## 5 Results and Discussion
### Sedimentation Patterns and Flushing Efficiency  
To elucidate the erosion and deposition patterns, numerical simulations of the movable bed are performed, ranging from \(\alpha\) = 0\({}^{\circ}\) to 30\({}^{\circ}\) at an interval of 5\({}^{\circ}\). With the gradual change in orientation, changes in reservoir sedimentation are easily observed. The modeling indicates that at \(\alpha\) = 0-5\({}^{\circ}\), the reservoir is still subjected to siltation, implying that the deposition on the left side is not removed. Local deposition, though not significant, occurs at the trailing edge of the vane. A similar pattern was also observed by [PERSON] [4] through laboratory tests. The orientation is not effective. At \(\alpha\) = 15-20\({}^{\circ}\), the results show that improvements induced by vanes become significant, in which the siltation in the reservoir is mitigated, especially in the previous circulation zones. Figure 15 compares, at \(\alpha\) = 15\({}^{\circ}\), the bed-elevation changes in the form of isolines. In either case, the results refer to the initial bed elevations minus the resulting bathymetry. The 2015 underwater laser-scanning data (Figure 3) are the initial condition. Positive values denote erosion and negative ones deposition. The vane-induced erosion is \(\sim\)1 m at maximum, which is positive. The two flow circulation zones, with an appreciable reduction in size, as also demonstrated in the physical tests, shift from deposition to erosion, and the eroded depth is on average \(\sim\)0.35 m. Local deposition in the vane vicinity is observed, which is somewhat enhanced with an increase in angle. Figure 14: Modified flow patterns in the presence of guide vanes at \(\alpha\) = 15\({}^{\circ}\). The water flows from right to left. (**a**) case 1; (**b**) case 2. At \(\alpha=25\)-\(30^{\circ}\), the vanes generate similar reservoir erosion and deposition pattern, as at \(\alpha=15\)-\(20^{\circ}\). However, the flow seems to be overly deflected. As a result, deposition occurs behind the first vane nearest the right bank, with a thickness of \(\sim\)1.20 m at maximum. The most undesirable is that flow separation and severe local scour take place around the four vanes on the right, which is unfavorable to the vane foundation and stability. The situation is even checked at the high river discharge (case 3), showing that the trend is enhanced and becomes more obvious, which is undesirable. The sediment flushing efficiency, i.e., the ratio of bypassed to incoming sediment flux (\(q_{\rm t}\)) including both bed load (\(q_{\rm b}\)) and suspended load (\(q_{\rm s}\)), is an essential index in the context. For both cases, Figure 16 plots the \(q_{\rm t}\) changes as a function of \(\alpha\). The results show that the flushing efficiency, similar at low and medium flows, becomes enhanced with an increase in guide-vane angle. Up to about \(\alpha=20^{\circ}\), the increment is almost linear. At \(\alpha=30^{\circ}\), the flushing efficiency amounts to 65-70%. Figure 17 shows, as a function of guide-vane angle, the flux of the bypassed sediment through the shuice structure. The incoming bed-load flux is limited in the reservoir, and  
Figure 16: Sediment flushing efficiency as a function of guide-vane angle. Figure 15: Riverbed changes in the reservoir with the installation of seven guide vanes, all aligned at \(\alpha=15^{\circ}\). (**a**) case 1; (**b**) case 2. Compared with the 2015 river bathymettical data, a positive value denotes erosion and a negative one deposition. the overwhelming portion is suspended load. It is essential that the guide vanes function satisfactorily for suspended sediment. Considering the effectiveness of sediment mitigation and also the structural stability of the guide vanes, they are preferably not aligned more than \(\alpha=20^{\circ}\) with the approaching flow direction. The layout of \(\alpha=15^{\circ}\) is thus recommended, suitable at both medium and low discharges at which the flushing efficiency amounts to 50%.