A viscous drag concept for flow resistance in vegetated channels

A conceptual model of hydraulic resistance for vegetated channels is proposed based on an assumption that the total drag is dominated by viscous (or skin) drag on plant surfaces while form (or pressure) drag can be neglected. The double-averaging methodology provides a consistent way for linking spatially averaged flow properties with vegetation roughness characteristics and the bed shear stress obtained for the same averaging domain. Applying this methodology we show that the hydraulic resistance factor can be expressed as a function of the effective plant surface area where viscous drag acts. As direct measurements of the effective plant surface area are difficult, we propose several relationships expressing the effective area through such easy-measured parameters as the plant biomass density, plant total volume and the ratio of the site-averaged canopy height to the mean flow depth also known as the relative roughness or blockage factor. The model is tested using laboratory and field data collected by the authors themselves as well as published data. Our results indicate that the viscous drag is the key contributor to the total drag in streams with natural aquatic vegetation, as opposite to a common perception that the form drag is most significant. Thus, the data provide convincing support to the model, and show that it may offer an alternative way of interpreting hydraulic data for natural vegetated channels. The results also indicate that there is a self-regulation mechanism that makes an areal density of wetted plant area approximately the same among species and sites.