Next Meeting May 26, 2010

The next meeting will be on the campus of Colorado State University (see details on the right). Our speaker will be Dr. Brian Bledsoe. The title of his talk, and a brief abstract can be found below:

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

ABSTRACT

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 typed 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 models are applied in two sets of simulations to improve mechanistic understanding of patterns in the field data and examine: 1) spatial scale dependency between channel width and vegetation effects, and 2) the co-evolution of flow hydraulics, channel form, and vegetation establishment. 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. In general, the field data and CFD simulations indicate a significant scale-dependent effect of bank vegetation that has not been previously accounted for in downstream hydraulic geometry relationships, regime slope models, and shear stress partitioning schemes for gravel bed rivers. Accordingly, the scale-dependent influence of bank vegetation also has important implications for stream restoration designs based on tractive force, regime, and analytical approaches.