Scale Effects of Patch Configuration and Density on Vegetation Drag in Floodplain Flows

Vegetated floodplains modulate river hydraulics through drag forces that affect flow conveyance, sediment transport and floodplain ecosystems. Non-uniform vegetation patches are widespread in natural floodplains and challenge drag prediction models, as the scale-dependent effects of patch configuration, canopy density and submergence remain inadequately quantified. Here, we directly measured drag forces on nature-like flexible woody vegetation across individual plant, isolated patch and reach-scale patch set-ups using a novel multi-scale drag measurement system in a recirculating flume. Investigations span emergent (H/h_d ≈ 1) and submerged (H/h_d ≈ 2) regimes, with patches varying in planform configuration and density. Results show that drag increased non-linearly with spatial scale. For instance, reach-scale patches having the same number of plants and canopy density (LAI) as the isolated ones exerted 1.1–2.2 times greater drag than isolated patches due to canopy continuity and wake interactions. Normalization of drag by one-sided leaf area revealed strong dependence on patch configuration and flow velocity, whereas normalization by newly defined configuration–density parameter (D_W) reduced inter-configuration variability, providing a scaling framework from emergent to submerged conditions. For patches submerged to twice the deflected canopy height, the total drag was reduced by roughly half compared with emergent condition. These findings show that vegetation-induced drag depends jointly on spatial scale, configuration, plant density and submergence. Overall, the results establish a physical basis for linking vegetation-induced drag to flow resistance and flow distribution, which together determine key ecohydrological functions such as flow partitioning, sediment retention and hydraulic connectivity across vegetated floodplains.