When computing a model using the Explicit solver, one of the following errors occur:
"The SWMM engine only supports a single outflow/inflow link to an outfall."
"ERROR 209: undefined object (outfall label) at line #### of [PUMP] section"
The Explicit (SWMM) solver in SewerGEMS and CivilStorm does not allow for more than one pipe connected to an outfall. For instance, if you have two conduits or two pressure pipes coming into an outfall, you would see an error like the above.
If possible, you can try using the Implicit solver, which does allow for this configuration. If you need to use the Explicit solver, try including an intermediate node where the flow from the two pipes will come together, similar to the screenshot below.
Differences between solvers: GVF-Convex vs. GVF-Rational vs. Implicit vs. Explicit (SWMM)