The Impacts Analysis of Plant Shape and Spatial Distribution on the Flow Properties

Abstract: Vegetation in shallow water systems, such as rivers and wetlands, plays an important role in altering flow resistance and turbulence. Submerged or emergent vegetation reacts to the drag of water by either remaining erect, oscillation in response to turbulent fluctuations, or bending. There are extensive research focused on the plant-flow interactions. However, given the variety for plants and their temporal and spatial variability, in addition to the wide rand of flow and morphological conditions in nature, significant knowledge gaps still exist. To get the details of the velocity distribution around the plants in three-dimensional direction, Large Eddy Simulation is used in this study. Three plant shapes with different configurations are formed with Immersion Boundary Method. The river bed is formed by five layer packed particles so that both permeability and roughness can be considered. Based on the LES model, three dimensional velocity is simulated. The relationship between the turbulent events and eddy and the plants shapes are analyzed. Moreover, using the double average method, the form-induced normal and shear stresses are calculated for different plant shapes and different spatial distribution. It can be seen the significantly influence under and over-canopy flow behaviour, as well as the interfering wakes among the stems. Element scale (stem scale) turbulence is generated within the canopy. These phenomena will affect the sediment transport and the mass diffusion. Then, the drag and the drag coefficient distribution are calculated in different cases. The results show that not only the additional drag exerted by plants can reduces the mean flow velocity, but also the plant form and the spatial distribution can have a significant effect on the mean flow field. These results are applied to the ecological restoration of the Yongding River in Beijing. The plant types and density are suggested for the engineering design and the evaluation of the flood risk and the sediment transport.

Keywords: Plant shape; spatial distribution; flow properties; Large-eddy simulation; double average method