Development of sampling system for consolidated mud and understanding on settling mechanism of floating mud using the sampling system

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By investigating the sediment at the sea bottom of the Hiroshima Bay, we were able to show the seasonal change mechanism of the sediment with the pore water movement.The turbidity increasing does not only depend on the velocity above the bottom but also on the movement of pore water in the consolidated sediment. According to the experiment, the fluid mud doesn't well up when the velocity of under 6cm/s is occurred above the bed. In these results, the bottom flow, observed at the head of Hiroshima Bay, cannot cause the upwelling of the bottom sediment.Wet density, water content and specific gravity in surface sediment seasonally changed in over 20 cm depths. Water content increased more from summer to fall, and was over 500% at the fluid mud layer in fall. The high turbidity is generated in water content increasing period.The generation process of the high turbidity is hypothesized that firstly, the pressure different between sea and ground generates in the stratified season because of the water heating etc., secondly, the pore water moves cyclically with tide, finally, the fluid mud wells up with the pore water flowing up from the sediment.In the consolidated sediment, DO above the sediment are consumed resulting to oxygen-deficient water mass occurs at the sea bottom. According to the numerical simulation, the estimated amounts of the pore water moving explain the turbidity variation and water content profiles of the sediment. These phenomena can be explained by the pore water movement between the sea above the bottom and the sediment layer.