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Highlights: [•] New insights into the flow resistance of foliated vegetation were obtained. [•] We measured drag and alternative reference areas for natural poplars in a flume. [•] Partly and just submerged plants had similar drag and frontal area per reference area. [•] Drag and reconfiguration were controlled by the foliage–stem reference area ratio. [•] The alternative reference areas satisfactorily predicted the drag and flow resistance. [ABSTRACT FROM AUTHOR]

• Novel flume setup combining high-accuracy pressure and force sensors. • Extension of LAI-based modelling of vegetative resistance to low and high densities. • Flow resistance contribution by the understory grasses should not be neglected. River flows are greatly influenced by floodplain vegetation with implications on hydrological and hydraulic conditions from cross-sectional to river reach scales. Flow models need to reliably reflect changes in the riverine environment, such as vegetation growth associated with altered flow regimes, increased sediment loads and eutrophication. Leaf area index (LAI) based approaches are increasingly used as tools to predict the flow resistance caused by natural vegetation. However, current LAI-based modelling involves uncertainty at low and high vegetation densities and flow velocities due to a lack of research and validation at the outer ranges. The aim of this paper is to investigate the flow resistance for a mixture of flexible floodplain vegetation consisting of woody plants and understory grasses of low to high densities (LAI = 1–5) over a wide range of mean flow velocities (0.05–1.2 m/s) at low relative submergences of 1–2. A novel flume setup was designed by using high-accuracy pressure sensors to measure flow resistance and force sensors to measure plant drag forces. Flow resistance decreased by 35–90% when the submergence H / h v increased from 1 to 2, which is a highly relevant range for floodplain flows. The results provided new evidence that LAI-based modelling of vegetative friction factors can be reliably extended from low to high LAI values for non-submerged vegetation. However, adjustments to existing LAI-based approaches are required for water stages higher than the vegetation height. Furthermore, the friction by the understory grasses cannot be neglected as often assumed in literature especially for cases of low plant densities and low relative submergences. [ABSTRACT FROM AUTHOR]