flow. Notably, the falling jet of sediment-laden flow was smoother than that of fresh water, and the \begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**No.**} & **Concentrations** & **Upstream** & **Downstream** & \multirow{2}{*}{**No.**} & **Concentrations** & **Upstream** & **Downstream** \\  & **of Sediment** & **Water** & **Water** & & **of Sediment** & **Water** & **Water Depth** \\  & **(kg/m\({}^{3}\))** & **Head (m)** & **Depth (m)** & & **(kg/m\({}^{3}\))** & **Head (m)** & **(m)** \\ \hline
1 & 0 & 1.80 & 0.1 & 7 & 150 & 1.80 & 0.1 \\
2 & 0 & 1.85 & 0.1 & 8 & 150 & 1.85 & 0.1 \\
3 & 0 & 1.90 & 0.1 & 9 & 150 & 1.90 & 0.1 \\
4 & 50 & 1.80 & 0.1 & 10 & 250 & 1.80 & 0.1 \\
5 & 50 & 1.85 & 0.1 & 11 & 250 & 1.85 & 0.1 \\
6 & 50 & 1.90 & 0.1 & 12 & 250 & 1.90 & 0.1 \\ \hline \end{tabular}
\end{table} 
Table 1: Experimental cases. Figure 2: Relationship between flow rates and upstream water head to deviations at different sediment concentrations. small waves appeared slightly later. The third region was characterized by the transition of the surface waves into circumferential vortex elements. Jet break-up was clearly observed and was less intense in sediment-laden flow than in fresh water, as was the air entrainment. This behavior has been attributed to the higher viscosity of sediment-laden flow [46]. Figure 4 presents the relationship between viscosity and sediment concentration at 10 \({}^{\circ}\)C, 20 \({}^{\circ}\)C, and 30 \({}^{\circ}\)C. Clearly, the viscosity of flow increases with increasing sediment concentration and decreases with increasing temperature. The increasing viscosity of the sediment-laden flow likely results in an increase in surface tension [9], and, as a result, the falling jets exhibit different nappe regimes at the different sediment concentrations. When a falling jet impacts the floor slab of a plunge pool of sufficient depth, the flow regime in the plunge pool can be divided into the three regions shown in Figure 5 to determine the hydraulic pressure distribution. The first region (I) is the submerged jet region. In this region, the flow velocity of the main stream remains approximately linear; the center of the main stream exhibits the highest velocity while the edge of the main stream exhibits the lowest velocity. The second region (II) is the main stream impact region, in which the velocity of the main stream decreases rapidly and streamlines abruptly upon meeting the slab, so the pressure on the slab of the plunge pool increases sharply. The third region (III) is the wall jet region, in which the main stream is ejected and flows to the wall and along the slab of the plunge pool. Here, spirals begin to roll and diffuse. Looking at these three regions, clearly the design impact pressure is caused by the effects of the jet in the main stream impact region (II), where the maximum pressure on the slab occurs, and instability and failure of the slab are most likely. Figure 4: Relationship between various sediment concentrations and fluid viscosity.