RMA2 - boundary conditions

[snowbell]

I would like to modeling a river which flows through a town. I have managed to create a mesh with the river and the possible flood plain area.

But I have a problem with the boundary elements which are not come in touch with the flow, which stay dry. The calculation stops with the error message: "Iterations terminated by excessive change in elevation".

How is the best way to define the boundary condition for outflow elements when there is not a parallel flow?

And which data is necessary to become an stabel model include the dry boundary elements?

I hope you can help me.
Thank you very much for your answer!