An Experimental Study on Longitudinal Dispersion in Various Vegetation Arrangements

Aquatic vegetation significantly affects the dispersion of scalars in a channel. In the river and ocean, vegetation grows with diverse characteristics such as density, submergence and flexibility. And the scalars disperse in the different magnitude depending on those characteristics. Therefore, the previous studies have investigated the effect of geometric vegetation on longitudinal dispersion, which is one of the characteristics in vegetation field. Since the previous works mainly focused on the effect of population and density of vegetation, most of them deployed the vegetation evenly in the uniform or random pattern. However, vegetation in nature is spatially distributed in patched or heterogeneous form rather than in homogenous form due to the environmental conditions of habitat or the kind of vegetation. Thus, the present work investigated the effect of spatial arrangement of vegetation on longitudinal dispersion by conducting the laboratory experiments with artificial emergent vegetation.

Various types of arrangements are described by the concept of isometry and allometry, which are originated from the study of ecology and biology, and are quantified numerically by the standardized Morisita index. The results of experiments reveal that the vegetation arrangement changes the various hydraulic parameters such as mean velocity, drag coefficients, longitudinal dispersion coefficient etc. The mean stream-wise velocity decreases as the standard Morisita index increases. Emergent vegetation makes the vertical distribution of stream-wise velocities uniform but causes fluctuation in lateral direction. In particular, for the 2D allometric arrangement which containing 2D clumps, the spatial heterogeneity of velocity in the lateral direction increases evidently. The mean drag coefficient in the isometric arrangement is always larger than allometric arrangement by a factor of 2. The longitudinal dispersion coefficient varies from 13.5 to 101.7 cm²/s depending on the standardized Morisita index. It can be explained from the variation of spatial heterogeneity of velocity in both lateral and vertical directions, which depends on the type of vegetation arrangement.