## 1 Introduction
Installing more groins may prolong the regulating length of the river and cause more a complicated turbulent flow pattern, but this is difficult to measure via laboratory experiment. Employing a numerical model validated by experimental data, the complex recirculating flow inside multiple groins field was successfully captured by [PERSON] et al. [35], [PERSON] et al. [36], [PERSON] et al. [37] and [PERSON] et al. [38]. The simulated results show that the center of the downstream recirculation zone moves towards the groin as groin length increases, and the recirculation zone between groins increased with the spacing between two neighboring groins. Contrary to the ipsilateral layout, groin groups constructed on both sides of a river is a so-called bilateral layout. The bilateral layout of groin groups is generally performed in two forms, i.e., the alternate layout and symmetrical layout, which depends on the relative spacing between groins on both sides. The spacing between groins is a crucial consideration to retain an appropriate channel depth and cost-effective construction. The open-channel flow with groins in the alternate layout was experimentally investigated by [PERSON] and [PERSON] [39]. The experimental results show that the spacing between groins has a significant effect on the location of the reattachment point and maximum velocity downstream, leading to change the scale of the recirculation zone formed behind groins. Further, [PERSON] et al. [40] proposed the spacing threshold of two adjacent groins, which is the spacing when the lateral distribution of velocities adjacent to two groin sections becomes coincides, and compared the velocity distribution of two groins with the ipsilateral layout and alternate layout via numerical simulation. It shows that the spacing threshold of alternate groins is smaller than that of ipsilateral groins. Following that, a contrastive analysis between the hydraulic performances of groins placed ipsilaterally and alternately was carried out by [PERSON] et al. [41], focusing on water-level fluctuation, velocity distribution and bed patterns around groins. From the results, the alternate arrangement of groins showed a greater increase in water levels and velocities upstream. In terms of flow field characteristic of symmetrical groins, the recirculation zone width induced by groins is highly sensitive to the length of groins, especially the length of the groin placed in middle [42]. Both the recirculation width and length of symmetrical groins at downstream increased with decrease in the cross-section of the main flow, and the average recirculation width and length were 0.4 and 8 times the groin length, respectively [43]. The alternate groins layout was widely used on training large rivers, such as the Mississippi River [44; 45; 46], Rhine River [47; 48; 49], Yellow River [50; 51] and Yangtze River [52; 53; 54]. Due to the asymmetric layout, there were significant differences in flow properties between ipsilateral groins and alternate groins. According to previous research [55; 56], the extension of the horizontal vortex downstream of a groin can be used as a criterion for selecting the suitable spacing between groins. In spite of all this extensive research, it is still difficult to make a direct estimation of the backflow scope behind groins deployed at both riverbanks. For instance, even for a single groin, the surrounding 3-D turbulent flow change is very complex, not to mention the accumulative interaction with the integrated groin group in a natural river system. Moreover, some questions remain to be answered: how do the backflows generated by alternate groins affect each other? Is there a critical spacing between groins that maximizes or minimizes the respective recirculation zones behind groins? How do they differ from each other in the velocity distribution? To answer these questions, further research is needed to investigate the backflow interaction mechanism of bilateral groins.