Pressure inlet BC in open channel flow?
 

Hi everybody! I hope someone can explain this to me.

We have been modelling an open channel flow in a flood discharge tunnel. Our professor has told us to choose "pressure inlet" as the boundary condition for the tunnel roof, even though it is a closed tunnel, and IMO a wall BC would do just fine.
I have tried to read about this, but I can't really understand why this should be the appropriate setting. Do any of you know this?