Recently, it is found that the bottom sediments of many harbors of Japan are contaminated with toxic chemicals. Extremely fine material such as clay and silt adsorbs the toxic chemical, and has a tendency to quickly go into suspension during the dredging process. Therefore, sediment resuspension largely depends on the dredging technique adopted. In the grab dredges, the operation of a bucket controls the dispersion of dredged material. This paper discusses the relationship between operating conditions and the turbidity cased by grab dredge using the watertight bucket at the closed water surrounded by revetment.
Recently, dioxin that exceeded environmental standards in some places is found by the investigation of the nationwide public waters. The main causes of dioxin pollution of Japan are the agricultural chemicals used in the past, and discharge from the incinerator. Therefore, the Japanese Government must do the countermeasure of dioxin immediately. It is important to reduce the occurrence of the turbidity by dredging because dioxin has the property that sticks to the fine sediments easily. The grab dredging is one of the methods of removing the contaminated sediment that contains dioxin. But, the research on sediment release from dredgers in recent years is paucity in Japan. Many of researches in recent years are the monitoring techniques of the turbidity and the researches on the diffusion forecast. The research that decreased the generation of the turbidity by construction was done in Japan in the 1970's when Marine Pollution Prevention Law was enacted. Koiwa (1977)'s research clarified that the turbidity is evaluated by the parameter of (grabbed weight per cycle) X (hoisting speed), and there exists the operational conditions that reduce the turbidity to minimum. However, the relation between using parameters and sediment release was indefinite, and the resuspension mechanism by dredging was not discussed.