# Effects of Outlet Shrinkage on Hydraulics in Hyper-Concentrated Sediment-Laden Flow
## 5 Conclusions  
The effect of sediment concentration on energy dissipation is an urgent problem for high dams in hyper-concentrated sediment-laden rivers. Thirty-six experiments were conducted using overflows with three contraction ratios and four sediment concentrations (0 kg/m\({}^{3}\), 50 kg/m\({}^{3}\), 150 kg/m\({}^{3}\), and 250 kg/m\({}^{3}\)) under three upstream water heads (2.10 m, 2.15 m, and 2.20 m). The experimental results were used to study the hydraulic characteristics of energy dissipation in terms of flow discharge, jet properties, and hydrodynamic pressure. The main conclusions were as follows:(1) The discharge flow was related to the upstream water head, regardless of the sediment concentration. The flow coefficient of different overflow shapes generally first increased and then decreased with an increase of upstream water head. When the water head of weir was below 10 cm, the discharge capacity was basically not affected by the width of the outlet. With an increase of water head, the influence of outlet shrinkage on the discharge capacity gradually increased. As the contraction ratio decreased, the water depth became greater, affecting the discharge capacity, and the flow coefficient decreased accordingly. (2) The effect of outlet shrinkage on flow regime decreased with increasing sediment concentration. In freshwater and sediment-laden flow (50 kg/m\({}^{3}\)), the water nappe became narrower with outlet shrinkage, air entrainment increased, and the length of the jet trajectory increased. In a sediment-laden flow (150 kg/m\({}^{3}\) or 250 kg/m\({}^{3}\)), it can be seen from the completeness of the nappe that the length of the jet was reduced because the air entrainment was significantly reduced and the influence of gravity gradually increased. As the sediment concentration increased, the diffusion of the water nappe decreased, and the effect of the outlet shape on the nappe reduced. (3) Fluctuating pressure is the result of the movement and interaction of vortices of different scales in the pool. The time-averaged pressure and fluctuation pressure both exhibited peaks, describing the impact of the jet on the bottom of the plunge pool. As the width of the outlet became narrower, the air entrainment of the water nappe increased, and the relative time-averaged pressure on the bottom of the plunge pool dropped. The difference between the maximum RMS is similar to the different overflow weir shapes with different sediment concentrations. The shape change causes a change in fluctuating pressure, and as the shape of the outlet becomes narrower, the fluctuating pressure decreases. (4) When considering hydrodynamic pressure, the main frequency of the fluctuating pressure was from 0 to 0.2 Hz. The outlet shrinkage caused the frequency range to expand. The dominant frequency became lower with the increasing sediment concentration. The variation of the autocorrelation coefficient distribution was similar to the power spectral density. With increased sediment concentration, the effect of the overflow shape on the vortex scale was reduced and the autocorrelation coefficient distribution decreased more slowly. The findings of this study provide invaluable insights into the effects of overflow shape on energy dissipation in high dams on sediment-laden rivers. The behaviors and relationships determined in this study establish a foundation for the informed design of dissipator structures such as plunge pools, improving the operational safety of the dam. All authors contributed to the research work. [PERSON] and [PERSON] conceived and designed the experiments; [PERSON] and [PERSON] performed the tests; [PERSON] analyzed the data; and [PERSON] and [PERSON] drafted the paper. All authors have read and agreed to the published version of the manuscript.