# Effects of Outlet Shrinkage on Hydraulics in Hyper-Concentrated Sediment-Laden Flow
## 4 Discussion
### Comparative Analysis  
In practical projects, a method of outlet shrinkage should be employed, such as a flaring gate pier. [PERSON] [37; 38] studied the flow discharge with contraction of jet and water nappe characteristics in a high arch dam by using a flaring gate pier. The research results indicated, that due to the shrinkage of the cross section, the direction of water particles changed at different depths vertically along the exit cross section, and the water nappe was obviously made longer, with dispersion characteristics in the longitudinal direction. The discharge capacity was affected by the contraction ratio, and with the increase of water head on the weir, the effect of outlet shrinkage on the discharge capacity was more obvious. [PERSON] [39] analyzed the characteristics of hydrodynamic load under the X-shaped flaring gate pier in a short stilling basin. The maximum fluctuating pressure increased with the widening of the outlet at the bottom of the stilling pool. Previous studies [37; 38; 39] stated the influence of the flaring gate pier on the flow regime of nappe, discharge capacity, and fluctuating pressure on the bottom of the pool. Although different objects were studied, and different experimental details were produced, the effect of outlet shrinkage of the overflowing building in the literature correspond well with those results. [PERSON] [40] used a large-scale model to test the same shape of bucket with fresh water. Two physical models with different scales produced consistent results when comparing sediment-laden flow with fresh water. The results pointed out that the discharge capacity gradually decreased as the contraction ratio decreased in a relatively high water head of a weir, and that outlet shrinkage had little effect on the position of the pressure peak in the plunge pool.  
In terms of numerical simulations, [PERSON] [41] studied the typical three-dimensional flow pattern in the plunge pool and energy dissipation of an arch dam. The research results revealed that an energy transition occurred mainly in the region near the jet axis, while only a little water mass in the plunge pool took part in the energy dissipation. The distributions of turbulent energy and its dissipation rate were revealed, and energy dissipation was more difficult with concentrated jets. The way to optimize the plunge pool and dam outlets should be to expand the range of the shear dissipation region; the outlet shrinkage used in the literature can disperse the water nappe and accelerate energy dissipation.  
The flow fluctuating load was caused by various vortices. The fluctuating pressure was the exact reflection of different scale vortices on the solid boundary. [PERSON] [42] pointed out that a large-scale vortex component with low frequency was present in the bottom of the plunge pool. [PERSON] [43] investigated the characteristics of pressure fluctuations in the three-phase flow of water, air, and sand based on experimental data. Air entrainment and sediment had finite effects on the distribution of the probability density function of the fluctuating pressure. This was roughly consistent with the normal distribution, as in fresh water. According to a spectrum analysis of experimental data, the fluctuating energy of high-velocity sediment-laden and aerated flow was widely distributed in frequency. The power spectrum of fluctuating pressure was basically the same as that of fresh water when the sediment concentration was lower than 100 kg/m\({}^{3}\). [PERSON] [21] and [PERSON] [26] conducted model tests with a hyper-concentrated flow (sediment concentrations of 250 kg/m\({}^{3}\) or 400 kg/m\({}^{3}\)). The results of [21; 26; 43] revealed that large eddies existed in the sediment-laden flow and conformed to the research result. However, the effect of outlet shrinkage was also considered in the research. The outlet shrinkage decreased the dominant frequency at a hyper-concentrated flow, and the large-scale vortex increased. When the sediment concentration was low, outlet shrinkage had little effect.