Turbulent anisotropy and coherent structures in flow through a symmetric compound channel with emergent floodplain vegetation

This experimental study advances understanding of the impact of homogeneous grass (HG), homogeneous shrub, and mixed (heterogeneous) vegetation (MV) on floodplains (FPs) and the flow dynamics within a symmetric compound channel. Velocity measurements were conducted using an Acoustic Doppler Vectrino Profiler in the main channel (MC), slope region (SR), and FP to compute depth-averaged mean streamwise velocity, Reynolds stress anisotropy tensors, and coherent structures. The analysis revealed a 54% increase in depth-averaged velocity at the MC centerline with MV compared to the no vegetation (NV) case, marking the highest increase observed among all vegetation scenarios. Reynolds stress anisotropy showed that streamwise turbulence anisotropy dominated in both the MC and SR, particularly with MV, reflecting enhanced momentum exchange due to vegetation. The anisotropic invariant map demonstrated a shift from one-dimensional to two-dimensional (2D) turbulence states and the formation of cigar-shaped turbulence structures, especially in the SR. This shift was attributed to increased momentum exchange and resistance. Quadrant analysis revealed that vegetation on the FP intensified bursting events, enhancing sediment transport in the near-bed region compared to the NV case. At the free surface, outward interactions were more frequent than inward interactions, indicating upward water movement. Higher vortex frequency and energy dissipation observed in the SR with vegetated FP confirmed the presence of quasi-2D coherent structures, facilitating the breakdown of larger vortices into smaller ones. These findings provide critical insights for river system management, emphasizing vegetation's role in altering flow dynamics and enhancing sediment transport.