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Screw Pump Outfall Elevation-Flow Curve (SewerGEMS)

Hello,

I am modelling a collection system flowing to a screw pump (Archimede's screw) which lifts sewage to a treatment plant. I am not modelling anything downstream of the screw pump, and have the screw pump defined as an Outfall with an elevation-flow curve boundary condition. The maximum flow rate of the screw pump is 59.5-CFS at elevation 918.52. During high flow events, when inflow into the screw pump exceeds pump capacity, the screw pump should continue to discharge at a constant 59.5-CFS above elevation 918.52 as sewage begins to back-up into the collection system. Instead, SewerGEMS, is allowing the Outfall node to pass flow in excess of 59.5-CFS while artificially lowering the HGL at the outfall.

See the attached image for flow and HGL at the Outfall node along with the screw pump rating curve. Any idea what is going on here? Is the Outfall attempting to lower the HGL to the normal or critical depth elevation in the outfall pipe? My goal is have the screw pump discharge at a constant 59.5-CFS whenever the outfall elevation is above 918.52 so I can study the back-up in the collection system under various storm events.

Thank you.