Comparative Tidal Current Observation in Ariake Bay with the Previous Results
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AbstractAriake Bay is a semi-enclosed bay located in the west coast of Kyushu, where it is well known that the largest tide in Japan occurs. It is called Shimabara-Wan in nautical chart and its inner area Ariake-Kai, but we call it Ariake Bay in this paper as used for general purpose. In autumn 2000, seaweed plants in Ariake Bay were severely damaged by extra-ordinary blooming of phyto-plankton, and people were afraid of the marine environment getting worse. Recently, tendency of tidal amplitude reduction has been pointed out and suspected as one of the causes of marine environment deterioration. Furthermore, the reduction of tidal current is also doubted concerned to the tidal amplitude reduction. Hydrographic and Oceanographic Department, JCG, was requested to carry out tidal current observation in order to check the change comparing the previous tidal current in the bay observed in August - September 1973. Therefore HOD carried out tidal current observations of 15 days at 12 stations from 10th to 28th in May 2001, including vertical current profile observations at 3 stations using WorkHorse ADCP. Their locations were set as close as the previous stations. The data obtained in previous observation were re-processed using the same calculation software as for new data. Considering the result that the tidal currents were getting strong in some place and weak in other place, it was not concluded that the tendency of tidal current reduction was distinguished so definitely. However, mean current flowing out along the peninsula coast of Shimabara was definitely reduced as 1/3 as that of previous one. Comparing the vertical profiles at the off-Shimabara station, it was remarked that tidal current was getting strong in the surface layer 3m below the sea surface, but unchanged in the sub-surface layer of 10m. While low saline water was distributed in the surface layer of the central area of the bay in 1973 and 2001 observations, its value was getting saline in 2001. Besides, the river discharge in 1973 was about 4 times larger than that in 2001 and possibly made the coastal sea water less saline. Less saline water possibly tends to flow in surface layer on the saline and denser water in the subsurface layer as density current. Therefore such larger river discharge might strengthen the mean current flowing in the surface layer in 1973. Furthermore, the flood current may be suppressed by this strengthened mean current in the surface layer but not so influenced in the subsurface layer. Then the reason of weak tidal current in the surface layer in 1973 compared to that in 2001 could be explained by this larger river discharge too. Although the difference of river discharge was not made clear by seasonal or year-toyear change, it was indicated that the river discharge could influence not only to the less saline water distribution and the mean current flow but also to the tidal current through the vertical distribution change.
JournalReport of Hydrographic and Oceanographic Researches
Page Rangepp. 33-61