Linking canopy density and submergence level to vegetative drag and flow dynamics in fluvial systems

Vegetation enhances hydraulic resistance by removing momentum from the flow; however, the effective drag response depends on canopy configuration, including canopy density and submergence. This study examines how the reach-averaged vegetative drag coefficient, CD, varies with vegetation density and submergence ratio for an idealized rigid-stem canopy, correlating the drag coefficient with observed flow patterns. Laboratory experiments were performed in a closed-loop, recirculating flume, utilizing staggered arrays of rigid cylindrical stems to simulate high-stiffness vegetation canopies. Two-dimensional particle image velocimetry measurements were conducted in both vertical and horizontal planes to collect hydrodynamic data and estimate CD from a reach-scale momentum balance with stem Reynolds numbers, Red≈200–800. Velocity fields show that canopy density modifies the spatial distribution of momentum within the patch through vegetative drag, while turbulence statistics vary across the water column and differ between canopy densities and flow regimes. Across all scenarios, CD decreased as Red increased, reflecting the transition from viscous to inertia-dominated flow. Sparse canopies exhibited higher CD than dense arrays due to reduced wake sheltering. Conversely, submerged canopies maintained higher CD than emergent ones because downward vertical momentum flux from the canopy-top mixing layer partially offsets the momentum loss from upstream stem sheltering. Finally, this study identifies a density threshold where the balance shifts between competing sheltering and blockage effects on CD. The results provide new insight into the combined effects of canopy density and submergence on reach-scale drag, with implications for improved parameterization of vegetated flows in hydraulic modeling and river restoration.