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I currently have a terrain that changes from northwest to southeast. Normally, after I set the lower right boundary as an open boundary for free outflow, the rainfall should flow out from the lower right. However, in reality, water is flowing in from the lower right open boundary for free outflow. Could you please explain why this is happening?Thank you so much

Hard to see with no info on the elevation of the terrain at the boundary and the eleveation specified for the boundary. if water flows in at a free flow boundary that norammly means your water elevation at the boundary is higher then inside you model (terrain as inital depth = 0).

I had a similar issue when using the Finite Volume kernel.
Are your boundary conditions set to free and open depth and velocity (4 4)?

Such a setting can result in incorrect water inflows.

I recommend you check this previous topic: v8p4r0 causes liquid boundary errors

www.opentelemac.org/index.php/kunena/16-...quid-boundary-errors

Try the BC recommendations suggested in the post.
It would also be nice to definitively know what the correct BC settings are for such a model type.