Port and harbors have been constructed more than one hundred for commercial use around the Japanese coast and still developed and utilized for the local and national economic activity. Several ports locate at the area of shallow coast and estuary, where inevitably suffer from back siltation in dredged navigation channels and turning basins. Minimizing the harbor siltation, therefor, is key factor for the efficient port operation by reducing the cost for maintenance dredging and for the requirement of the limitation of the damping site of dredged sediments as is often the case in the world.
The purpose of the study is reviewing the current situation of the dredging activities for the maintenance of navigation channels for the Japanese ports and analyzing the characteristics of siltation mechanisms of them. The target sites are mainly in the western part of Japan, where many ports are struggling with severe back siltation sediment by the sedimentlogical and topographical reasons.
The analyses of the study include not only characterizing the general aspect of the siltation problems under the typical conditions of surrounding topography and sediment supply processes but also illustrating the sedimentary dynamics and their transport process around the navigation channels dredged in the shallow muddy coast based on the field monitored data. We finally discuss on the application of the nautical depth approach for the case of siltation by muddy sediment.
Typical topographical conditions surrounding the ports in the study area include the shallow muddy coast with the intertidal mud flat developed under the macro-tidal condition with the tidal range of around 5 m during the spring tide period. In addition to the force condition due to the tidal current, wave forces are also key to the sedimentary processes especially during the storm events due to the passage of typhoons. The port, which is located near river mouth, should be affected by the sediment discharge through the river and it is also intermittent event rather than steady phenomena since Japanese rivers are of short length and steep slope. The siltation process in the ports are strongly dominated by the intermittent events and it is one of the reasons that the planning countermeasures are not straightforward.
Through the field monitoring in the specific site at the port of Kumamoto in the Ariake Bay, the characteristics of the sediment transport processes were elucidated due to the tidal current and storm event. The monitoring campaign during the two weeks captured resuspension dynamics of the bottom sediment due to the currents and waves by the measurement of near bottom mounted current and turbidity sensors. The sediment fluxes were quantitatively estimated through the data analyses and it provides the dominant effect of the storm event. Another specific finding from the monitoring is that the wave induced by the ferry boat cause the suspension of the sediment in the area.
We also show the sedimentary characteristics of the monitoring site with the measurement of bulk density of the bottom sediment. The data were obtained by analysis of sediment core samples and by an in-situ densimeter also. In cases of the mud with higher water content, the data shows gradual increase of bulk density in vertical and the so-called fluid mud layer was detected. The measured fluid mud layers were around the thickness of 10 to 20 cm in the site. Due to the existence of the fluid mud layer, the difference between the bed levels of the acoustic bottom and the lead line detection and it becomes as much as 50 cm in the highest water content mud case.
In order to characterizing the harbor siltation and reveal sedimentary dynamics and structures in the Japanese ports, after the general classification of siltation mechanisms at the typical conditions of surrounding topography, the specific mechanisms of sediment transport around navigation channel were analyzed with the field data monitored in the Ariake Bay and the analysis demonstrated the relative importance of the intermittent storm event rather than the cyclic tidal event.
Finally, field data of muddy sediment characteristics were also demonstrated in the port of Kumamoto, where bulk density were measured with the in-situ densimeter. The data showed that the existence of fluid mud layer and it should make difficult to detect or judge the sea bottom with any acoustic devices. Since the fact is crucial for the maintenance of navigation channels and water ways, the present monitoring techniques can be applied for further discuss on the applicability of the nautical depth concept in Japan.