The influence of vegetation canopies on the flow structure in streams, rivers, and floodplains is heavily dependent on the cumulative drag forces exerted by the vegetation. The drag coefficients of vegetation elements within a canopy have been shown to be significantly different from the well‐established value for a single element in isolation. This study investigates the mechanisms that determine canopy flow resistance and proposes a new model for predicting canopy drag coefficients. Large Eddy Simulations were used to investigate the fine‐scale hydrodynamics within emergent canopies with solid area fractions ( λ) ranging from 0.016 to 0.25. The influences of three mechanisms in modifying canopy drag, namely, blockage, sheltering, and delayed separation, were investigated. While the effects of sheltering and delayed separation were found to slightly reduce the drag of very sparse canopies, the blockage effect significantly increased the drag of denser canopies ( λ≳0.04). An analogy between canopy flow and wall‐confined flow around bluff bodies is used to identify an alternative reference velocity in the definition of the canopy drag coefficient; namely, the constricted cross‐section velocity (Uc). Through comparison with both prior experimental data and the present numerical simulations, typical formulations for the drag coefficient of a single cylinder are shown to accurately predict the drag coefficient of staggered emergent canopies when Uc is used as the reference velocity. Finally, it is shown that this new model can be extended to predict the bulk drag coefficient of randomly arranged vegetation canopies.

Shear stresses on vegetated beds play an important role in driving a wide range of processes at the sediment‐water interface, including sediment transport. Existing methods for the estimation of bed shear stress are not applicable to vegetated beds due to the significant alteration of the near‐bed velocity profile and turbulence intensities by the vegetation. In addition, bed shear stress distributions are highly spatially variable in the presence of vegetation. In this study, computational fluid dynamics simulations were used to investigate the spatial variability of bed shear stresses in the presence of emergent vegetation (modeled as arrays of circular cylinders) and the variation of bed stress with characteristics of both the bulk flow and the array. A recently proposed model that assumes a linear variation of stress in the viscous layer immediately above the bed is shown to be a reliable tool for estimating the spatially averaged bed shear stress over a wide range of flow conditions and vegetation densities. However, application of this model is found to be restrictive due to the lack of a reliable predictive tool for the thickness of the viscous layer. Based on a balance between turbulent kinetic energy production in the vegetation stem wakes and the viscous dissipation of turbulent kinetic energy at the bed, an enhanced formulation is proposed to predict the thickness of the viscous layer, which significantly improves the accuracy of model predictions. This improved model enhances the predictive capability for important benthic processes (such as sediment transport) in vegetated aquatic systems.

Aquatic vegetation provides numerous ecosystem services in riverine and coastal environments. For instance, vegetation can help to stabilize mobile sediment beds by transforming an erosional bed into a depositional one (Shields et al., 1995). The enhanced drag forces induced by the vegetation can also modify circulation and sediment transport pathways, which over time can shape the morphological evolution of riverine and coastal systems

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