Shoitiro Yokosi

Characteristics of suspended sediment and turbulence in a tidal boundary layer

High-resolution measurements of three velocity components and the concentration of suspended sediment (SS) have been performed in the Ota diversion channel through a tidal cycle. Data are collected with an acoustic Doppler velocimeter at various distances from the bottom. Turbulence measurements are extended to the immediate vicinity of the bottom. Turbulent fluxes of SS concentration are directly estimated from the fluctuations of concentration and velocity. Both the mean concentration and the vertical turbulent flux increase with the Reynolds shear-stress, though the mean concentration lags the shear stress. The frequency range in which the fluctuations mainly contribute to the vertical turbulent fluxes of SS concentration is higher than that of the Reynolds shear-stress ρu*2 near the bottom. The settling velocities of SS, ws, are estimated from the transport equation of suspended sediment. The values of ws decrease during the large shear velocity. The vertical profiles of vertical eddy diffusivity are shown. The ratio of the momentum and sediment diffusivity coefficients, β = Nz/Kz, decreases with increasing values of u*/ws and the SS concentration.