## 1 Introduction  
In low-land flood-prone areas, construction of a sluice structure is a common engineering practice for regulation of flood discharge and water stage in a river. Being site-specific, it may also have other functions. The structure consists usually of a number of sluice gates that cover part of the river width. In most cases, the water head is in the range of 5-20 m.  
Two distinct features characterize the multigate sluce structure in an alluvial plain [1,2]: (i) the flow often conveys a significant amount of sediment in form of both bed load and suspended load, and (ii) the reservoir is bent, rather than straight, in planform, generating thereby a main current with flow circulations upstream of the structure and resulting in varied sediment-carrying capacity in cross-section. Even asymmetrical operation of the gates generates uneven velocity distributions [3]. As a result, the sediment has a tendency to deposit on one side of the reservoir. With the elapsing of time, the sediment deposition may affect the sludge operation if countermeasures are not properly taken. As an effective instream training solution, guide vanes modify flow fields and control sediment transport in rivers [4, 5, 6, 7]. They are often vertically placed in the riverbed, either submerged or non-submerged, depending on the local flow situations. Aligned with an attack angle to the approach flow, they induce a secondary flow component, which changes the magnitude and direction of bed shear stresses and leads to a redistribution of flow and sediment [7,8]. The first known installation, proposed by [PERSON] and [PERSON] [9], is to reduce the bank erosion in a river bend, where the vanes are laid out so that vane-generated secondary currents eliminate the centrifugally induced ones, which is the root cause of bank undermining [10,11]. Likewise, they divert flow away from eroded banks and stabilize the lateral migration. In a curved flume, [PERSON] and [PERSON] [12,13] performed experiments with submerged vanes of small, slender, and airfoil shapes. These were shown to modify near-bed flow patterns, redistribute sediment transport, and thereby mitigate channel erosion. To redirect the flow through a bend, vanes are arranged typically in one array or multiple arrays if necessary [14,15]. Many successful applications of the vanes have been made to counteract unfavorable flow and sedimentation patterns [5,16,17,18]. In the connecting straight channel downstream of the bend, guide vanes also facilitate obtaining a relatively uniform flow [19]. The vanes deflect the flow from the bend, re-distribute the main current, and alleviate the uneven sedimentation in cross-section. They are effective in sediment redistribution at diversions [20] and water intakes [8,21]. Strategically placed, the vanes prevent the sediment, especially for bed load, from entering the structures. By modification of flow fields, guide vanes are also an efficient approach for sediment removal. For example, for reservoirs and water intakes facing loss of function with severe deposition, installation of vanes can mitigate its occurrence [21,22,23]. Their efficiency is dictated by several factors, including layout and configuration. Many applications have a simple, rectangular, flat-panel design. Parallelogram and tapered plates and even curved vanes are also used in some cases [4,11]. The vane size, number, and spacing are governed by river width and bend curvature [5,24,25]. Typically, the submerged vane height is 0.2-0.4 times the local water depth. For the non-submerged vane, its crest is usually at the same elevation as the water surface. To minimize the flow interference from the neighboring vanes, both the vane length and lateral spacing are 2-3 times the height. The vane thickness is 0.05-0.50 m, depending on the material. The structure is installed at an attack angle of 10-20\({}^{\circ}\) with the flow [4,26]. However, site-specific conditions may require adjustments to these dimensions.