Flow Resistance Decomposition in the Presence of Leafy Flexible Vegetation and Sand Dunes

In open‐channel flows, hydraulic resistance is influenced by various factors, including sediment, channel geometry, and vegetation. Understanding how total resistance partitions into skin friction and form drag is fundamental for improving sediment transport predictions and advancing river morphodynamics knowledge. This study investigates the composition of total bed shear stress in environments featuring leafy flexible vegetation and sand dunes. Laboratory experiments were conducted under both mobile‐bed and fixed‐bed conditions using artificial plants with removable leafy branches. For mobile‐bed experiments, bed shear stress components were predicted using literature models for skin friction and form drag associated with bedforms and vegetation, whereas for fixed‐bed experiments they were derived from drag measurements. Results showed that the linear superposition principle fails when leafy vegetation is present. In such cases, total bed shear stress, estimated using the depth‐slope product, deviated, on average, by 52% under mobile‐bed conditions and 35% under fixed‐bed conditions from the sum of the individual stress components. In contrast, deviations averaged 15% in leafless setups. Under fixed‐bed conditions, total bed shear stress, inferred from drag measurements, exceeded model predictions by a factor of 2.2–3.3. Although the same dune model was used in all fixed‐bed setups, resistance coefficient associated with dune‐induced form drag increased exponentially with vegetation roughness density. This indicates existing models may underestimate dune‐related drag when leafy vegetation is present. Results highlight the role of foliage configuration in controlling total bed shear stress through non‐linear interactions with dune‐related form drag and indicate the need for predictive models accounting for such coupled effects.