We investigated how the water exchangeable breakwaters promote seawater exchange between a harbor and its outer region using numerical simulation. The flow was calculated with nonlinear baroclinic two layers model taking into account the flow through a water exchangeable breakwater. In this model, a breakwater was expressed as a line on the grid space without thickness. In order to calculate the water level at inside and outside of a breakwater, water level was defined with 4 divisions at 1 point. The mass flux through a water exchangeable breakwater is calculated by an energy equation of non-steady flow through a tube. Characteristics of seawater exchange at a harbor were investigated by many water particle trajectories. Distribution of water residence time and variation of exchange ratio were evaluated. Dominant horizontal oscillating flow had little effect for water exchange; therefore, residual flow including vertical component played on important role for it. When exchangeable breakwaters were arranged to promote tidal residual circulation, water qualities of the harbor was improved effectively.
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