how to model inlet which is also an outlet
 

Hello,

I'm just starting with Fluent and am making a model of a rectangular tidal harbour with one entrance. I don't really know how to model this inlet which has to be an outlet at the same time.

Through this inlet the water comes in for 12 hours (high tide) and after this, the water has to go out through the inlet for 12 hours (low tide). How can I model this in Fluent?