Matching the Vegetation Component of Flow Resistance in SRH-2D with a Porous Media Model in OpenFOAM

In the depth-averaged 2D open channel flow software, SRH-2D, flow resistance caused by granular sediment bed roughness and vegetation that may protrude upwards through a large fraction of the flow depth is characterized by a composite Manning number consisting of a sediment grain roughness, a bed form roughness, and a vegetation component. In the 2D flow model, these effects show up in a composite shear stress that is the sum of grain, bed form, and vegetation components. In a 3D CFD flow model, the grain roughness is characterized in the bed boundary condition with a roughness height, and its effect shows up in the bed shear stress. The flow resistance of vegetation, however, is distributed through the volume of the domain from the bed to the top of the vegetation, which for bushes or trees may be the entire depth of the flow. Its presence causes a volumetric momentum loss that can be modelled using a momentum sink term derived from the Manning equation. Results of both 2D and 3D simulations are compared for a main channel with adjacent flood planes with vegetation. Vegetation resistance on the flood plains causes some of the flow over the flood plains to shift to the main channel compared to flood plains with no vegetation. Shear stress from sediment grain roughness agrees reasonably well between SRH2D and OpenFOAM away from obstructions that intersect the bed, such as piers, and the effect of the vegetation resistance part of the Manning number on the energy grade line also agrees well between 2D and 3D software with a porous media model for vegetation upstream of piers and abutments.