Assessment of different vegetation friction approaches in numerical hydrodynamic modeling

The correct representation of the interaction between flow and vegetation in numerical hydrodynamic modeling has been of growing importance in recent years. As conventional roughness approaches like Manning-Strickler or Nikuradse fail to represent the hydraulic resistance of vegetation for various hydraulic conditions in depth-averaged models, vegetation approaches must be applied. These approaches calculate the time-variable hydraulic resistance of plants according to the plants' characteristics and the hydraulic conditions within the numerical hydrodynamic simulation. In the open-source numerical modeling software openTELEMAC-MASCARET, eight vegetation approaches are implemented. These approaches account for flexible and rigid plants in emergent and submerged conditions. In addition, a biomorphodynamic model was integrated into openTELEMAC-MASCARET. This biomorphodynamic model uses a drag force approach combined with Stone and Shen's (2002) approach to calculate the flow velocity in submerged flow conditions to account for the hydraulic resistance of plants. In this study, we compare the representation of vegetation in numerical modeling as a drag force according to the biomorphodynamic model with the implementation as a friction approach. This way of implementation also allows a comparison with the above-mentioned vegetation friction approaches. Additionally, Stone and Shen (2002) have developed an approach for calculating the hydraulic resistance, which we also implement to compare the performance of this approach. Therefore, we first convert the drag force approach of the biomorphodynamic model into a friction approach according to existing approaches. The resulting approach and the original hydraulic resistance approach of Stone and Shen (2002) are then implemented in openTELEMAC-MASCARET. In order to evaluate the performance of these two approaches, we compare the resulting Darcy-Weisbach friction factors to those of the existing approaches. Using a simplified test case within openTELEMAC-MASCARET, we calculate the bed shear stress in vegetated areas and compare it with the results of the biomorphodynamic model. The results indicate a good agreement between the newly implemented vegetation approaches and the existing ones and the biomorphodynamic model. This study thus lays the foundation for further numerical investigations using vegetation approaches, especially concerning the interaction between vegetation, sediment, and flow.