From the blade scale to the reach scale: A characterization of aquatic vegetative drag

Previous studies have considered vegetative drag at different scales, the blade scale, the patch scale, and the reach scale, but few studies have considered the connection between these scales. In this paper, we develop simple, physically-based models that connect processes affecting the drag generated by aquatic vegetation at the blade and patch scale to the hydraulic resistance produced by vegetation at the reach scale. For fully developed flows through submerged patches of vegetation, velocities can be successfully predicted using a two-layer model in which momentum transfer from the unobstructed flow to the vegetation patch is characterized using a constant friction factor. To account for vegetation flexibility in this two-layer model, we develop an iterative procedure that calculates the reduction in plant height and drag for a given flow speed based on the plant material properties, and feeds this information back into the momentum balance. This simple iteration accurately predicts vegetation heights and velocities for submerged flexible vegetation. Finally, we consider the effect of varying vegetation distribution patterns by extending the two-layer model to account for more complex channel and patch geometries. The total hydraulic resistance produced by vegetation depends primarily on the blockage factor, i.e. the fraction of the channel cross-section blocked by vegetation. For a constant blockage factor, the specific distribution of vegetation can also play a role, with a large number of small patches generating more resistance than a single large patch. By considering models with different levels of complexity, we offer suggestions for what field measurements are needed to advance the prediction of channel resistance. (C) 2012 Elsevier Ltd. All rights reserved.