Field-Scale Analysis of Flow Dynamics and Turbulent Structures in a Compound Channel with Emergent Rigid Floodplain Vegetation

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.