In this study, we present a three-dimensional numerical model for the interaction of flow with submerged flexible vegetation, based on a large-eddy simulation and the immersed boundary method. The model innovatively realises the interaction between the flow and highly flexible vegetation with clustered leaves. Besides being a three-dimensional model of motion with full degrees of freedom, this study improves the consideration of the motion of the vegetation in all directions, and in addition the energy and momentum transfer in the spanwise direction. Furthermore, we perform a flume experiment for the flow with submerged flexible vegetation, the results of which are used to validate the simulation effects of the numerical model. It is found that the numerical model can effectively simulate the velocity profiles and the movement of vegetation induced by the flow. Using the model to analyse the flow–vegetation interaction, we find that the movement of vegetation is closely related to the flow velocity. As the flow velocity increases, both the offset angle and the vegetation swaying amplitude increase. Compared to vertical rigid vegetation, the tilting of flexible vegetation does not significantly change the velocity difference and the magnitude of the turbulent kinetic energy between the inside and the outside of the vegetation canopy, but it does weaken the disturbance to flow, thus reducing the resistance to flow. However, the swaying of vegetation dose significantly increase the velocity difference between the inside and the outside of the canopy. It forms Kelvin–Helmholtz hairpin vortices intensifying the turbulence production, and enhancing the disturbance and resistance to flow.

Submerged flexible vegetation occupies the core of aquatic ecosystem research. Hydrodynamics of submerged flexible vegetation and its interaction with the flow in an open channel are of great significance in studying the mass and momentum transports in the flow. In this study, a numerical model for highly flexible vegetation based on large eddy simulation and the immersed boundary method was used to simulate the flow–vegetation interaction. It is recognised that alternate vortices with opposite sense rotations appear at the flow–vegetation interface. These vortices prompt the vegetation canopy to have wave-like coherent waving motion, commonly called the monami phenomena. The spatial scale and the spreading velocity in the streamwise direction of these vortices determine the wavelength, frequency and amplitude of the vegetation coherent waving motion. In this study, the fast Fourier transform method was applied to analyse the factors affecting the characteristics of the vegetation coherent waving motion. It is revealed that, as the flow velocity increases, the wavelength of the coherent waving motion decreases, while the frequency and amplitude increase. Besides, as the vegetation spacing increases, the wavelength and amplitude of the coherent waving motion increase, but the frequency decreases. Furthermore, an increase in the relative density of vegetation magnifies the amplitude of coherent waving motion without affecting the wavelength and frequency.

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The Yarlung Zangbo River originates from the Qinghai-Tibet Plateau, and runoff consists of the rainfall, melting of snow and glaciers in the upper high-altitude areas. The flow and sediment transport induced by flow in the Yarlung Zangbo River is highly sensitive to changes in climatic conditions. Changes in flow and sediment transport will also have an impact on the downstream wetland ecosystem. In order to study the changes of flow and sediment transport in the Yarlung Zangbo River, Niyang River wetlands under the changes of climatic conditions is used as the considered case. The two-dimensional shallow water equation and the non-equilibrium bedload sediment transport model are proposed in the Yarlung Zangbo-Niyang River. The model is discretized using the unstructured grid finite volume method (FVM). The model includes approximately 355 km length and covers an area of approximately 673 km2. According to the measured time series of flow data at Nuxia hydrology station and the simulated data under the climate change conditions by the climate change model, the water level, flow velocity, erosion and siltation, and sediment gradation change in the river channel are calculated. The calculation results show that by the year 2025, the water level of Niyang River wetlands will increase by 2m under RCP8.5 (Representative Concentration Pathway), and accordingly the total area of the wetlands will decrease by 5%. The results show that climate change has a significant impact on the flow and sediment transport and downstream ecological environment of the Yarlung Zangbo River.