## 2 Project Background  
The hydropower project encompasses a water impounding dam, a riverbed-type power plant, a power transmission and transformation project, and additional buildings designed primarily for power generation and meeting downstream comprehensive water consumption requirements. It is classified as a large (2)-type hydropower station, with permanent main buildings designed as grade 2 structures and secondary structures designed as grade 3. The hydropower station comprises left and right bank dam sections, a riverbed power plant, a flood discharge dam section with seven sluice gates, and other buildings. The flood discharge dam section includes four sluice gates in the riverbed and three sluice gates built across the diversion channel on the right bank, as illustrated in Figure 1. Energy dissipation and anti-scour structures downstream of sluice No.1#-7# are composed of a stilling basin, continuous sill, open channel apron, and its extension. Figure 2 shows a typical section of the sluice No.5#-7#, which is 74.6 m long along the dam axis and has a sluice chamber of 60.0 m in length in the flow direction. The maximum gate height is 58.0 m, and the weir crest elevation is 994.0 m. The sluice has a flat bulkhead and radial working gates, with a width of 16.0m and an orifice size of 21.0 m high. The pier is designed as a flared gate pier, commonly used for auxiliary energy dissipation in many Chinese projects [8]. The pier's width gradually increases from around the post-median position to its end, leading to a gradual decrease in the chamber's width, from 16.0 m to 9.0 m in the present case. Near the end of the gate chamber, the middle pier's width is 12.6 m, and that of the side pier is 8.08 m. The upstream weir surface is an arc with a radius of 4.0 m, followed by a 7.0 m long top horizontal part. This is then followed by an arc with a radius of 3.0 m, connected to a flip bucket with a radius of 40.0 m via a sloping chute with a slope ratio of 1:2.08. The stilling basin is 65.0 m long, and its base slab elevation is 982.00 m, the same as the open channel base slab. Sluice No.1#-4# commenced operations in November 2014, and sluice No.5#-7# began operating in October 2015. By the end of 2018, sluice No.1#-4# had operated for 12,437 h, and sluice No.5#-7# had operated for 2261 h, covering four complete flood seasons. Initially, during low-flow flood discharge conditions without power generation, gradual erosion occurred in the energy dissipation zone, leading to an increase in the water head difference between the upstream and downstream water levels. Furthermore, due to a cut by fault F1 and discharging flow in the open channel, the foundation of sluice No.1#-4# was partially scoured and eroded under the left guide wall, posing a severe threat to structural stability. Thus, reinforcement work was necessary for the weak and eroded area of the guide wall foundation to ensure structural safety. Additionally, to enhance safety, reliability, and flexibility of flood discharge at the power station and to meet the low-flow flood discharge demand of sluice No.5#-7#, technical reconstruction work for EDFs in the stilling basin at the downstream side of sluice No.5#-7# was carried out under the guidance of the Power Station Administration Office from 1 November 2019 to 30 May 2020. The reconstruction work involved installing anti-slide piles along the axis of the left guide wall through the guide wall into its foundation and constructing a continuous sill and reinforcement base slab along the open channel, as shown in Figure 3.  
Figure 1: Plane layout of the hydropower station. Figure 2: Typical cross-section of the stilling basin. Following the technical reconstruction of sluice No.5#-7#, the discharge flow from the sluices is locally dammed under the continuous sill, creating conditions for a forced hydraulic jump in the stilling basin.