## 4 Discussion  
### Effect of Backflow Patterns of Groin B on Recirculation Zone Behind Groin A  
The flow interaction between bilateral groins determines the contraction extent of the main channel by groins with different spacing, which further influences the downstream backflow affected by the streamline curvature. It implies that the deformation of streamlines passing groins is important to the development of the recirculation zone behind Groin A. When Groin B is placed upstream of Groin A, the outward normal direction of flow at the end of the recirculation zone induced by Groin B points toward Groin A and the convex recirculation zone behind Groin A concurrently (see Figure 17a). It can be interpreted as the flow pointing towards the rear of Groin B along the section in the outward normal direction, which provides additional momentum to downstream recirculating flow induced by Groin A, resulting in a larger velocity component pointing to the opposite river bank. With Groin B approaching Groin A, the width of the section along the outward normal direction decreases, but the current velocity on the section increases correspondingly to conserve mass, especially the velocity component in the transverse direction (i.e., spanwise velocity) because the angle between the outward normal direction and the flow direction become larger. As previously mentioned in Section 3.2, the spanwise velocity near the groin tip has a positive effect on the scale of the recirculation zone formed behind Groin A (see Figure 11), so that the laterally confined flow promotes the development of backflow behind Groin A. However, if Groin B is nearly symmetrical with Groin A on both banks, the outward normal direction of flow along the recirculation behind Groin B is no longer pointing to Groin A but to the downstream backflow of Groin A, as shown in Figure 17b. Eventually, the longitudinal propagation of the recirculation zone is limited, and the reattachment point moved towards Groin A.  
Figure 17: Schematic diagram of the influence of Groin B on the backflow behind Groin A in different cases, (**a**) Groin B in the upstream far-field of Groin A, (**b**) Groin B in the upstream near-field of Groin A, (**c**) Groin B in the downstream near-field of Groin A, (**d**) Groin B in the downstream far-field of Groin A.  
Conversely, when Groin B is located in the downstream near-field of Groin A, the velocity distribution at the cross-section of Groin A would be shifted by the separating flow in front of Groin B, which is caused by the upstream corner vortex due to the blockage of Groin B. The outward normal direction of flow at the upstream part of the recirculation zone behind Groin B accordingly points to Groin A and the downstream recirculating flow induced by Groin A, see Figure 17c. With the joint actions of the flow at the front and rear of Groin B, more flow energy is dissipated near the tip of Groin A, where the maximum turbulent intensity appeared. Consequently, the spanwise velocity near the groin tip decreases further, and thus, the recirculation length of Groin A becomes smaller due to the dissipation of turbulent mixing energy. When groins are further apart (over 0.15 m), the angle between the downstream direction and current direction becomes smaller, leading to the angular momentum of mean flow increasing along the outward normal direction of recirculating flow induced by Groin B. Similar phenomena were observed by [PERSON] and [PERSON] [39] for groins in alternate layout. The front streamlines of separating flow induced by Groin B become gradually skewed and almost parallel to the rear streamlines of the recirculation zone behind Groin A. The outward normal direction of the front streamline along the recirculation zone of Groin B mainly points to backflow behind Groin A, resulting in more momentum concentrated in the recirculation zone (see Figure 17d). Thus, the spanwise velocity along the outward normal direction of streamline increases and transfers more kinetic energy to enhance the development of backflow behind Groin A.