The role of increasing riverbank vegetation density on flow dynamics across an asymmetrical channel

Abstract

Over the last two decades, the role of vegetation in the environmental and ecological restoration of surface water bodies has received much attention. In this context, the momentum exchange between the flow through the main channel and the riparian zone is a key mechanism. The primary goal of this study is to investigate the role of bank vegetation density on flow dynamics across the whole channel. This experimental study presents the major findings from a series of flow measurements across a channel having a sloping bank with vegetation at varying densities. The experiments are conducted under the same, uniform flow and fixed bed conditions, for a range of six linear and rectilinear arrangements of incremental streambank vegetation densities. A set of ten velocity profiles is obtained across the test cross-section of the channel, including the riverbank, for each vegetation density. These flow measurements are analyzed to derive roughness coefficients, which are related to the bulk flow velocities through the main channel and the riverbank and discuss the redistribution of flow velocities. An approximate doubling for the estimates of time-averaged boundary shear stress at the main channel, is observed for the case of no to dense vegetation, which enable further discussing implications for the stability of bed surface material. It is found that the vegetation arrangement, in addition to vegetation density, can have a strong impact in modifying the mean flow velocity at the main channel, for low riparian densities (φ < 0.6%). Highlights Flow dynamics are measured across the whole channel, including the vegetated riverbank. As stem density increases, mean flow velocity in the main channel increases while mean flow at the riverbank decreases. The arrangement of riparian vegetation can be as important as that of the density, in modifying the mean flow field of the main channel, for low riparian densities. Bed shear stresses at the main channel are estimated to increase with riverbank vegetation, reducing the stability of the stream’s bed surface.

Characterizing Erosion and Deposition in and Around Riparian Vegetation Patches: Complex Flow Hydraulics, Sediment Supply, and Morphodynamic Feedbacks

“…This indicates that the flow conditions in and around the vegetation patch were primarily a function of vegetation characteristics rather than reach‐scale conditions or bed morphology (e.g., Diehl, Wilcox, et al., 2017; Etminan et al., 2018; Ji et al., 2023; Västilä & Järvelä, 2018), at least at the scale of the morphologic changes observed in this study. In both experiments, streamwise velocity within the vegetation patch was reduced to 60%–75% of that in the freestream owing to increased vegetative roughness, while flow constriction in the adjacent main channel increased the velocity by approximately 5%–20% (Figure 2b), which agreed with previous research on sparse uniform vegetation in open channels (e.g., Caroppi et al., 2022; Kim et al., 2015; Lightbody et al., 2019; Valyrakis et al., 2021). Interestingly, the side channel had reduced flow velocities compared with the freestream, which contrasts with previous observations of flow acceleration ($$\left\langle \frac{u}{{u}_{0}}\right\rangle $$ > 1) in the gap between narrowly spaced dense patches (e.g., Meire et al., 2014; Vandenbruwaene et al., 2011).…”

Impacts of Vegetation on Dryland River Morphology: Insights From Spring‐Fed Channel Reaches, Henry Mountains, Utah

“…This direction of change is consistent with Trail Canyon reaches with denser bank vegetation (Figure 10). Increased bank vegetation has also been shown to increase bed shear stress through feedbacks with depth and velocity (Valyrakis et al., 2021). Interestingly, Figure 9 indicates that the width scaling of sparsely vegetated Woodruff and Trail reaches is similar in spite of grain size differences.…”

“…A proposed mechanism is increased hydraulic roughness from vegetation leading to higher adjacent turbulence and shear stresses, including near river banks (e.g., Hopkinson & Wynn, 2009; Yager & Schmeeckle, 2013). However, bank vegetation can slow near‐bank velocities and reduce local shear stresses as well (e.g., Valyrakis et al., 2021). Bed vegetation may also enhance bed deposition in places, potentially shallowing and widening channels (e.g., Molina et al., 2009; Tooth & Nanson, 2004).…”

Turbulence structure and bank erosion process in a dredged channel

“…The vegetation can instead destabilize the riverbank by scouring the toe region and leading to mass failure. Their study idealized the bank as non‐erodible and the vegetation as rigid cylinders (Li, Liu, & Huai, 2022; Liu, Valyrakis, & Williams, 2017; Meftah, de Serio, & Mossa, 2014; Valyrakis, Liu, Turker, & Yagci, 2021; Yang, Bai, & Xu, 2018). Studies have suggested that tension cracks occur in a non‐cohesive soil riverbank (Arai et al, 2018; Liang, Jaksa, Ostendorf, & Kuo, 2015).…”