Numerical study on mixed layer vegetation in open channel flow

Vegetation in river bed produces high resistance to flow and has a great impact on flow characteristics in rivers, especially during floods. The resistance due to vegetation in open channels reduces the flow discharge, which can lead to remarkable changes in physical and biological processes in aquatic environments. Numerical modelling analysis is undertaken for two and three layers vegetation in open channel flow. First, a model was developed to represent the contribution of vegetation in the RANs models through an additional momentum or resistance by modelling it as porosity domain. Then, the numerical models were tested and verified using the experimental data of Liu et al. (2008), and was then used to study for various other scenarios proposed. The modelling results by Ansys Fluent showed that the velocity profile is mostly uniform over the depth in both cases, except at location 1 for three layers vegetation. The increase of velocity profile above the short vegetation in three layers vegetation condition is much larger than that in two layers condition. Generally, in both cases the flow velocity inside the vegetation layer is significantly smaller than that in the surface layer (i.e. non-vegetation layer). A near-constant velocity dominates inside the vegetation layer, and then starts to increase near the interface at the top of vegetation. There is a sudden change in the shape of the velocity profile near the top edge of vegetation. The results also showed that for both two and three layers cases, the flow velocity is strongly dependent on locations, and that the distributions of the turbulent intensity attains maximum just around the edge of vegetation height.