Author(s): M. Higashino; T. Kanda
Linked Author(s): T. Kanda, Makato Higashino
Keywords: Diffusional mass transfer; Dissolved substance; Sediment-water interface; Bottom sediment; Trench; Reattachment; Cavity
Abstract: A fundamental study has been performed on diffusional mass transfer from bottom sediment to overlying water for turbulent flow in a rectangular trench. The effect has been experimentally investigated of the flow velocity just above the trench, of the depth of the trench and of physical property of the sediment like the water content on the release flux of dissolved substance. We have also examined how diffusional mass transfer is related to the change of flow pattern going from a reattachment type to a cavity type. The experimental results show that the release flux increases as the flow velocity increases in almost all the experiments and that the release flux tends to be larger when the trench depth becomes smaller. In addition, the difference of the dependence of the flow velocity on the release flux between the case of shallow trench (depth of 1～4cm) and deep trench (depth of 5～8cm) is exhibited. These results suggest that the characteristic of diffusional mass transfer shifts according as the flow pattern changes from reattaching flow to cavity flow when the trench depth increases. It is also found out that the release flux in the case of the same trench depth increases when the water content of the sediment becomes larger. From these results, it can be said that the effect of the flow velocity and the trench depth on the release flux is caused by the enhancement of the exchange between the water body near the sediment-water interface and bulk flow due to the increase in the flow velocity, or the increase in disturbance when the trench depth decreases. It is also considered that the release flux varies with the water content because mass transfer with adsorption-desorption between solute and sediment particles in the sediment affects the vertical diffusive flux at the sediment-water interface.
Year: 2001