Assessing bed shear stress effects on flow resistance of vegetated channel beds through Leaf Area Index (LAI)

Flow resistance in vegetated rivers is generated by both the drag force exerted by plants and bed friction. Nevertheless, since surface friction is often neglected in the literature, drag is considered the preeminent resistance source. Flume tests were analysed in this study to assess the flow resistance component due to surface friction over a rough bed for different arrangements of artificial flexible plants that resemble plants species typical of vegetated floodplains and riparian areas. The plant drag was measured through drag force sensors and the bed friction factor was estimated by applying the linear superposition principle. It comes out that the contribution of the bed friction must be properly considered in the estimation of the total flow resistance, especially in low-density plants arrangements, where it reaches up to 25 %. Furthermore, the Leaf Area Index (LAI) can serve as an indicator for assessing the variability of bed shear stress across different leaf mass conditions. This study also indicates that LAI is a reliable predictor of plants density classification in vegetated beds, being a robust canopy biomass proxy, independent of the experimental hydrodynamic conditions, much more easily retrievable than the plants density parameter ah, especially for flexible plants and in reach-scale ecohydraulic studies. Specifically, LAI << 0.41 indicates sparse plants, LAI > 0.91 represent dense plants, and values in between correspond to transitional plants density. Finally, it is demonstrated that the classical logarithmic and power flow resistance formulae are not sufficiently accurate to predict bed friction for plants species typical of vegetated floodplains and riparian zones, leading to substantial underestimations (up to 95 %).