Drag coefficients and water surface profiles in channels with arrays of linear rigid emergent vegetation

• Knowledge of the drag coefficient is necessary in channels with rigid vegetation. • A drag coefficient was proposed in the past as function of vegetation density only. • This drag coefficient allows two different sets of profiles to be drawn correctly. • The effective width was taken into account in the profile computation. • In profile computations, the boundary condition in subcritical flows must be downstream. Although vegetation in watercourses has an important ecological function, from a hydraulic point of view, it increases flow resistance, determinates higher water levels and generates a greater risk of flooding. In engineering practice, to determine water levels, the drag coefficient must be known, whose experimental values, in literature, are provided for uniform or quasi-uniform flow. In this paper, the expression of a drag coefficient, previously proposed by the authors for the case of emergent rigid vegetation arranged in a linear manner, was used to simulate experimental water surface profiles, employing the effective width for the first time. The formula was validated by simulating 26 experimental profiles observed at the University of Calabria, over 1100 points in total, plus 8 published in literature. In some of these applications, the vegetation density was much higher than that for which the drag coefficient expression was derived. For this reason, it is possible to consider a new, wider field of validity for it. The proposed equation is independent of the Reynolds stem number, so that it can be used in natural streams and rivers, provided that viscosity effects can be considered negligible. Finally, some comments are offered on the application of a new formula proposed in literature regarding the computation of the profile when downstream depth is taken as a boundary condition.