Detention pond with outlet pipe to chamber to outfall

Question

Originally posted to this link: communities.bentley.com/.../pond-s-water-level-stays-stagnant-over-time-even-though-there-is-culvert-to-empty-it 
 
 Dear Scott, 
 could you please tell how we set up composite structure in SewerGEMS for detention pond with out let pipe to chamber to outfall? would you please share example model?

Craig Calvin · Answer

Hello Mohamad, 
 By looking at the image you provided, I think that the part you labeled "outlet structure" would be best modeled as a manhole or catch basin. I attached an example model and a profile to show my recommended approach. 
 
 Detention Example.zip

Jesse Dringoli · Answer

If water in the pond can overflow the dam and spill into the top of the "outlet structure", then you could try modeling that as a second pond outlet (likely with a weir) , connected to the "outlet structure" via a channel > cross section > channel, with the downstream end of the second channel set to the rim elevation of the catchbasin node used to represent the "outlet structure" 
 You could also try modeling the "outlet structure" as a separate pond, but that may prove to be difficult to solve hydraulically and you may need to adjust calculation options to achieve stability per this article , or use the SWMM solver with a small routing step (also mentioned in the article) 
 If the top of the "outlet structure" is sealed and pond overtopping does not spill into it, then the manhole approach should be more stable than attempting to model both as ponds. Or, you could combine this into one outlet structure, in which case you would need to decide how to represent it. If the second outlet pipe has adequate capacity and the storage effects of the "outlet structure" are negligible, you could simply model this as a single orifice representing the restriction from the first opening. Or, model it as two orifices in series, by way of the "downstream ID" field in the composite outlet structure manager.