## 1 Introduction
[25] found that the recirculating flow patterns in the groin fields depend mainly on the ratio between groin length and groin field length. The aspect ratio of 0.5 is the critical value at which the flow pattern changes from one-vortex to two-vortex in the groin fields. Subsequently, more details regarding vortex flow in groin fields were experimentally investigated by [PERSON] [26]. By increasing the distance between groins further, the flow pattern on the downstream side of the groin field changes while, on the upstream side, the primary and secondary vortices remain qualitatively the same [26]. More flow details near river groins were further experim  
Figure 1: Schematic sketch of flow separation from a groin (Top view). [PERSON] [27] for emergent and submerged groins. The flow circulation within the emerged groin field is largely driven by the main stream via momentum exchange through the interfacial mixing layer, while the flow in the submerged groin field does not show the same circulation pattern as that in the emerged groin field. A three-dimensional (3D) view of the flow and turbulent structures is necessary for better understanding the hydrodynamics and particular features of groin-induced flow patterns, particularly concerning the shallow flow associated with low-land rivers [28]. The shallow water depth implies that the horizontal distribution of flow velocities is strongly influenced by the local depth, curvature bed roughness, and the presence of obstacles such as groins. Due to the complexities, the 3D flow in shallow fluids is predicted generally by high-resolution experiment instruments and validated numerical models for large temporal and spatial scales. Using Stereoscopic Particle Image Velocimetry (SPIV), [PERSON] et al. [29] measured the horizontal flow velocities around a dipolar vortex generated by a single magnet with pulsed forcing in a shallow fluid layer. The flow exhibits significant 3D features from the patches of downward flow in the vortex cores. Following that, [PERSON] et al. [30] investigated the interaction of a shallow-layer vortex dipolar with a lateral wall, and observed that the influence of the wall on the vertical motion inside the dipolar vortex becomes stronger with decreasing water depths via SPIV measurement and numerical simulation. [PERSON] and [PERSON] [31] reviewed the research work on complicated vortices evolving in shallow fluid layers, and suggested the importance of vertical secondary flows on the surface of primary horizontal motion. Furthermore, [PERSON] et al. [32] noticed that the discrepancy of the 3D vortex structure occurred possibly in two-layer and single-layer fluids, and substantiated that 3D vortex structures and their evolution are resembled in both fluids via experimental and numerical simulations. The influence of a single groin is apparently limited in altering the geometry or flow dynamics of the river channel. In actual practice, groins exert influences on the river system usually in the form of groups, i.e., groin groups, to systematically reshape the river channel. The spatial layout of a groin group is an important factor to realign the river channel and improve the navigation condition, including ipsilateral and bilateral layouts. The ipsilateral layout is several groins deployed on one side in order to regulate the riverbank or coastline. Based on laboratory experiments, [PERSON] et al. [33], [PERSON] et al. [34] studied the flow patterns around an ipsilateral groin pair, focusing on the impact of groin length and distance between groins. Their results indicated that the effective distance between two groins ranges from three to four times the groin length. [PERSON] [10] examined the flow patterns induced by groin groups with different types, i.e., permeable, impermeable and inclined crest groins, and chiefly observed the vortex shedding and recirculation in the groin field.