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Tokyo Bay is one of the estuaries in Japan with a high population of almost 26 million people in the basin area. One of the major concerns for the environment in this water area is the decreasing ecosystem functions including the deterioration of water and sediment qualities caused by various anthropogenic activities. Since the bottom sediments around almost the entire area of the inner bay consist of fine materials with a high organic content, which cause the deterioration of water quality through processes such as hypoxia, an understanding of the fine sediment dynamics in the Bay is crucial for an environmental assessment of the water area. This paper proposes a model for the key processes of fine sediment dynamics, which reflects field data about muddy bed structures and their dynamics obtained during the monitoring campaign in 2007. One of the specific features of the sediment in the Bay at present is the persistent existence of fluid mud layers (water content over 300 %) with a thickness of around a few decimeters, which might be caused by deposition of abundant organic particles due to eutrophication. The present study shows that diffusion flux model delivers quite reliable results for estimating erosion flux from the top of fluid mud layers after calibrating the model parameter against the time series data of vertical flux measured by an acoustic Doppler velocimeter system. This study also derives analytical solutions, based on the Bingham fluid concept, of advection flux in the fluid mud layer on which external shear stress force is applied.
相似文献For rocks of similar types and mineralogies the clay content of slip-surface materials can be directly related to the displacement and history of slide movement. This study indicates that the type of slip surface ranges from: (1) striation type (showing only striations on bedrock without clay), to (2) brecciated type (clayey breccia), (3) mylonite type (clay with breccia), and to (4) clay type (clay without breccia). If the slip surface occurs in tuff or tuffaceous materials and is subjected to increasing slide movement, the smectite content in the slip-surface clay increases, while the silicate mineral content decreases. Furthermore, the smectite content is observed to increase within the same landslide from the head to the toe portion of the landslide (direction of groundwater flow). The groundwater along the slip surface (above the impervious zone) contains an increased concentration of Ca++ ions. As the concentration of HCO3− ions exceeds 40 mg/l, an environment is created that promotes the formation of smectite. This process indicates that there is an active ion exchange within the groundwater near the slip surface that forms smectite. Thus, the formation of smectite in the slip surface requires the presence of tuff and a sufficient concentration of HCO3− ions in the groundwater.
The study results presented here indicate that frictional resistance decreases as clay content increases. The reason for repeated movement of Japanese landslides in Tertiary tuffs or tuffaceous rocks at or in the immediate vicinity of the slip surface is attributed to: (1) an increase in clay materials along the slip surface as the slide movement is repeated and (2) the presence of groundwater that promotes the formation of smectite. As is commonly known, smectite is one of the clay minerals that has lowest frictional resistance. 相似文献