can't find "is diversion?" in the pipe properties

Please check if the numerical solver is set to GVF-Convex solver for modeling diversion.
Please go through the support solution mentioned below , whcih explains about modeling split flows.
communities.bentley.com/.../8582

To add to what Sushma provided here, the Implicit solver in SewerGEMS doesn't have a property field for "Is Diversion?" because the Implicit solver can model split flows. SewerCAD and StormCAD do not model split flows, thus a diversion needs to be set up.

So the SewerGEMS will model split flows automatically. You can add control structures to conduits if there is a weir on one of the conduits where the split occurs.

Regards,
Scott