In Search of Friction Laws for Vegetated Flow within 2D Large Scale Applications

As restoration and conservation of the ecosystem in federal inland waterways in Germany are of growing importance, vegetation in open-channels plays an increasingly important role. New approaches to estimate the vegetation effects on the flow within large-scale applications are needed, e.g. for flood protection planning and for more nature-friendly designs of inland waterways. In an ongoing research at the BAW, five promising approaches to estimate the flow resistance due to vegetation are examined with numerical simulations of experimental data using Telemac-2D software first and are then analytically investigated. The essence of the governing physics of vegetated open-channel flows is given first with a focus on the most challenging issue of this subject, i.e. vegetation flexibility, which can significantly alter the flow resistance due to vegetation. In the numerical simulations, the approaches of Järvelä (2004), Whittaker et al. (2015) and Baptist et al. (2007) performed best and showed good agreement with the measured values from low to high vegetation friction. The analytical investigation was performed using three vegetation types. Results showed similar behaviors of the friction coefficients estimated by the approaches in the case of non-submerged vegetation, but widely dissimilar behaviors in the case of submerged vegetation. Also, results showed high sensitivity of the Järvelä (2004) and Whittaker et al. (2015) approaches to the empirical flexibility parameters used. In terms of large-scale applications the leaf-area index seems to be a suitable parameter to determine the vegetation density.