## 5 Suggestions on Flood Discharge Mode and Discussion  
To ensure the safety of discharging floods in the stilling basin, three key points must be met simultaneously: (i) a reasonable operation mode of the sluice gate; (ii) reliable energy dissipation and anti-scouring facilities in the stilling basin; and (iii) good stability of the continuous sill and reinforcement base slab. Based on hydraulic prototype observation results in various flood discharge conditions, the following suggestions for the flood discharge mode can be proposed:  
1. From the results of the HPO test, it was found that the flow state in the open channel is closely related to the opening mode of the sluice gate and the discharge rate in the open channel. When the discharge rate of any single sluice gate reaches 500 m\({}^{3}\)/s, the impact on the continuous sill by the discharging water is more severe than in other FDCs. Therefore, the discharge rate of a single sluice gate should be reduced to 250 m\({}^{3}\)/s. On the contrary, when the discharge rate in the open channel exceeds 2000 m\({}^{3}\)/s, the discharge of water into the stilling basin starts to tumble violently, and the fluctuation of the free water surface at the end of the open channel is significantly intensified, leading to worse downstream river flow conditions. Consequently, a discharge rate in the open channel of more than 2000 m\({}^{3}\)/s should be avoided.
2. When the opening mode of sluice gates No.5#-7# is not symmetrical, an asymmetrical flow of discharging water in the open channel may occur, and the transverse backflow phenomenon may occur in some areas of the stilling basin. Therefore, if sluice gates No.5#-7# at the head of the open channel must be opened for flood release, it is strongly recommended that they be opened evenly and symmetrically.  
During our PHO tests, the work conditions in the field were relatively tough. When the sluice gate is opened for discharging water, the hydraulic jump and turbulence in the open channel are violent, threatening the personal safety of staff for in situ measurement data. Consequently, collecting a multitude of data points is a challenging task. Many hydraulic properties are investigated to overcome a host of difficulties, but other parameters, such as flow velocity and water depth at the outlet of the gate chamber, the length of the hydraulic jump, air concentration in the discharge flow, and the dimension of the bubble in water, are challenging to measure. Meanwhile, the free water surface stagnation is challenging to identify by naked-eye observation of the turbulent free water surface fluctuation. Accordingly, some uncertainties are introduced when measuring the subsequent depth at the end of the hydraulic jump.  
The mode of the sluice gate opening can influence the characteristics of the hydraulic jump produced in the stilling basin. When the gate structure consists of multiple side-by-side undershoot sluice gates, single sluice gates can probably be opened based on the operation requirement. If only one sluice gate is needed to be open, the discharge flow from the gate chamber can cause transverse expansion in the stilling basin when releasing. Under this condition, the hydraulic jump produced in the stilling basin is entirely different from that in the condition where all gates are open. Nevertheless, research on the effect of gate asymmetry and uneven opening on the hydraulic jump is lacking and challenging for researchers.