Hydrodynamic Modelling of Flexible Woody Vegetation With <scp>Delft3D FM</scp> : Implementing Physically‐Based <scp>2D</scp> Flow–Vegetation Interaction Processes

ABSTRACT Riparian vegetation provides crucial ecosystem services but the reliable estimation of its impacts on the hydrodynamic and transport processes is limited by the oversimplified description of plants in numerical models. The aim of this paper is to improve the representation of the interactions between flow and flexible woody vegetation in the 2‐dimensional hydrodynamic model D‐Flow FM. We generalized the existing total resistance approach, implementing a two‐way flow–vegetation interaction process allowing to correctly describe effective bed shear stress. The novelties compared to other numerical models are the realistic representation of woody plants by considering the leaf and stem area indices, the full reconfiguration process including the streamlining of leaves and bending of the stems under flow forcing, and the flexibility‐generated modifications specifically for submerged conditions. 1D simulations representative of floodplain shrubs and young trees demonstrated the 3–7 times higher accuracy of the new model compared to the original model describing plants as rigid cylinders. The consideration of the foliation process supports reliable simulations from leafless to densely foliated conditions. As one of the first validation studies for submerged vegetation, we found that the proposed von Kármán scaling factor improved the predictions. Further, we preliminarily validated the new bed shear stress computation method for the rarely measured case of complex reconfiguring vegetation. We support the mainstreaming of these developments by integrating the described numerical scheme into the open‐source Delft3D FM software, making it accessible to end‐users. Finally, the developed model decreased the uncertainty in flow hydraulics, indicating an improved description of the physical system. Thus, the advances serve for improving understanding of the impacts of varying vegetation conditions on broader hydrological, transport and water quality processes in rivers and floodplains. As the developed model has constant vegetative parameter values under variable conditions, it is expected to improve predictions particularly under non‐calibrated conditions.