Influence of sediment supply on mean flows and turbulence in mountain stream

In mountain streams, turbulent shear stress and associated flow characteristics are highly sensitive to sediment entrainment, transport, and deposition. Notably, sediment entrainment and deposition induce significant energy exchange driven by interactions among transported particles, the fluid, and the bed, which substantially modify near-bed turbulence characteristics. This study experimentally investigates near-bed turbulence parameters under conditions with and without sediment entrainment and deposition in a gravel-bed stream. Experiments were conducted in a rectangular open-channel flume with a uniformly packed gravel layer. A sediment feeding hopper was installed at the upstream end to introduce sediment at uniform rates, and an acoustic Doppler velocimeter (ADV) was employed to measure instantaneous flow velocity components. Time-averaged velocity profiles were recorded under no-sediment-feed conditions and at four different sediment deposition elevation levels. In sand-filled gravel beds, the log-law streamwise velocity distributions exhibited a decrease in the von Kármán coefficient, while both the virtual bed and zero-velocity levels increased, attributed to progressive sediment infilling of gravel-bed pores. Higher sediment deposition levels resulted in significant damping of Reynolds shear stress profiles near the bed. Turbulent budget analysis revealed that particle entrainment, transport, and deposition substantially altered the rate of turbulent production, whereas turbulent kinetic energy dissipation rate was only weakly affected. The pressure energy diffusion rate in the vicinity of the gravel-bed surface was strongly modified by sediment entrainment and deposition processes. A correlation among all budget components was established, and the time-averaged pressure energy diffusion profiles for sand-filled gravel-bed streams indicated enhanced turbulence in response to particle entrainment and deposition.