Laboratory Investigation on Bed-Shear Stress Partitioning in Vegetated Flows

Vegetation exerts a strong control in the morphological evolution of fluvial systems. It is therefore important to include the effects of vegetation in fluvial studies and numerical models. By assuming a momentum conservation balance, a common way to analyze the flow resistance in vegetated channels splits the total shear stress, t, into shear stress due to vegetation (or vegetation drag), tv, and bed-shear stress, tb. However, there are no methodologies available to reduce the contribution of each bed-shear stress component, bed and vegetation, when the vegetation is sparse or dense. To study the latter effect, this work is based on an intense experimental investigation. The laboratory experiments were carried out in a tilting flume, using rigid vegetation at three different densities and considering submerged hydraulic conditions. The results of this investigation show that the bed-shear stress contribution reduces considerably in configurations where dense vegetation is present. A method to consider this reduction is proposed and tested with data gathered from the literature.