A tidal simulation of Ariake Bay—A tideland model

Abstract

A three-dimensional primitive σ-coordinate model is developed to allow for tideland. The model determines the coastline position each time step based on a minimum threshold depth, and extrapolates the three-dimensional predictive variables onto tideland only when the water depth exceeds that threshold value, assuring that the extrapolation is consistent with physics as well as with the numerical scheme involved. The model is applied to an M2 tide in the northern estuary of Ariake Bay characterized by the large tideland. The model successfully simulates flood and ebb tides during which a large area of tideland is covered and uncovered with water due to the large tidal difference in sea level. The model also reproduces a strong salinity front caused by the freshwater runoff from Chikugo river. The general patterns of model-computed tidal flows and density front are consistent with data available in this region. The mean flow field averaged over a twelve hour period shows a strong northward current along the slope accompanied by anticyclonic eddies over tideland, the latter feeding a southward transport along the eastern coast. It is shown that such a circulation pattern is enhanced by the joint effect of baroclinicity and bottom relief. Finally, some implications of model results are discussed in relation to the fishery.