Effects of Seawater Flow on the Soil Structure in Tidal Flat

摘要

The purpose of this study is to clarify the effects of flow of overlaying seawater on the soil structure in tidal flat. A natural-like man-made tidal flat and a typical man-made one, both located in a deep inlet an island of in Hiroshima Bay, were selected in this study. Bacterial population, organic matter content and silt content of the soil in the natural-like man-made tidal flat were higher than those in the man-made ones. The shear stress by seawater flow on the sediments of natural and constructed tidal flat was compared. The amounts of sediment trap and seawater flow rate in the typical man-made tidal flat were 4 times and 2 times, respectively, as high as these in the natural-like man-made tidal flat. The shear stress to the surface of the typical man-made tidal flat was higher than that of the natural -like man-made tidal flat. The frequencies of the shear stress higher than the critical value for - silt transport in the typical man-made tidal flat were 1 to 17 times higher than that in the natural-like man-made one. These results indicate that the difference in the soil structure between the two tidal flat was due to the difference in the shear stress caused by seawater flow. Furthermore, the effects of the seawater flow on other tidal flats in Hiroshima Bay were investigated. The results also indicated that the profiles of shear stesses in man--made tidal flats were higher than these in natural ones. It is probable that the seawater flow plays an important role in determining soil structure in tidal flats.