## 1 Introduction  
Groins, also termed spur dikes, have been used widely to protect coastlines and riverbanks from scour and erosion for centuries. They are typically built perpendicularly or at an oblique angle to the bank so that the flow is kept away from the bank [1]. Installing the groins, the laterally confined flow attains a sufficiently high velocity to rectify a navigable river [2]. Due to the blockage effect of the groin, a complex three-dimensional turbulent flow pattern is generated near the structure, resulting in local high velocity-gradient and sediment movement [3]. The flow field in vicinity of groins is a visual indicator of the turbulent flow characteristic and plays a key role in the understanding of hydrodynamics induced by the groin [4, 5]. The emerged and impermeable groin is the most common type of groin used in river regulation. The flow field around such a kind of groin can be divided into two subzones due to flow separation from the groin tip: the recirculation zone with a large-scale vortex downstream of the groin and the main stream with contraction and diffusion by the protrusion of the groin (see Figure 1 for detail). Previous studies have examined the two-dimensional (2D) flow fields around a single groin, the simplest layout, to interpret the mechanism of groin-induced turbulent flow. Most scholars focus on the flow separation from the groin tip [6], back water upstream of the groin [7], dead-water zones in the mixing layer [8], water surface oscillation [9] and the recirculation zone downstream [10, 11], as shown in Figure 1. Among issues above, the large-scale recirculation zone of the groin downstream is the hottest topic discussed due to its key role on the local scour near the structure. The earliest report on the recirculation zone of a thin plate groin was from [PERSON] et al. [12], which presented the structure of the recirculation zone behind the groin and preliminarily investigated the average length and maximum width of the recirculation zone but lacked flow measurement. [PERSON] and [PERSON] [13] further experimentally observed the velocity profiles near a thin-plate groin and the recirculation zone downstream. They stated that the length of this recirculation is about 12 the times groin length. Subsequently, [PERSON] et al. [14], [PERSON] and [PERSON] [3], and [PERSON] [15] also conducted a series of physical experiments to simulate recirculating flow varied by different impact factors, i.e., Froude number, approaching velocity and model scale. Their experimental data show that the downstream recirculation length varied from 7 to 16.6 times the length of the groin and suggested that downstream channel training is the key in controlling the recirculation length in natural rivers. These early experimental investigations gave a robust basis for the follow-up study of the recirculation zone induced by a groin-like structure, in particular providing validated data for numerical simulation. Considerable efforts have been devoted to numerical simulation of flow around a single groin with the rapid development of computation technology, presenting more detailed information regarding flow field structure around a single groin [16; 17; 18; 19; 20; 21; 22]. The flow within the groin field is virtually stagnant and has a permanent open connection with the main stream, resulting in a continuous exchange of mass and momentum between the groin field and main stream [23]. Using the planar velocity measurement with Particle Image Velocimetry (PIV), [PERSON] and [PERSON] [24] conducted an experimental work of the flow patterns around two groins and the mass exchange processes between the recirculation zone and the main stream. The exchange process was found to be governed by the 2D flow structure generated at the groin tip, which grows in the horizontal direction mainly between two groins because the flow is very shallow. Based on flume experiments and field measurements, [PERSON] et al.