Vegetation canopies in turbulent flows often undergo oscillatory motion that modulates drag and reorganizes turbulence near the canopy top. We perform large-eddy simulations of an open-channel flow in which the canopy is modeled as a continuous drag layer using a time-dependent variable-drag model (VDM). The form-drag coefficient is prescribed to vary sinusoidally about its mean value with amplitude $A$ and frequency $f$. We examine how these controls modify the mean flow, the turbulent kinetic energy (TKE) budget, and coherent structures from within the canopy to the outer region. Compared with a steady-drag canopy, temporally varying drag redistributes TKE production, transport, and dissipation: energetic activity shifts from the canopy-top shear layer into the canopy interior, intensifying in-canopy turbulence while slightly reducing the peak production at the canopy top. Increasing $A$ further enhances the TKE production at the canopy top and promotes the associated downward transport of TKE. In contrast, changes in $f$ have only a weak impact on the mean flow but strongly affect the canopy-top velocity statistics. An intermediate $f$ produces velocity PDFs closest to Gaussian, whereas both lower and higher $f$ yield more intermittent, non-Gaussian fluctuations and decrease the phase lag between the drag and canopy-top velocity. Phase-resolved analyses show that coherent structures transition from hairpin-dominated motions within the canopy during intermediate-drag phases to Kelvin-Helmholtz (KH) and KH-hairpin hybrid structures at peak drag, while low-drag phases resemble an open-channel state. These phase-dependent changes are accompanied by systematic variations in sweeps, ejections, and high-speed/low-speed streaks. Overall, the results clarify how temporally varying drag reorganizes shear-layer instability, TKE transport, and sweep–ejection dynamics, and they demonstrate that the VDM provides an efficient framework for isolating turbulence responses to prescribed canopy drag.

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• Kelvin-Helmholtz instability
• Three-dimensional turbulence
• Turbulence simulations
• Turbulent boundary layers
• Vortex interactions