Hydrodynamics of longitudinally discontinuous, vertically double layered and partially covered rigid vegetation patches in open channel flow

Abstract

The aim of this paper is to numerically investigate how the flow structures are affected through a longitudinally discontinuous and vertically two‐layered vegetation occupying half width of the channel, with steady flow rate and subcritical conditions. A three‐dimensional (3‐D) Reynolds stress turbulence model (RSM), incorporated by Computational Fluid Dynamics (CFD) code FLUENT, was first validated with the experimental data, and then used for simulation purpose. The results showed that the flow stream‐wise velocities within the gap regions are visibly slower than that in the vegetation patch regions. Along the cross section, the velocity in the vegetation region (VR) reduced significantly due to resistance offered by the vegetation, which affected the channel conveyance; as compared to the free (non‐vegetated) region. The flow instability in the lateral direction was triggered by the flow shear due to the presence of partly distributed vegetation, resulting in the formation of coherent vortices and exchange of momentum at the interface. The discharge percentage passing through the free region (FR) was found to be 144–525% larger than that passing through the VR. The flow resistance increased significantly with higher vegetation density, whereas it decreased when both the vegetation layers were submerged. Moreover, the flow characteristics profiles in large gaps were more stable than in small gaps. The turbulent kinetic energy (TKE) and turbulence intensity also increased significantly through the patch regions compared to that of the gap regions. The results indicated that the flow structures and the flow resistance are strongly influenced by partial and discontinuous vegetation.

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

“…Several previous researchers implemented flow simulation techniques through a uniform array of vegetation to simulate the turbulent flow (Cui & Neary, 2008; Ghani, Anjum, Pasha, & Ahmad, 2019; Lopez & Garcia, 2001; Neary, 2003). The numerical studies on investigating the flow structures in vegetated open channels utilizing CFD tool FLUENT have also been reported (Anjum & Tanaka, 2020; Lima, Janzen, & Nepf, 2015). However, in riparian environments or flood plains of several natural rivers, finite length vegetation can occur as partial distribution, with submerged as well as emergent condition of constant vegetation heights (Devi & Kumar, 2016; Devi, Sharma, & Kumar, 2019).…”

“…Previous studies have been discussed separately; however, the studies elucidating the influence of different vegetation formations and their submergence while keeping the same vegetation density in the pattern of discontinuous and partly distributed in open channel flows are limited. Although some of the flow structures through only double‐layered vegetation (DLV) configuration with a variety of patterns have been numerically clarified by Anjum and Tanaka (2020), the investigations of the turbulent flow behaviours through double‐layered and single‐layered vegetation (with both submerged and emergent conditions) while keeping the constant vegetation density is still limited. The flow mechanism (demonstrating the comparison) around partially distributed discontinued homogenous (constant formation) and heterogeneous (varying heights) vegetation would give an extra understanding of the riparian environment and a clear idea of true representation of the vegetal communities along the natural river flows.…”

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

“…can be found in natural environments. Shorter plants, like shrubs and grasses, are submerged by the flood flow during floods in riparian environments or on wetlands, whereas taller vegetation, like trees, maintain their emergent state [14].…”

“…Natural rivers typically have variety of discontinuous vegetation communities. As a consequence, a unique distribution of the flow structure would arise from the combination of discontinuous and vertically stratified vegetation (which is usual in natural channels), the complexity of which required to be investigated [14]. As mentioned above, there have been a few studies on turbulent flow in an open channel with partially covered, discontinuous,and layered vegetation patches for management of vegetation in the channel.…”

Impact of Different Vegetation Zones on the Velocity and Discharge of Open-Channel Flow

“…The flow interacts with differentlayered vegetation, thus leading very complex flow structure. To grasp the role of multiple-layer vegetation to the flow, there is some research on the flow of openchannel with the entire bed covered by an array of tall and short vegetation (e.g., [21,25,27,28]), whereas numerical modelling was carried out to reveal the flow characteristics of open-channels with patched double-layered vegetation [29]. Most recently, Tang et al [30][31][32] undertook an experimental investigation into the openchannel flow where the channel bed was partially covered with double-layer vegetation.…”