Investigating the turbulent flow behaviour through partially distributed discontinuous rigid vegetation in an open channel

Abstract

Riparian vegetation does not only affect the channel flow carrying capacity, but also plays significant roles in water management, stream restoration, and river rehabilitation. This study numerically investigates the flow characteristics through longitudinally discontinuous rigid vegetation occupying half width of the channel, with the help of three-dimensional software FLUENT in which a Reynolds stress turbulence model was adopted. Three varying conditions of vegetation were considered comprising of vertically double-layered vegetation (DLV), submerged vegetation (SV), and emergent vegetation (EV) while keeping the same vegetation density, as well as a varying discharge condition against DLV was also tested. The results indicated that the flow distribution becomes more complex through DLV and SV followed by multiple layers with an inflectional instability in the vertical velocity profile around submerged canopy top, as compared to the flow through EV where the uniform distribution of flow over the canopy column was observed. The velocity in the canopy zone decreased considerably because of the resistance due to vegetation, which influenced the channel carrying capacity, in comparison to that in the non-vegetated zone. The flow velocities through the obstructed part of the channel, that is, canopy zone, in DLV arrangement reduced by a percentage difference of approximately 42 and 37% compared to that of SV and EV arrangements, respectively; whereas it was reduced by approximately 55% when the discharge was twice while keeping the same configuration of DLV. The inflectional instabilities and estimated mixing layer over the interfacial zone suggested a stronger lateral exchange of momentum for DLV configuration in comparison to that of SV and EV. Within the gaps between the patch zones, the flow velocity, turbulent kinetic energy, and turbulent intensity reduced significantly due to blockage effect and sheltering offered by the vegetation patches, signifying a positive flow response towards aquatic life and sediment deposition.

Flow characteristics of open channels based on patch distribution of partially discontinuous rigid combined vegetation

“…For the simulated vegetation, since the object of this study is the combined vegetation patches of different stem thicknesses, and the vegetation stems are usually cylindrical, so this study uses cylinders with different diameters to simulate vegetation. In addition, in order to enhance the simulation and experimental effects and improve the efficiency, most of the current studies also generalize the whole plant to an effective water blocking cylinder ( Zhao and Huai, 2016 ; Anjum and Tanaka, 2020 ; Wang et al., 2021 ). In this study, a cylinder with a height of h v of 0.08 m was used to simulate open channel vegetation.…”

“…For example, Kazem et al (2021a); Kazem et al (2021b) studied the vortex structure in the channel of vegetation patch and the characteristics of turbulence gradually subsiding in the undeveloped area downstream of the patch by changing the patch size, and its research results showed that the presence of patches has a great influence on the flow structure inside and around the patches, and there are three different flow layers downstream of vegetation patches: wake layer, mixed layer and shear layer. However, studies on the patch distribution of partially discontinuous vegetation on open channel flow is not systematic enough, although this vegetation distribution pattern is more common in natural channels (Anjum and Tanaka, 2020;Li et al, 2020). In addition, most current studies use the uniform size of vegetation for generalized simulation processing, which does not conform to the natural situation of river vegetation (Sand Jensen and Madsen, 1992).…”

Numerical Simulation of Discontinuously Vegetated Open Channel Flow to Estimate Effects of Vegetation Condition on Flood Mitigation

“…Anjum and Tanaka [25] used the k − ϵ model to elucidate the flow structure through discontinuous vertically layered vegetation that had been covered the channel's entire width.Large eddy simulation (LES) model was also applied by Huai et al [26] to simulate turbulent flow in a partially vegetated open channel. Anjum et al [27] investigated turbulent flow behavior in an open channel with partially dispersed discontinuous stiff vegetation. They found that the velocity in plant patches was lower than that in the non-vegetated zone due to plant resistance, which affected channel carrying capacity.…”

Modeling depth‐averaged streamwise velocity in a channel with one‐line emergent vegetation patches

“…An array of porous vegetation patches with patch‐to‐patch interactions, such as reeds, is common in rivers but has not been adequately investigated. Few studies have investigated the effect of patchy vegetation on flow structures (Anjum & Tanaka, 2020; Ghani, Anjum, Pasha, & Ahmad, 2019). Providing accurate predictions about the depth‐averaged streamwise velocity profiles in the flow with an array of discontinuous vegetation patches remains challenging.…”

“…An array of porous vegetation patches with patch-topatch interactions, such as reeds, is common in rivers but has not been adequately investigated. Few studies have investigated the effect of patchy vegetation on flow structures (Anjum & Tanaka, 2020;Ghani, Anjum, Pasha, & Ahmad, 2019)…”