Laboratory measurements at 1:1 scale and model development on the deformation and roughness of flexible vegetation in rivers

Vegetation is becoming increasingly important in flood protection, as current hydraulic engineering projects also pursue ecological objectives and vegetation is an integral part of natural and restored rivers. In addition, vegetation functions such as shading and flow retention are deliberately utilised, which are becoming increasingly important in the context of climate change in view of rising water temperatures and increasing flood discharges. During flood events, vegetation interacts with the flow and sediment transport, and the changes in bed elevation and flow resistance caused by this interaction influence water levels and thus flood safety. Up to now, the flow resistances in hydrodynamic-numerical models have been taken into account by a roughness coefficient. This is usually assumed to be constant, whereas flexible vegetation adapts its shape and thus adjusts the hydraulic resistance.We present developments of a numerical model of the mechanical deformation of a plant under flow loading, which discretises the plant as a rod model using the finite element method. The geometry of the analysed plants was recorded by photogrammetry and supplemented with measurements of the stem and branch diameters and the foliage. Parameters concerning the stiffness against bending and torsion were recorded as a function of the diameters of the branches. The discharge of up to 10 m³ s⁻¹ available in the BOKU River Lab allowed experiments on a 1:1 scale on individual plants and entire vegetation patches. Measurements of the deformation and drag force of individual plants, as well as the flow velocities in vegetation patches, form the basis for testing the model. A conversion of the drag force into a roughness coefficient enables the model to be integrated into two-dimensional hydrodynamic-numerical models, which are frequently used in the simulation of flood discharges.