Isahaya Bay, freshwater lake to an estuary again

• Gate opening to reintroduce sea water into the fresh water reservoir in reclamation.
• Modelling currents, hydrography and sediment transport to assess the influences of the gate opening.
• Restricted opening would generate hypoxia in the reservoir.
• Full opening would generate erosion and sediment deposition over the clam culture ground.

The inner 1/3 of Isahaya Bay which is a tributary of Ariake Sea in Japan was shut off from the sea by a dike for the reclamation and disaster prevention in 1997. On the other hand, several environmental and fisheries problems occurred in Ariake Sea after 1990s. Some fishermen insisted that the major reason for the decrease of fishing must be the influence of the dike construction and filed lawsuits. Now the court decision is fixed and Japanese government must open the gates to reintroduce sea water into the reservoir. We made numerical simulations of currents, hydrography and sediment transport to assess the influences of the gate opening. To choose the environmentally wise procedure of gate opening, it is needed to reduce the erosion and deposition of bottom sediments caused by the enhanced tidal current and to minimize the occurrence of hypoxia in the reservoir.