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

Abstract

AbstractRiparian 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.