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

通过对广岛湾海底沉积物的调查，揭示了沉积物随孔隙水运动的季节变化机制，即沉积物浊度的增加不仅取决于海底流速，还取决于固结沉积物中孔隙水的运动。实验表明，当床层以上流速小于6cm/s时，泥浆不会发生上涌。结果表明，在广岛湾湾头观测到的底流并不能引起底质的上涌，表层沉积物的湿密度、含水量和比重在20 cm以上深度有季节性变化。含水量从夏季到秋季增加较多，秋季在流态泥层处超过500%。高浊度是在含水量增加期产生的，高浊度的产生过程假设为：首先，由于水体加热等原因，在分层季节产生海-地压力差，其次，孔隙水随潮汐循环运动，最后，淤泥威尔斯随沉积物上涌的孔隙水而上涌，在固结的沉积物中，沉积物上方的溶解氧被消耗，导致海底出现缺氧水团。根据数值模拟结果，估算的孔隙水运移量解释了沉积物的浊度变化和含水率剖面。这些现象可以用海底上方海水与沉积层之间的孔隙水运动来解释。

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.