Flow resistance of floodplain vegetation mixtures for modelling river flows

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.

Automated summary

This study investigates the flow resistance of flexible floodplain vegetation mixtures under a range of mean flow velocities and low relative submergences. The results show a significant decrease in flow resistance when the submergence increases, providing new evidence for extending LAI-based modeling to a wider range of vegetation densities. Adjustments to existing LAI-based approaches are needed for high water stages and the contribution of understory grasses to flow resistance should not be underestimated.