Un Ji

Understanding flow behavior in compound channels with floodplain vegetation is essential for modeling river hydraulics and ecohydraulics processes. This study conducted a controlled large-scale flume experiment in an asymmetric trapezoidal compound channel to examine the effects of emergent rigid cylindrical vegetation on mean flow, turbulence, momentum exchange, and coherent structures. Vegetated and nonvegetated sections were compared using water-level measurements and three-dimensional acoustic Doppler velocimetry. Floodplain vegetation intensified the velocity gradient at the main channel–floodplain interface, increasing turbulent kinetic energy and streamwise and lateral Reynolds stresses in the transition zone. The vegetated section developed a broader mixing layer and stronger coherent structures, as indicated by larger autocorrelation time scales and peaks in the lateral velocity spectra. Although normalized turbulence statics were consistent with laboratory-scale studies, the spatial and temporal scales of coherent structures were notably larger in the present large-scale experiment. These scale-dependent differences highlight that the need for scale-aware interpretation when using laboratory-derived turbulence and resistance parameters in compound-channel modeling. The data set provides a useful resource for validating numerical models and upscaling laboratory findings to field-relevant two-stage channel configurations.

Vegetation is often retained in river channels for habitat conservation and restoration and to facilitate nature-based flood management, and its hydraulic effects are typically represented using flow resistance coefficients in two-dimensional (2D) models. This study evaluated two Manning’s roughness parameterizations in HEC-RAS 2D against large-scale experimental data for grouped and isolated willow patches under high- and low-flow conditions. A distributed approach, which represents spatially varying total resistance using a momentum-based resistance formulation, was compared with a spatially uniform bulk coefficient applied to the entire reach. Because the bulk coefficient was back-calculated from the measured experimental data, it served as an observation-based benchmark rather than an independently derived prediction. A sensitivity analysis established a terrain resolution of 0.001 m and a mesh size of 0.25 m as appropriate for the simulations. The distributed approach consistently underestimated water levels, exhibited an incomplete but directionally correct response to changes in the drag coefficient for grouped patch configurations, and was negligibly sensitive to changes in the drag coefficient for isolated patch configurations. In contrast, the bulk approach reproduced water levels more closely but overpredicted them for grouped patch configurations and could not resolve local flow structures. Patch arrangement appeared more influential than vegetation density; however, their individual effects could not be separated because the grouped layout was also the densest. These findings provide guidance for selecting resistance parameterizations in 2D models of partly vegetated channels.

Riparian vegetation plays a fundamental role in alluvial channel evolution by modifying flow and sedimentation dynamics. To elucidate the roles of sediment supply, flow‐dependent transport capacity, and morphodynamic feedbacks on the evolution of emergent vegetation patches over a single hydrograph, we conducted two experiments with different reach‐scale sediment supplies: a high‐supply experiment (HSE) and low‐supply experiment (LSE). We measured flow velocities, bedload transport, and topographic changes around a full‐scale patch of live emergent willows in an outdoor laboratory flume. Erosion occurred in the patch‐adjacent channel areas irrespective of the sediment supply, whereas deposition within the patch interior was suppressed in the LSE and enhanced in the HSE. The magnitude of patch deposition in each experiment was controlled by the local sediment supply to the patch and local sediment mobility during each discharge in the hydrograph. The local sediment supply was affected by bed morphodynamics at the patch head, which modulated the reach‐scale sediment supply by redistributing the bedload in the channel. Sediment mobility within the patch was flow‐dependent and a function of velocity and turbulent kinetic energy. For different discharges, the velocity in the patch changed proportionally with the freestream velocity; however, the turbulent kinetic energy was more sensitive, being elevated compared with the freestream during high flows and inhibited at low flows. Therefore, deposition and erosion within vegetation patches are not simply functions of the reach‐scale sediment supply or patch flow characteristics, as is often assumed, but additionally depend on the local sediment supply to the patch interior.

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.