Analysis of flow velocity for free-surface flow interaction with vegetation configuration and bed slope in an open channel using CFD

This study investigates the combined effects of bed slope, vegetation stem diameter, and stem count on velocity distribution in open-channel flow using FLOW-3D. A 3 × 3 × 3 simulation matrix was developed using three bed slopes (S = 0.005, 0.015, and 0.025), three stem diameters (d = 0.0025, 0.0035, and 0.0045 m), and three stem counts (N = 70, 80, and 90). The numerical model, employing the RNG k–ε turbulence model and Volume of Fluid method, was validated against published laboratory measurements. Results showed a strong effect of bed slope on streamwise velocity. For d = 0.0025 m, velocity at a depth of 0.12 m increased from 0.48 m/s at S = 0.005 to 1.21 m/s at S = 0.025. At the mildest slope, increasing stem diameter from 0.0025 to 0.0045 m reduced near-bed velocity by approximately 50%. Increasing stem count generally reduced velocity for the larger stem diameters, whereas its effect for the smallest diameter depended on bed slope. Overall, vegetation configuration effects were more pronounced under mild-slope conditions, whereas the influence of bed slope became increasingly dominant as slope increased. The results quantify the combined effects of channel slope and vegetation configuration on open-channel velocity distributions.