A numerical method is developed for calcutating the tides in Ariake Bay, which is well known to have large tides and vast tidal marsh. The basic equations are transformed into the finite difference equations and they are calculated by ADI method. Because most significant wave period of the tides in Ariake Bay is about 12 hours, effects of tidal marsh are treated by an assumptin of the quasi-statics. Boundaries between land and ocean are moved at every time steps due to the topography and water level which is obtained by the extrapolation from the calculted values at ocean side. Tidal levels obtained by this method show good agreement in comparison with the observations given by tidal harmonic constants. Some flow patterns and water level distributions are presented graphically.
