Vegetation in general and mangrove, in particular, plays a significant role in the protection of the coastal and estuarine regions from erosion. In particular, estuarine mangrove forests can effciently reduce the impact of a strong along-bank flow during high tides and high river discharge, creating shelter regions for the development of the ecological system. As estuarine mangrove is usually inundated during high tides and exposed to strong tidal flows, the hydrodynamic of estuarine mangrove forest is similar to that of a vegetated compound channel. In order to gain more insight into this field of the research, a hydraulic laboratory experiment of a shallow flow field in a vegetated compound channel has been conducted. Experimental results confirm a pronounced existence of large horizontal coherent structures (LHCSs). Although the presence of the large horizontal coherent structures in the vegetated channel has been well recognized, it is still unclear how they affect the flow field, mass and momentum exchange in the vegetated compound channel. Detailed analyses of the experimental data reveal more about the role of the large horizontal coherent structures. The flow field under the effect of the large coherent structures shows a spatially and temporally cycloid motion, contributing a large part to the momentum and mass transfer. The quadrant analysis of the Reynolds shear stresses has been applied to examine the connection between the motion of the large horizontal coherent structures and their associated cycloid flow events.