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

Abstract

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.

Longitudinal dispersion affected by willow patches of low areal coverage

“…For example, patch widths as low as 1–2 m create wake flows extending up to tens of meters, so that the velocity does not fully recover between the patches (e.g. Caroppi et al, 2022; Marjoribanks et al, 2019). The uncertainty in modelling the mean velocity distributions in partly vegetated flows (e.g.…”

“…Boxall & Guymer, 2007), which are all influenced at several scales by plants (Marion et al, 2014; Shucksmith et al, 2011). 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).…”

Simulation of Accelerated Subcritical Flow Profiles in an Open Channel with Emergent Rigid Vegetation

“…The analysis of different methods for estimating the drag coefficient described in Section 2.2 is limited exclusively to the case of emergent rigid vegetation when the flow does not affect the leaf apparatus. In the presence of real herbaceous vegetation, the drag coefficient is much lower than that in case of rigid vegetation [41], while it increases significantly when the flow affects the foliage [42][43][44]. It must be said, however, that foliage reconfiguration and inflection of the stems cause a reduction in the area of the affected plant and the drag exerted on it [45,46].…”

Shear layer over floodplain vegetation with a view on bending and streamlining effects

“…Vegetation greatly impacts hydrodynamic processes at multiple scales [17,18], influencing flow organization, geomorphic changes, and habitat dynamics [19,20]. Vegetation introduces additional flow resistance, influencing water-surface elevations and channel conveyance capacity [21,22].…”

“…Consequently, we expect canopies in transitional regime to be rare in such hydro-environments, under comparable submergence conditions. However, seasonality driven changes in vegetative drag, due for example to shedding of leaves [18], can profoundly alter the canopy drag [54,75]. In these conditions, or for higher flow velocities, the bed drag would assume a higher importance, with the transitional and the sparse regime being relevant for describing the flow structure.…”