Process-Based Response of Flow-Vegetation Interactions in Bifurcated Channel Systems: A Laboratory Investigation

Abstract This study examines the flow dynamics over and around a midchannel sandbar under both vegetated and nonvegetated conditions. Although previous research has predominantly focused on bare sandbars, this work provides new insights into how variations in vegetation density alter the turbulence characteristics. Laboratory flume experiments were conducted under three conditions: bare, sparsely vegetated, and densely vegetated sandbars. High-resolution velocity measurements obtained using an acoustic Doppler velocimeter at seven cross sections were analyzed to evaluate streamwise velocity distribution, secondary currents, turbulent kinetic energy (TKE), Reynolds shear stress (RSS), and quadrant-based turbulent events. Results revealed that the bifurcated channel separated into a divergence zone, characterized by flow separation near the banks and a convergence zone where flow separation was absent. Streamwise velocity in the convergence zone was nearly 2–2.5 times higher than in the divergence zone. Increasing vegetation density reduced velocity over the sandbar while enhancing velocities in the bifurcated branches, indicating a redistribution of flow momentum. Secondary currents weakened above the sandbar but strengthened within the branches as vegetation density increased. TKE peaked near the sandbar elevation in the branches, with additional localized intensities near the surface and within the canopy; however, overall TKE declined under dense vegetation. RSS also peaked at the sandbar elevation, and its direction was strongly influenced by vegetation density. Quadrant analysis showed that sweep and ejection events dominated, with flexible vegetation amplifying their frequency and strength. The findings underscore the role of instream depositional units in shaping turbulence, momentum exchange, and flow structure in bifurcated channels.