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Problem modeling pond outlet structure - SewerGEMS

I am trying to model the existing condition for a detention basin.  The existing outlet structure is as follows:

1)  Three grates aligned vertically to the flow measuring 5'w x 3'h (Elev. = 255.70)

2)  Three grates aligned horizontally to the flow (riser?) measuring 5' x 3' (Elev. = 258.80)

These are in a concrete box structure and really only act as a trash grate without any real attenuation of flow.  This is consistent with observation and data received from the town.  The discharge from the box is a 24" RCP (Inv. = 254.90).

Now I know that for events greater than 5-year storm the 24" RCP outlet is surcharged which produces a tailwater at the pond outlet control structure against which the water surface in the pond has to work against to push the flow through.  My model would indicate otherwise so I know there must be something wrong with how I am modeling it.  My profile through the pond shows a higher HGL in the structure than in the pond which doesn't make any sense.

How does SewerGEMS use the computed HGL at the upstream end of the 24" RCP when computing the discharge through the outlet control structure?  Seems that if the 24" RCP outlet is undersized and the HGL in the outlet structure is above my grates (or at least impinging on them) then the discharge is really controlled by the limited capacity of the 24" RCP outlet pipe.

I am currently modeling the outlet structure with the two grates as area orifices and then the 24" RCP outlet pipe is just another element in the SewerGEMS model which leaves the structure.

Thanks for any help.