Stream-scale flow experiment reveals large influence of understory growth on vegetation roughness

Water Resources Research

Gualtieri

et al. 2021

In a variety of environmental settings, including vegetated channel banks and rivers, and their associated floodplains and riparian areas, vegetation occurs along river margins, partially obstructing the cross-section. In these systems, referred to as partly vegetated channels, the long regions of finite width of emergent vegetation laterally interact with the flow, deeply altering the mean and turbulent flow structure with implications on the conveyance capacity of the channel, the water levels, and the mass and momentum exchange processes (

Section: Methodsmentioning

confidence: 99%

Comparison of Flexible and Rigid Vegetation Induced Shear Layers in Partly Vegetated Channels

Caroppi

Water Resources Research

Gualtieri

et al. 2021

“…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

Järvelä

et al. 2022

Hydrological Processes

Self Cite

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.

“…We acknowledge that our data covers a limited range of conditions but expect that the results can be extended to slightly outside the studied ranges of flow velocity, plant density and coverage. The study setting lacking grassy understory likely under-estimates D x for the commonly observed case with grasses growing below the foliage zone of woody shrubs (Berends et al, 2020;Carling et al, 2020). It 4) and ( 5)), because they do not describe the changes in the differential advection.…”

Section: Benefits and Limitations Of The Experimental Methodologymentioning

confidence: 99%

Longitudinal dispersion affected by willow patches of low areal coverage

Sonnenwald

et al. 2022

Hydrological Processes

Self Cite

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.