E-proceedings of the 38 th IAHR World Congress September 1-6, 2019, Panama City, Panama doi:10.3850/38WC092019-0438 5610 LABORATORY INVESTIGATION ON BED-SHEAR STRESS PARTITIONING IN VEGETATED FLOWS MATEO MUNAR-MARTINEZ (1) , ANDRÉS VARGAS-LUNA (2) & ANDRÉS TORRES (3) (1) Master candidate in Hidrosistemas, Pontificia Universidad Javeriana. Bogotá, Colombia. E-mail: munar_williamm@javeriana.edu.co. (2) Associate professor, Pontificia Universidad Javeriana, Bogotá, Colombia. E-mail: avargasl@javeriana.edu.co. (3) Full professor, Pontificia Universidad Javeriana, Bogotá, Colombia. E-mail: andres.torres@javeriana.edu.co ABSTRACT 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, , into shear stress due to vegetation (or vegetation drag), v, and bed-shear stress, b. 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. Keywords: Vegetated flows, shear-stress partitioning, rigid vegetation, laboratory experiments. 1 INTRODUCTION The presence of vegetation in natural channels affects flow conditions, sediment transport and the morphological development (Hickin, 1984). Field measurements, for example, have shown that vegetation exerts a certain control in the stability of the river banks and the final planform of a river (Kleinhans, 2010). Previous studies have shown that the presence of riparian vegetation can reduce the braiding index of rivers and that its establishment on sediment bars is one of the main factors affecting the migration of river meanders. Vegetation also influences the mitigation of floods and sedimentological balances (Gurnell, 2014). Locally, vegetation increases the resistance to flow, increasing the local hydraulic roughness, generating a decrease in flow velocity and promoting sedimentation (Nepf, 2012). In fact, it has been shown that vegetation alters the vertical velocity profiles, an aspect that radically modifies the hydrodynamics of natural channels (Chen and Kao, 2011; Guo and Zhang, 2016; Tsujimoto, 1999). The effects on velocity profiles depend on the hydraulic conditions of the plants in relation to the water depth, which can be: emergent (when vegetation protrudes above the free surface) and submerged (when vegetation is completely below the water surface) (Vargas-Luna et al., 2015b). At present, there exist models that allow estimating the hydraulic roughness in vegetated channels which include separately emergent and submerged conditions and a few models that take into account both cases (Vargas- Luna et al., 2015a). All existing models are based on a momentum balance, for uniform flow conditions, described by Eq. [1] (Baptist, 2005):  =  ℎ  =   +   [1] where  is the density of the fluid (kg / m 3 );  is the acceleration of gravity (m / s 2 );  ℎ is the flow depth (m);  is the channel slope (m / m);   is the shear stress on the bed (N / m 2 ) and   is the shear stress absorbed by the vegetation (N / m 2 ). From Eq. [1], the reduction of the bed shear stress in a vegetated channel is easily identifiable due to the presence of plants and the associated sedimentation mentioned above. However, recent research has identified that the shear stresses distribution in the bed and vegetation described by Eq. [1] may not be applied in a generalized manner (Thompson et al., 2004; Vargas-Luna et al., 2016, 2015b). This assumption is supported by considering that the shear stress absorbed by the plants is dependent on its physical properties (height, diameter, density, etc.) and that an increase of vegetation density results in a reduction of the influence of the