Evaluation of Methods for Predicting Velocity Profiles in Open Channel Flows with Submerged Rigid Vegetation

This paper evaluates the four analytical models of Klopstra et al. (1997), Defina & Bixio (2005), Yang et al. (2010) and Nepf (2012) for predicting the velocity profiles in open channels flows with submerged rigid vegetation against a wide range of experimental data available in the literature. The 17 datasets used include various relative depths of submergence [flow depth (H) /vegetation height (h) =1. 5 ~ 5], different vegetation densities (defined as a, the frontal area of the vegetation per unit volume) (a =1~ 10 m -1) and bed slopes (S o = 1. 8x10 -6 ~1. 6x10 -3). For submerged vegetated flow, the velocity profile can often be described by two layers, the vegetation layer in the lower region and the surface layer in upper non-vegetated region. The vegetation retards flow by exerting a drag force, thus inducing different velocities, between the vegetation layer and the upper surface layer, arising from the presence of Kelvin-Holmtz (KH) vortices. Based on a mixing-layer analogy, different analytical models have been proposed for the velocity profile in the two layers, and the four models used herein were chosen to test their applicability when tested against more datasets than used previously. Studies show that none of the models can predict well for all datasets. The three models of Klopstra, Defina, Nepf performed reasonably well in some of the cases, whereas Yang’s model failed in all the cases examined herein. It was also found that the Defina model is almost the same as the Klopstra model despite some simplification made in the latter, if the same mixing length scale of eddies (O) is used. Finally close examination of the mixing length scale of eddies (O) in the Defina model showed that when O/h = 0. 025 (H/h) 1/2 was used, the model could predict velocity profiles well for all the datasets used.