Study of the submerged dike roles for protecting siltation in harbours and access channels

Cohesive sediment accumulated in navigation channels and harbours defined as silta- tion, is commonly the main cause of the navigation problems, and this takes place especially in shallow water areas. To take countermeasures against siltation, mainte- nance dredging is commonly carried out. However, maintenance dredging is expensive because it has to be repeated year after year.

The major source of siltation is considered due to fluid mud invasion by waves and currents, and some special engineering devices are required to cater to this problem. One of the devices is submerged dikes. A method to check invasion of fluid mud by submerged dikes was adopted in Kumamoto Port, Japan in 1992 and follow up survey has been conducted so far and the effectiveness has been confirmed. However, the mechanism of submerged dikes to control fluid mud inflow is still unsatisfied.

This paper aims to investigate the fluid mud behavior around submerged dikes and to observe the roles of submerged dikes for protecting siltation. Further, to explain the fluid mud role in siltation and its behavior around the submerged dikes, some experiments were done using special flume (slim-tank). In order to solve and to support about the phenomena that still unclear from the physical model, the numerical model was also held. In one of the results of study; it is concluded that if submerged dikes were higher than the fluid mud height, the capability to control fluid mud inflow would be much higher. Moreover, we found that the 'inverse v' shape was considered as the most excellent submerged dike type to protect the channel from fluid mud invasion.

In order to release the effect on fishery work and ship navigations causes by the limitation of dike height or in areas where coastal water is very shallow, the combination of double rows i.e. 'inverse vT' is one type of submerged dike to be considered.