Experimental Measurements of Vertical Concentration Profiles of Saltating Grains under Bare and Vegetated Bed Conditions

Saltation is a dominant bedload sediment transport mode in waterways, and its vertical concentration profile (VCP) holds key insights into grain dynamics, yet it remains underexplored for vegetated beds. This study investigates VCPs of saltating grains via laboratory flume experiments with bare beds (BBs) and vegetated beds (VBs, simulated by emergent cylindrical rods), using particle image velocimetry and particle tracking velocimetry for measurements. For BB, VCPs featured a near-bed linear concentration increase to a maximum followed by an exponential decay, well-fitted by a lognormal function. For VB, emergent vegetation induced significant flow heterogeneity, decoupling shear stress from bulk momentum and shifting saltation to a turbulence-dominated regime. VB-VCPs became depth-invariant for relatively deep flows and deviated from the lognormal function at higher elevations due to elevated turbulence. Saltation height, length, and velocity for VB increased with flow depth, while impact angle varied across measurement planes. These findings advance understanding of saltation in vegetated environments and provide quantitative bases for improving sediment transport models.