## 4 Observation Results
### Water Surface Change  
Based on the results of the HPO tests, the free water surface changes in the open channel under different FDCs are shown in Figure 6. The free water surface in the open channel continuously rises with an increase in flood discharge. Moreover, the greater the discharge flow, the more significant the hindrance effect of the continuous ridge, and the more obvious the forced hydraulic jump phenomenon would be. Three FDCs, i.e., FDC No. 1, No. 9, and No. 16, are selected to analyze the hydraulic jump phenomenon. In FDC No. 1 and No. 16, the hydraulic jump is formed only near the exit of the sulice chamber due to the low discharge rate. It is called a submerged hydraulic jump, which disappears very soon, and the free water surface levels off quickly. In FDC No. 9, a violent forced hydraulic jump is formed by the hindrance effect of the continuous ridge under significant discharge conditions. The waves roll violently, and an obvious waterfall is formed behind the continuous ridge, resulting in a second forced hydraulic jump in the stilling basin, as shown in Figure 5e,f. The characteristics of a hydraulic jump in the stilling basin are highly complex and are influenced by various factors such as water head, sulice gate open mode, depth of the downstream river, the geometry of the stilling basin, the roughness of the base slab, etc. Therefore, analyzing a hydraulic jump using a theoretical approach is challenging, and current research has focused on experimental approaches and numerical methods. In our work, we briefly analyzed a hydraulic jump based on the results of the HPO tests. The schematic diagram of a hydraulic jump is shown in Figure 7. The average width of the stilling basin is 65.0 m, and the continuous ridge is 8.0 m high and 3.0 m wide on the top. The water depth in front of the hydraulic jump is represented as \(h_{\rm c}\), with a flow velocity of \(v_{\rm c}\). The water depth behind a hydraulic jump is represented as \(h_{2}\), with a flow velocity of \(v_{2}\). The hydraulic parameters of the three FDCs are given in Table 2. The flow velocity at the narrowest position of the sloping chute at the gate chamber exit is estimated according  
Figure 6: Water surface changes in the open channel under different FDCs. to its width, upstream water level, and the opening degree of the gate, which is around 20 m/s. Froude number (\(Fr_{c}\)) at the contracted section before the hydraulic jump can be calculated by \(Fr_{c}=\frac{v_{c}}{\sqrt{g^{2}h_{c}}}\), and Reynolds number (Re) can be obtained by \(\text{Re}=\frac{v_{c}h_{c}}{v}\), where \(v\) is the kinematic viscosity of water, \(v\) = 1.01 \(\times\) 10\({}^{-6}\) m\({}^{2}\)/s at 20 \({}^{\circ}\)C; \(v_{c}\) is flow velocity before the hydraulic jump; \(hc\) is contracted water depth before the hydraulic jump. The Froude number (\(Fr\)) can be used to distinguish the type of hydraulic jump. The higher \(Fr\) is, the more obvious the hydraulic jump would be, and the better energy dissipation effect would be achieved for discharge flood. When \(Fr\) < 2.5, the energy dissipation efficiency is less than 20%; when \(Fr\) > 9.0, the efficiency of energy dissipation reaches up to 85% or even more. The Froude and Reynolds numbers in front of the hydraulic jump are about 2.5-6.0 and 2.0 \(\times\) 10\({}^{7}\)-1.5 \(\times\) 10\({}^{8}\), respectively. In the current case, \(Fr\) rises with the decrease in flood discharge rate in the open channel, which means higher energy dissipation efficiency, and vice versa. Therefore, a low-flow flood discharge is more suitable for this working condition. The judgment of the flow pattern across the continuous ridge could be made as below: When \(\frac{h_{t}-c}{H_{10}}\leq\) 0.45, the flow pattern across the continuous ridge is free overflow.