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.
Hello Jason,
SewerGEMS computes a EQTW table for the pond outlet structure, which is used to determine the outflow for a given headwater and tailwater. So, the tailwater from the conduit should be accounted for.
If the grates in your outlet structure aren't attenuating any flow, then you could consider ignoring their effect, by choosing "no" for the "has control structure?" attribute of the pond outlet node. In this case, the outflow hydraulics will be determined by the conduit capacity and the tailwater should still be accounted for.
With that being said, there could be a problem with your data input, or possible some instability in your model results, leading to the problem at hand. I would first recommend checking if you're using the latest version available. You can check this under Help > About SewerGEMS. The latest version is 08.11.02.46. A thorough check of the data input for the surrounding area would be a good next step. Next, you may need to try tweaking some of the advanced calculation options. Please see the help documentation for guidance.
Beyond this, you should send your model to technical support for further help. You can easily do this by zipping up the .swg and .swg.mdb files, then attaching them to a service ticket along with a description of the problem. You can submit a service ticket using the service ticket manager.
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Thank you Jesse.
I get good results - expected results - by choosing "no", as you suggested, for the "has control structure?" attribute of the pond outlet node. However, I must show the town that modeling the structure as it exists - with the grates that do nothing - provides no attenuation of the flow. This said, I would expect to get the same outflow hydrograph doing it both ways and I am currently not. The inflow hydrograph is good so there must be somthing going on with how I defined the outlet control structure and how it's linked to the 24" outlet pipe. I just started using SewerGEMS six months ago and have been able to figure most stuff out but this one has me stumped.
Thank you.
Jason,
Please send your model over to technical support and we'll help you out.