Decrease in Tide and Tidal Current in Ariake Bay

There has been much discussion about the cause of the decrease in M2 tidal amplitude that has resulted from the reclamation of Isahaya Bay in Ariake Bay. As it has not been clarified yet, we tried to solve this problem using two approaches: (1) by analyzing observational data at tide stations near Ariake Bay and (2) by numerical calculations using a three-dimensional variable mesh model. Applying these two approaches to tidal amplitude at three stations in Ariake Bay from 1981 to 2001, it was shown that the 25% decrease in M2 tidal amplitude at Oura from 1981 to 2001 is caused by the Isahaya dike, and the remaining 75% decrease is caused by the attenuation of tide in the East-China Sea. This conclusion is verified by both approaches. Although the decrease of tidal amplitude in Ariake Bay responds linear to the external effects, the tidal amplitude changes caused by internal effects such as the Isahaya dike depend on the location; that is, there is a decrease at Oura, no change at Misumi and an increase at Kuchinotsu. This is explained by the change in the tidal oscillation pattern in Ariake Bay due to the reclamation of Isahaya Bay