Measurement of Lateral and Wake Flows Associated With Streamscale Willow Patches

Västilä, Kaisa; Lee, Chan-Joo; Kim, Donggu; Kim, Sungjung; Kim, Jongmin; Järvelä, Juha

doi:10.3850/38wc092019-1109

Cited by 3 publications

(5 citation statements)

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“…Flume studies are by definition unable to capture all the spatial scales that may be relevant for hydrodynamic processes in natural systems (Bouma et al, 2007). Field and large‐scale studies involving patches of real‐scale riparian vegetation remain scarce (Berends et al, 2020; Ji et al, 2016; Västilä et al, 2019; Zhang et al, 2021). Simulating vegetation with simplified rigid surrogates in physical and numerical models disregards the seasonality and flexibility‐induced flow modifications that may be relevant for understanding and modelling hydromorphological and ecological processes in natural systems (Caroppi et al, 2021).…”

Section: Introductionmentioning

confidence: 99%

Flow and wake characteristics associated with riparian vegetation patches: Results from field‐scale experiments

Caroppi

Västilä

Järvelä

et al. 2022

Hydrological Processes

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Riparian vegetation patches growing on river banks and floodplains influence in‐channel and overbank hydromorphological processes. The current knowledge on patch‐scale hydrodynamics is largely based on laboratory flume experiments with simplified vegetation. The aim of this study is to provide new understanding of the flow and wake characteristics for real riparian vegetation patches based on field‐scale experiments with natural willows, in order to inform hydromorphological and ecological modelling. The focus was placed on the effects of foliage as the main driver of the seasonal changes in vegetation and on the influence of the flexibility‐induced reconfiguration on the flow in the wake and around the patches. The patch drag, defined by its flow blockage factor, was increased by 3.0–4.4 times by the presence of foliage and decreased by up to 60% because of the streamlining and reconfiguration of foliage with increasing flow velocity. Such large changes in the patch drag altered the flow and wake characteristics, affecting the onset of a patch‐scale vortex street. Seasonality and flexibility modified the patch sheltering effect, that is, the magnitude of velocity, turbulent kinetic energy, and bed shear stress reduction in the wake, relative to the background level. In the presence of foliage, mean flow velocity and bed shear stress in the wake were reduced on average by ~50% and ~70%, respectively. The sheltering effect was lower for the leafless conditions than for the foliated conditions. For the foliated cases, the spatial extents of the over‐depth and the near‐bed sheltered region were on average 1.5 and 1.8 times larger than in the corresponding leafless cases, respectively. Overall, seasonal changes in vegetation and flexibility‐induced mechanisms were identified as key controls for the flow associated with patches of riparian vegetation, with major implications on developing models for predicting hydromorphological processes and the potential to preserve and create habitats.

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Section: Introductionmentioning

confidence: 99%

Flow and wake characteristics associated with riparian vegetation patches: Results from field‐scale experiments

Caroppi

Västilä

Järvelä

et al. 2022

Hydrological Processes

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“…Combining the results of Ref. [39], sparse or lowcoverage canopies situated in the center of cross sections enhance velocities close to banks, and bankside patchy vegetation accentuates the cross-sectional velocity variability, thereby increasing the dispersion intensity. It could be preliminarily inferred that the shear stresses of velocity along the transversal direction are the main mechanism contributing to higher x k in channels with vegetation patches.…”

Section: Influence Of Vegetation Characteristics On the Longitudinal ...mentioning

confidence: 64%

“…However, the large Reynolds number and complex canopy structure in natural rivers makes it difficult to predict the velocity variations accurately. For example, the velocities in the vegetated regions are constricted by the drag force of canopies, leading to the regions with low or no flow, termed dead zones, which remarkably increases the resistance time of fine particles and contaminants in the river, floodplains, and wetlands [39] . Considering that the flexible plants in natural rivers are more common and grow in community patterns [40] , many studies have focused on the dispersion coefficient with plant clumps or flexible canopies [41][42] .…”

Section: Introduction mentioning

confidence: 99%

Longitudinal dispersive coefficient in channels with aquatic vegetation: A review

Yang¹,

Fang²,

Yang³

et al. 2023

J Hydrodyn

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The determination of longitudinal dispersion coefficient in rivers is necessary for pollution control, environmental risk assessment, and management. In rivers with aquatic vegetation, the flow field is remarkably modified by canopies, which affects velocity profiles and dispersion characteristics dominated by the heterogeneity of the velocity field. The dispersion is deduced from lateral and vertical longitudinal velocity gradients for compound channels with vegetated floodplains and rectangular channels with river-wide vegetation, respectively. Although many efforts have been exerted to clarify the dispersion process in different conditions and predict the diffusion of contaminants in vegetated rivers, no studies have introduced it systematically. This study reviews the dispersion coefficient characteristics, including magnitude, main impacted factors, and relationships with flow and vegetation features, in channels with aquatic canopies considering the variation of impact factors changing with the different vegetation and river morphology scenarios. Several typical methodologies for determining longitudinal dispersion coefficients are also summarized to understand the dispersion processes and concepts. Apart from the pioneer outcomes of previous studies, the review also emphasizes the deficiency of existing studies and suggests possible future directions for improving the theory of dispersion in vegetated channels.

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“…For instance, patches modify the cross‐sectional velocity distributions (Caroppi et al, 2022; Yamasaki et al, 2019) and generate turbulent fluctuations through the vortices at the lateral shear layers between each patch and open water and in the patch wake regions (e.g. Västilä et al, 2019). Smaller‐scale turbulent fluctuations are caused by wake flows associated with stems and branches, dynamic movements of flexible leaves, and coherent motions of flexible interfaces (Caroppi & Järvelä, 2022).…”

Section: Introductionmentioning

confidence: 99%

“…90 and particularly U d indicated that shear dispersion was the main mechanism contributing to higher D x in the presence of patches. While all riverine patches generally reduce the local streamwise velocities (Figure5;Västilä et al, 2019), sparse or low blockage (B x ⪅ 0.3) patches situated in the central part of the cross-sections increased velocities close to banks, thus generating only slightly higher differential advection compared with non-vegetated conditions. By contrast, patches located particularly in low-velocity regions, such as near the banks, accentuated the cross-sectional velocity variability, hence enhancing dispersion.…”

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confidence: 96%

Longitudinal dispersion affected by willow patches of low areal coverage

Västilä

Sonnenwald

et al. 2022

Hydrological Processes

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Vegetation notably influences transport and mixing processes and can thus be used for controlling the fate of substances in the hydro-environment. Whilst most work covers fully vegetated conditions, the novelty of this paper is to focus on flows with real-scale flexible willow patches. We aimed to investigate how longitudinal dispersion varies according to the spatial distribution, density and coverage of the patches and to evaluate the explanatory power of predictors that consider the hydraulics, vegetation and channel geometry. Salt tracer experiments were performed in a trapezoidal channel where we established 3-4 m long and 1-1.6 m wide patches of artificial foliated willows that reproduced the shapes and plant densities observed on woody-vegetated floodplains. We examined sparsely distributed patches with low areal/volumetric coverage of 6-11%, and non-vegetated conditions for reference.Flow depths and surface widths were 0.7-0.9 and 6-7 m, respectively, and the mean flow velocities ranged at 0.3-0.6 m/s. The emergent patches generated from a negligible to over a four-fold increase in the longitudinal dispersion when compared with non-vegetated conditions. The patches with a preferential location in low-velocity areas, such as near banks, or with a high plant density and a blockage of the crosssectional flow area ⪆0.4, led to the largest dispersion and residence times. Patches under such configurations enhanced the normalized differential velocity defined as the difference between the highest (90th percentile) and lowest (10th percentile) cross-sectional flow velocities divided by the mean velocity, thus increasing shear dispersion. As existing analytical predictors failed to estimate the effect of different patch configurations, we proposed the change in the normalized differential velocity between vegetated and corresponding non-vegetated conditions as a basic predictor of the reach-scale longitudinal dispersion coefficient under patchy vegetation. In contrast, we observed no clear relationship between flow resistance and dispersion. Thus, our findings indicated that bankside vegetation may allow for reduced peak concentrations and lengthened residence times, supporting pollutant management, while ensuring good flow conveyance. Such rare field-scale analyses improve the estimation of solute transport in real vegetated flows.

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Measurement of Lateral and Wake Flows Associated With Streamscale Willow Patches

Cited by 3 publications

References 0 publications

Flow and wake characteristics associated with riparian vegetation patches: Results from field‐scale experiments

Flow and wake characteristics associated with riparian vegetation patches: Results from field‐scale experiments

Longitudinal dispersive coefficient in channels with aquatic vegetation: A review

Longitudinal dispersion affected by willow patches of low areal coverage

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