Flow Characteristics of Flow Through Vegetation of Rigid-Base-Flexible-Top

Vertically stratified riparian vegetation typically combines rigid stems near the bed with a more flexible upper canopy, yet its hydrodynamic effects are less understood than those of uniformly rigid canopies. Experiments were conducted in a water flume on its half-bed covered with a terrace-like vegetation patch composed of tall and short rigid-base-flexible-top elements at two flow depths (15 and 22 cm). Streamwise velocity profiles (with a 1 cm vertical resolution) were measured at 27 locations throughout the entire cross-section, including positions behind vegetation, behind the gap in vegetation, and in the free-flow zone. The measured results show that the velocity profiles in the far-end free-zone (FZ2) are C-shaped, whereas near the vegetation–free interface, they deviate from the log-law due to lateral shear and mixing. Within the patch of vegetation, the gap–wake contrast depends on canopy type: BG (behind vegetation gaps) and BST (behind vegetation) reverse with height in tall vegetation but show the opposite pattern in short vegetation, reflecting differences in drag distribution and effective porosity. Under complete submergence, both subzones exhibit a four-layer structure; however, only the short-canopy area-mean profile shows a clearer J-shaped recovery and a limited log-law tendency, whereas the tall-canopy case remains influenced by canopy-top mixing. Depth-averaged velocity is strongly reduced in the vegetated zone and recovers sharply across the transition, with an additional near-wall reduction in the free zone consistent with sidewall boundary-layer effects.