Characteristics of Turbulence in the Downstream Region of a Vegetation Patch

In presence of vegetation patches in a channel bed, different flow–morphology interactions in the river will result. The investigation of the nature and intensity of these structures is a crucial part of the research works of river engineering. In this experimental study, the characteristics of turbulence in the non-developed region downstream of a vegetation patch suffering from a gradual fade have been investigated. The changes in turbulent structure were tracked in sequential patterns by reducing the patch size. The model vegetation was selected carefully to simulate the aquatic vegetation patches in natural rivers. Velocity profile, TKE (Turbulent Kinetic Energy), turbulent power spectra and quadrant analysis have been used to investigate the behavior and intensity of the turbulent structures. The results of the velocity profile and TKE indicate that there are three different flow layers in the region downstream of the vegetation patch, including the wake layer, mixing layer and shear layer. When the vegetation patch is wide enough (Dv/Dc > 0.5, termed as the patch width ratio, where Dv is the width of a vegetation patch and Dc is the width of the channel), highly intermittent anisotropic turbulent events appear in the mixing layer at the depth of z/Hv = 0.7~1.1 and distance of x/Hv = 8~12 (where x is streamwise distance from the patch edge, z is vertical distance from channel bed and Hv is the height of a vegetation patch). The results of quadrant analysis show that these structures are associated with the dominance of the outward interactions (Q1). Moreover, these structures accompany large coherent Reynolds shear stresses, anomalies in streamwise velocity, increases in the standard deviation of TKE and increases in intermittent Turbulent Kinetic Energy (TKEi). The intensity and extents of these structures fade with the decrease in the size of a vegetation patch. On the other hand, as the size of the vegetation patch decreases, von Karman vortexes appear in the wake layer and form the dominant flow structures in the downstream region of a vegetation patch.

Formation of Coherent Flow Structures beyond Vegetation Patches in Channel

By using model vegetation (e.g., synthetic bars), vortex structures in a channel with vegetation patches have been studied. It has been reported that vortex structures, including both the vertical and horizontal vortexes, may be produced in the wake in the channel bed with a finite-width vegetation patch. In the present experimental study, both velocity and TKE have been measured (via Acoustic Doppler Velocimeter—ADV) to study the formation of vortexes behind four vegetation patches in the channel bed. These vegetation patches have different dimensions, from the channel-bed fully covered patch to small-sized patches. Model vegetation used in this research is closely similar to vegetation in natural rivers with a gravel bed. The results show that, for a channel with a small patch (Lv/Dc = 0.44 and Dv/Dc = 0.33; where Lv and Dv are the length and width of patch and Dc is the channel width, respectively), both the flow passing through the patch and side flow around the patch have a considerable effect on the formation of flow structures beyond the patch. The results of further analysis via 3D classes of the bursting events show that the von Karman vortex street splits into two parts beyond the vegetation patch as the strong part near the surface and the weak part near the bed; while the middle part of the flow is completely occupied by the vertical vortex formed at a distance of 0.8–1 Hv beyond the vegetation patch, and thus, the horizontal vortexes cannot be detected in this region. The octant analysis is conducted for the coherent shear stress analysis that confirms the results of this experimental study.

Numerical analysis of hydrodynamics influenced by deformed bed due to near-bank vegetation patch

This study with a 2D hydro-morphological model analyzes hydrodynamics over flat and deformed beds with a near-bank vegetation patch. By varying the patch density, the generalized results show that the hydrodynamics over deformed beds differs a lot from those over flat beds. It is found that the deformed bed topography leads to an apparent decrease in longitudinal velocity and bed shear stress in the open region and longitudinal surface gradient for the entire vegetated reach. However, the transverse flow motion and transverse surface gradient in the region of the leading edge and trailing edge is enhanced or maintained, suggesting the strengthening of secondary flow motion. Interestingly, the deformed bed topography tends to alleviate the horizontal shear caused by the junction-interface horizontal coherent vortices, indicating that the turbulence-induced flow mixing is highly inhibited as the bed is deformed. The interior flow adjustment through the patch for the deformed bed requires a shorter distance, La, which is related to the vegetative drag length, (Cda)−1, with a logarithmic formula (La = 0.4ln[(Cda)−1] + b, with b = 3.83 and 4.03 for the deformed and flat beds). The tilting bed topographic effect in the open region accelerating the flow may account for the quick flow adjustment.