| Research Abstract |
Mass transfer at the sediment-water interface plays an important roll in material balance in aquatic environments. Sediment oxygen demand (hereafter called SOD), for example, is a dominant sink of dissolved oxygen (hereafter called DO), and DO depletion in eutrophic lakes and estuaries has been a serious problem, as it damages fisheries and ecosystems. Recently, the effects of the hydraulic conditions on SOD have been pointed out by some investigators.
In the first part of this study, a general model which predicts SOD and phosphate release rate is proposed. This theoretical model is based on Nakamura and Stefan (1994) for smooth surface, and Dade (1993) for rough surface. According to this model, SOD and phosphate release rate is described as a function of DO concentration in the bulk water, the shear velocity, Schmidt number, equivalent sand roughness and reaction kinetics in the sediment.
In the second part, three kinds of experiments were performed to investigate the effect of flow v
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elocity and surface roughness on the mass transfer. The first one was an experiment in a continuous flow system with nondisturbed core sample of sediment. A stirring device was used to agitate overlaying water. The second was performed with a rectangular, closed flume in which flow velocity can be controlled. Rectangular roughness elements were put on this flume floor, to investigate the effect of surface roughness. The third one was an experiment using microelectrodes in an open flume, in which flow velocity was controlled. Microelectrodes enabled precise measurements of DO concentration profiles near the sediment-water interface. These experiments showed a monotonous increasing tendency of SOD,for smooth surface, as flow velocity increased, while phosphate release rate had a maximum value at an intermediate velocity. Mass fluxes had a maximum value in a buffer region between smooth and complete rough surfaces. These results show that model predictions well reproduced the SOD obtained by the experiments. Less
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