Effects of bed permeability on the hydrodynamic characteristics in a channel with a vegetation patch: A modeling study

Vegetation patches grow on permeable beds, but the effects of bed permeability on the hydrodynamic characteristics of channels with vegetation patches are rarely discussed. In this study, emergent rigid cylinders are used to represent a vegetation patch, and porous media are utilized to represent the sediment. Multiple groups of conditions with various bed permeabilities are simulated. The results show that when the bed permeability is high, a jet flow forms downstream of the patch. With increasing bed permeability, the velocity distribution of the Karman vortex block is closer to the level in front of the patch, and the peak value of the turbulent kinetic energy increases and extends downstream. However, when the bed permeability is low, the hydrodynamic characteristics in a channel with a vegetation patch (HCCVPs) change negligibly with the bed permeability. The bed permeability has a threshold effect on the HCCVPs. A further discussion suggests that the permeability Reynolds number Rek is suitable for characterizing the bed permeability and its threshold effect. The magnitude of the Rek threshold that corresponds to the change in hydrodynamic characteristics is 1. These new results also explain the process differences in sediment transport and patch expansion under different bed permeability conditions.