Effects of Flow Depth and Sediment Size on Near Bed Hydraulics and Sediment Mobility in Open Channel Flow

Accurate prediction of sediment mobility in open channel flows is essential for effective river engineering and sediment management. This study examines the combined influence of flow depth and sediment grain size on near bed hydraulics and sediment mobility using high-resolution Acoustic Doppler Velocimeter (ADV) measurements in a controlled laboratory flume. Experiments were conducted over uniform sand beds with median grain sizes of d₅₀ = 0.321 mm and d₅₀ = 0.81 mm under four different flow depths (12cm, 15cm,18cm,21cm) and a range of flow velocities. Three dimensional velocity components were measured at multiple vertical locations throughout the flow depth, while water surface elevations were continuously monitored. Depth resolved ADV data were used to compute mean streamwise velocity, Reynolds shear stress, friction velocity, and turbulent kinetic energy for each sediment size and flow depth. Sediment mobility was assessed using the Shields parameter, estimated from ADV-derived bed shear stress, and compared with the critical Shields parameter at multiple velocity points for each depth. The results indicate that coarser sediment beds exhibit increased near-bed turbulence intensity and higher friction velocity across all flow depths, while yielding lower Shields parameter values relative to finer sediment beds. Comparisons across the four flow depths reveal that sediment mobility transitions from stable to mobile conditions depending on the combined effects of flow depth, sediment size, and local velocity magnitude. At lower velocities, Shields parameter values remain below the critical threshold, indicating stable bed conditions, whereas higher velocities at the same depth result in Shields values exceeding the critical limit, signifying active sediment motion. Depth wise velocity and turbulence profiles demonstrate that both flow depth and sediment roughness significantly modify near-bed hydraulic structure and bed shear stress distribution. The findings highlight the importance of accounting for depth-dependent flow structure and sediment characteristics when evaluating sediment mobility. This study provides a robust experimental framework for identifying stable and mobile sediment regimes and estimating sediment transport potential using high-resolution ADV measurements without direct sediment transport observations.