Hydrodynamics of Three-Dimensional Flow Structure in a Rigid Patchy Vegetated Open Channel With Bank Roughness Elements

Abstract

A numerical modelling approach has been applied in this study to investigate the three-dimensional flow structure and turbulence characteristics in an open channel with both bank roughness elements (REs) and circular vegetation patches under varying submergence conditions. The bank REs, with lengths of 0.06, 0.04 and 0.02 m, were analysed at pitch-to-height ratios (p/k) of 9.67, 10.33 and 11 to assess their hydrodynamic impact. Computational fluid dynamics (CFD)–based simulations reveal significant velocity reductions in vegetated regions, with submerged vegetation decreasing velocities by up to 35% due to increased drag, whereas emerged vegetation induces localized flow accelerations. Furthermore, submerged vegetation reduces Reynolds shear stress, whereas emerged vegetation increases it. Moreover, turbulence intensity is observed to be 5.8% in emerged vegetation cases, whereas submerged vegetation cases exhibit turbulence intensity of around 5.31%. Results also indicate strong secondary current circulations downstream of vegetation patches, with higher turbulence and vortex formation in emerged cases compared to submerged cases. Turbulence intensity and Reynolds shear stresses are primarily dictated by the submergence state of vegetation patches, with negligible dependence on RE size. These findings underscore the critical role of vegetation in modulating flow resistance, turbulence and sediment transport, offering valuable insights for eco-hydraulic engineering, riverbank stability and sustainable river management.