## 2 Reservoir and Sluice Structure  
The sluice facility in question was commissioned in 1960 and comprises six bulkhead gates connected on each side with an abutment to the riverbank. After almost four decades in service, it was refurbished during 2007-2008, with replacements of all the gates and hoisting equipment and reinforcement of the concrete structure. Figure 1 shows a photo of the facility seen from upstream; Figure 2 illustrates its layout with dimensions. At the sluice site, the river is ~120 m wide. The multigate sluice is centrally placed and occupies approximately half the river width. As shown in the cross-sectional profile, the gate threshold is placed at elevation +0.00 m. The gates are founded on a platform sloping at rise:run = 1:4 both up- and downstream. The river has a roughly trapezoidal cross-section, with the bank slope rise:run = 1:3 on each side. Seen from upstream, the gates are numbered 1-6 from left to right. The gates are mounted along the same axis along the structure, equipped with wire hoists for operation. Five piers are 1.60 m thick, and two piers adjacent to the central pier are somewhat thicker--2.40 m. The gates, all 10 m high, have the same net width--\(b_{\text{g}}\) = 8.00 m. Including all the seven piers, the shuice structure is 60 m long. Based on underwater laser-scanning data in 2015, the river bathymetry is shown in Figure 3. The river course is relatively uniform in cross-section and bends \(\sim\)41\({}^{\circ}\) to the left in the reservoir area. Immediately downstream of the shuice, the riverbed is lowest in elevation, corresponding to the location of the stilling basin. In the reservoir, the lowest bed elevation ranges between 0 and +2 m MSL. The design retention water stage (DRWS) is +11.37 m MSL, at which the water-surface width varies between 90 and 150 m and the water depth amounts to 10-11 m at maximum. During a year, the river runoff varies significantly. The multiyear average river discharge is \(\sim\)38 m\({}^{3}\)/s, and the discharge during a wet season can be as high as 400 m\({}^{3}\)/s. Based on multiyear flow data, the discharge variations and the flow frequency are shown in Figure 4. The representative operation conditions of the shuice are at low and medium flow discharges (denoted as cases 1 and 2). The flow carries fine sediment in the form of both bed load and suspended load. Field measurements are conducted to determine its composition, which shows that the grain size is \(D\) = 0.002-0.90 mm and the median grain size is \(D_{50}\) = 0.014 mm (Figure 5). In light of the grain-size distribution, the sediment material is classified as sand (0.05-2 mm), silt (0.005-0.05 mm), and clay (\(<\)0.005 mm) [27]. On average, the suspended load of cohesive silt and clay accounts for \(\sim\)85% and the bed load of non-cohesive sand is \(\sim\)15%. Table 1 summarizes the operational parameters of the three typical cases examined, in which \(Z\) = water level (m ASL), with subscripts u and d denoting up- and downstream, \(Q\) (m\({}^{3}\)/s) = river-flow discharge, \(S\) (kg/m\({}^{3}\)) = suspended load concentration, \(e\) (m) = gate opening height, and \(b\) (m) = total net flow width at the shuice, depending on the number of opened gates (\(N\)). The first two cases are the design conditions for the proposed guide vanes, while case 3 is for checking their local scouring risk. Governed by the operation requirements, three gates are opened at low discharges (\(N\) = 3). At medium and high discharges, all the gates must be operated (\(N\) = 6). Once opened, all the gates have the same \(e\) value. Figure 1: Sluice structure centrally placed in the river course, with six bulkhead gates and connecting abutment on each side (looking downstream). Figure 3: Reservoir and tailwater bathymetry (based on the 2015 underwater laser scanning). Figure 2: Layout of the shuce structure. (**a**) cross-sectional view (looking downstream); (**b**) plan view. Including all the piers, the concrete shuce structure is 60 m long. Figure 4: Hydrological data at the shuce site.