Numerical study of mean and turbulent flow adjustments in open channels with limited near-bank vegetation patches

Abstract

Artificially planted nearshore limited-size vegetation patches are widely used as near-natural measures for bank protection and habitat creation, highlighting the importance of understanding their impact on local flow structures. This study conceptualizes a square vegetation patch located near the sidewall of an open channel as a porous array composed of rigid cylinders of equal diameter. Large eddy simulation is conducted to investigate the effects of varying vegetation density (λ) on the mean and turbulent flow fields. Differences caused by stem arrangement patterns are also considered. The simulation results indicate that, at λ ≥ 0.097, the group effect of the array becomes apparent, forming corner recirculation vortices upstream of the array, which contribute to flushing low-velocity fluid from lower heights to near the water surface, and a patch-scale horseshoe vortex system. Both the flow deflection occurring upstream of and within the array intensify with increasing vegetation density. For the same λ value, vegetation patches with stems arranged in a linear pattern experience weaker lateral flow deflection but stronger lateral shear layer turbulence compared to those with staggered arrangements. No recirculation bubbles were observed in the mean flow field downstream of the porous array within the parameter range covered, due to strong longitudinal bleeding flow. Compared to a solid square cylinder of the same size, the porous patch reduces bed shear stress near both sidewalls while only considerably increasing bed shear stress in the center of the channel. Additionally, the staggered arrangement pattern achieves the same reduction in bed shear stress with a lower vegetation density compared to the linear arrangement with a higher λ value.