Turbulence structure and near-wall suppression in equilibrium and nonequilibrium sediment transport: An experimental study

Sediment transport in open channels plays a significant role in shaping turbulent flow structures, influencing sediment dynamics and flow resistance. Transport regimes are classified into equilibrium, where sediment inflow and outflow are balanced, and nonequilibrium, characterized by bed degradation. This study experimentally investigated the turbulence characteristics of sediment-laden, low-velocity open-channel flows under two conditions: sediment-feeding (SF) flows representing equilibrium and nonsediment-feeding (NSF) flows representing degradation-type nonequilibrium conditions. Laboratory experiments were conducted in a 10-m recirculating flume using a 16-MHz acoustic Doppler velocimeter (ADV). Velocity and turbulence profiles were collected under fixed- and movable-bed configurations using two sediment types (d50 = 1.55 and 1.85 mm) simulating tropical riverbeds. Analyses of velocity profiles, turbulence intensities, Reynolds shear stress, mixing length, eddy viscosity, energy spectra, velocity correlations, and turbulence scales were performed. The results reveal clear distinctions between the SF and NSF flows, particularly near the bed. Sediment feeding reduces the near-bed velocity gradient (du/dy), suppresses near-wall turbulence, and shifts the turbulence intensity peak upward to y/H ≈ 0.15. It also significantly reduces the Reynolds shear stress, whereas changes in the eddy viscosity near the bed are less pronounced because of the dominant velocity gradients. A hybrid model combining exponential and power-law terms is proposed to better represent the turbulence intensity and shear stress profiles under sediment-feeding conditions. Spectral analysis confirmed that, despite the 50 Hz sampling limit of the ADV, the inertial subrange follows Kolmogorov's −5/3 law, although the dissipation range was not captured, and microscale estimations remain approximate. Compared with sediment feeding, increased bed roughness reduces turbulence scales, whereas bed mobility effects are secondary. Shear velocity estimates derived from the Clauser, energy gradient, and Reynolds shear stress methods indicate that turbulence-based methods yield more consistent results in sediment-laden flows. These findings advance the understanding of sediment–turbulence interactions and improve sediment transport modeling for low-velocity open channels. Furthermore, these insights can be applied to enhance predictive modeling, optimize sediment management strategies, and support the design of more resilient river engineering structures, particularly in tropical systems.