Scale-dependent Effects of Bank Vegetation on Channel Processes: Field Data and CFD Modeling

Bank vegetation substantially influences flow resistance, velocity, shear stress distributions, and geomorphic stability in many natural river settings. We analyze field data from gravel bed streams and rivers with various bank vegetation characteristics and employ three-dimensional computational fluid dynamics (CFD) modeling to examine whether the effects of bank vegetation on channel form and processes are scale-dependent. Field data from the US and UK indicate that mean bankfull dimensionless shear stresses are significantly higher in channels with thick woody vegetation, but only for channel widths less than ca. 20 m. Because specific mechanisms controlling the apparent scale-dependency are difficult to isolate in natural channels, we develop a method of representing coarse beds that, when coupled with a porous zone representation of bank vegetation, expands the utility of CFD for investigating physical processes in natural channels with variable bed roughness and bank vegetation. The CFD representation of bed roughness and bank vegetation is accomplished in the FLUENT application by computing drag forces using physical characteristics of bed material and vegetation, and applying these to the momentum equations. The CFD models are applied in case studies to improve mechanistic understanding of patterns in the field data. The scale-dependent bank vegetation effects on shear stress distributions in the CFD representations are consistent with field data from gravel bed streams and suggest that the length scale of bank vegetation protrusion relative to channel width is an important factor that could improve shear stress partitioning models. The scale-dependent influence of bank vegetation also has important implications for restoration design of gravel bed streams based on a "tractive force" or dimensionless shear stress criterion.