Tsunami protection breakwater is placed at the mouth of a bay in northenl Japanese coast Sanriku to prevent from threats of tsunami disaster from Pacific sea side. After the tsunami protection breakwater is constructed, a degradation of water quality in a bay is anticipated by decreasing the exchange rate of seawater. In this study, concentration of dissolved oxygen is used as an index of the deterioration of water quality. This study examined the effect of the tsunami protection breakwater on the temporal change of the dissolved oxygen concentration in a bay. Field observation and numerical calculations are conducted for Kamaishi Bay where the tsunami protection breakwater is under construction.
　We try to clarify the long-term change of the dissolved oxygen concentration by the installation of tsunami protection breakwater. First, Ohfunato Bay where the tsunami protection breakwater had already been set up was examined. A long-term trend of dissolved oxygen concentration change is not detected.
Therefore, it can be considered that the dissolved oxygen concentration in the bay is controlled by a balance between seawater exchange and load for one year. Next, the estimation of dissolved oxygen concentration in Kamaishi Bay was carried out numerically.
　A non-hydrostatic three-dimension model was used for the determination of the exchange rate of seawater. A one-dimensional vertical model was used for the prediction of dissolved oxygen concentration over a long period. The computations were carried out for five cases. Different settings of the breakwater include no breakwater case were examined.
　When the exchange rate of seawater decreases due to the construction of the tsunami protection breakwater, dissolved oxygen concentration in deeper part decreases in summer seasons. However, no rapid decrease of dissolved oxygen concentration as a long-term trend is predicted under the completion of the tsunami protection breakwater.