## 3 Results  
### Horizontal Flow Patterns  
#### 3.1.1 Flow Fields of a Single Groin and Symmetrical Groins  
Time-averaged horizontal flow fields of a single and two symmetrical groins on the same horizontal plane (\(z/\mathrm{H}=0.05\) m, where \(z\) is the height of velocity profile captured and \(\mathrm{H}\) is the experimental water depth) are shown in Figure 4. In the flow field of a single groin, nearly stagnant flow with the lowest velocity appeared in the lee of Groin A due to the blockage effect of the groin and strong backflow was generated downstream from the stagnant flow, which eventually caused a large-scale recirculation zone. The downstream recirculation zone formed behind a groin was subdivided into three main zones, i.e., shear layer, backflow, and reattachment zone. Velocity differences are evident between the mainstream and the backflow behind Groin A; therefore, the original smooth flow pattern became chaotic due to the strong shear stress within the mixing layer at the interface of the mainstream and backflow. Figure 4a also shows the time-averaged reattachment point, where the separated flow reattaches to the flume wall on the right side of the groin, and the velocity vector is as often in the upstream direction (recirculation) as in the downstream direction (main flow). The time-averaged velocity of the reattachment point is approximately zero. The streamline from the groin tip to the reattachment point forms a recirculation zone containing a large-scale inner vortex, as depicted in Figure 5. In this study, the distance from the reattachment point to the groin wall (the side of facing downstream) is defined as the length of the recirculation zone formed behind the groin, i.e., backflow length; the maximum width of the recirculation zone is the backflow width. The observed backflow length was over five times the length of a single groin. On one hand, the mainstream velocity significantly increases from the groin tip to the opposite bank due to the decreased cross-section caused by protrusion of the groin, with a velocity gradient rapidly increasing near the groin tip, resulting in the shear layer zone (0.089 m/s \(\geq v_{xy}\geq\) 0.030 m/s). On the other hand, the velocity behind the groin rapidly decayed, resulting in a low-velocity recirculation zone.  
Figure 4b shows the horizontal flow field of two symmetrical single groins installed in both banks. Because the cross-section of main flow was further contracted by two groins, the velocity of the main flow increased more than that of a single groin. Another large-scale recirculation zone was formed behind Groin B. Due to the symmetrical arrangement of two identical groins, the two recirculation zones behind the groins were also symmetrically distributed, with identical inner vortical structures rotating in the opposite direction. The cross-section of main stream is narrowed down and the velocity profile from the groin tip  
is reshaped by two recirculation zones, which would have significant effects on scouring in the main channel of a real-world river. The result is consistent with the study of [PERSON] et al. [42], which simply examined the impacts of groin length on flow fields around groin groups in a symmetrical configuration using Flow-3D software.