Large scale sand dredging and mode of slow recovery of sand dunes in the Seto Inland Sea, Japan

摘要

During the last 40 years, a huge amount of sandy sediments were dredged from the seafloor of the Seto Inland Sea and hence sea bottom environments have been suffering various kinds of damages. For example, bottom topography and sediments of Mihara-seto Sea have been greatly changed. In order to clarify the possibility and the speed of restoration of bottom topography and sediments, precise depth measuring survey using SeaBat in regular time intervals were carried out. Comparison of depth between the data of old navigation chart and our survey shows the disappearance of large sand dunes. At the central part of the surveyed area, where the water depth was less than 20 meters, water depth exceeds 50 meters at present. Sediment composition also changed in the dredged areas from sandy to gravelly bottom. Sand waves stretching in north-south direction are distributed at the northern part of the surveyed area. Result of recent depth comparison indicates shifting of sand waves from the channels to the area far away. Their shifting velocity is estimated to be a maximum 40 m per year. Their height, width and shifting velocity decreased toward the central part. However, other kinds of topographical changes were not observed in the surveyed area. In addition, based on bottom topography data obtained by SeaBat, it was found that the total volume of sediments in the area has not changed. Based on the data obtained during the last two years, it was also clear that the undulations made by sand dredging buried into flat bottom by the shifting of sandy sediments. Restoration of bottom topography in the area is less probable because of the little amount of sediments supplied by both channels but restoration of bottom sediments may be very much probable at areas of sand wave distribution.