Numerical analysis of hydrodynamic responses to bed roughness in vegetated open channels under subcritical flow conditions

Abstract Vegetation is widely recognized as an effective nature-based solution for flood mitigation because it increases hydraulic resistance and promotes flow energy dissipation. Although numerous studies have investigated vegetation-induced flow resistance and the influence of bed roughness independently, the coupled effects of vegetation and varying bed roughness under subcritical flow conditions remain insufficiently understood. This study employed a validated computational fluid dynamics (CFD) model based on the standard k-epsilon (k-ε) turbulence closure to investigate roughness-vegetation interactions in a vegetated open channel. The numerical model was validated against experimental streamwise velocity profiles under three subcritical flow conditions (Froude number (Fr) = 0.40, 0.44, and 0.48), yielding good agreement with laboratory measurements (correlation coefficient (R) = 0.91–0.96 and root mean square error (RMSE) = 0.0152–0.0259). Following validation, the model was applied to seven subcritical flow conditions (Fr = 0.40–0.65) and four equivalent sand-grain roughness heights (Ks = 0, 7.22, 14.32, and 24.47 mm) to evaluate their influence on velocity distribution, turbulent kinetic energy (TKE), turbulent eddy dissipation (TED), and bed shear stress (BSS). The results showed that increasing bed roughness enhanced flow resistance and significantly modified turbulence characteristics within the vegetated channel. Relative to the smooth-bed condition, the maximum velocity decreased by 6%, based on the validated velocity field and the associated k–ε turbulence closure, TKE was found to decreased by 31.4%, and the maximum BSS increased by 70% at the highest roughness height (Ks = 24.47 mm). These findings demonstrate that bed roughness substantially alters vegetation-controlled flow behavior and provide new insight into roughness–vegetation interactions for the design and optimization of sustainable nature-based flood mitigation systems.