Author(s): Xiaonan Tang
Linked Author(s): Xiaonan Tang
Keywords: Vertical velocity profile; Vegetated flow; Open channel flow; Rigid vegetation; Analytical model
Abstract: This paper evaluates the four analytical models of Klopstra, Defina, Baptist, and Nepf model for predicting vertical velocity distributions of flow in an open channel with submerged rigid vegetation, in order to examine their sensibility to vegetation density and submergence ratios (H/h), where H is flow depth and h is vegetation height. Fourteen experimental datasets used cover the ratios of H/h = 1.25 ~ 3.33, and various vegetation density of a = 1.1 ~ 18.5 m⁻¹ (a defined as the frontal areas of the vegetation per unit volume). For submerged vegetated flow, the vertical velocity distribution can often be described by two layers, the vegetation layer in the lower part and the non-vegetation layer in the upper part. The vegetation retards flow by exerting drag force on the flow, thus resulting in different velocities between the vegetation layer and the upper surface layer. Based on an eddy mixing-layer analogy, different analytical models have been proposed for predicting vertical velocity distribution in the two layers, and four models were chosen to examine their sensibility when tested against different independent datasets from those used in their original papers. Our studies show that the four models can predict reasonably well under a certain range of vegetation density and submergence, and that the prediction of all the models are less sensitive in the lower region. The studies also reveal that under the same submergence, the predictions of the four models in the upper layer are less sensitive to low vegetation density (e.g. ah < 1) than to high vegetation density (ah > 1), but that for the same vegetation density, all four models are much sensitive to the vegetation submergence ratio (H/h). It was also found that the Defina and Klopstra models are almost the same despite some simplification made in the Klopstra model, if the same mixing length scale of eddies (λ) is used. Finally, close examination on λ in the analytical models, and it was found that the predictions of the models (the Baptist, Defina and Klopstra models) are less sensitive to the vegetation density and submergence when λ is evaluated by 0.02√Hh.
Year: 2018