Flow out of detention pond decreases with less storage

Question

Bentley Product:: SewerGEMs V8i (Selectseries 5) Version 08.11.05.58 64 Bit 
 Project Notes: Analyzing Node DB-Detention Pond 
 Model A: Includes 8 detention systems to control runoff before it enters detention pond DB. 
 Model B: Includes 4 detention systems to control runoff before it enters detention pond DB. 
 Issue: 
 The flowrate into node DB does not change with additional detention storage under 10-Year Storm Model The flowrate out of node DB increases with additional detention storage (model A) under 10-Year Storm Model. 
 Additional notes: 
 Upstream detention nodes include orifice and weirs to control watershed runoff. 
 
 SewerGEMs - NEWR.zip

Jesse Dringoli · Accepted Answer

Suraj, 
 "Model A" is set to use a 1-year storm while "Model B" is set to use a 10-year storm. This means runoff will be higher from catchments in the Model B scenario. 
 I see that the "detention systems" that you added in the "Model A" are 4 additional ponds upstream, in series with the other ponds that ultimately discharge into the "DB" pond. It seems that this model distributes catchment runoff to these four additional ponds, to help prevent PO-6 from overflowing. If I change Model A to use the 10-year storm (to match Model B), I can observe this impact - PO-6 does not overflow and the overall volume of flow is the same (just attenuated from the ponds) 
 This does not necessarily decrease the peak flow into the "DB" pond though, because of a few factors: 
 
 The volume of water that was overflowing (and lost to the system) in Model B (with only 4 ponds), is now routed through the system and reaches the pond in the Model A with 8 ponds. 
 POS-19 is not configured with a pond outlet structure and is contributing to instability, impacting results. It is always best to set "Has Control Structure" to "True" and specify an outlet structure (for example even an orifice with the size size as the connected pipe). 
 If the continuity error is still above ~5%, the results may be further skewed. The Implicit dynamic solver tends to have more difficulty with complex interconnected pond systems so you may want to try using the Explicit (SWMM) solver. This is mentioned in the following article which has further troubleshooting tips: Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver 
 
 Additionally, I noticed that you're using quite an old version, which will no longer be supported soon. See this article for more about our Support Policy and this article about upgrading. We have introduced many improvements to the numerical solver in newer versions, which may impact the results for your model.

Jesse Dringoli · Answer

Hi Suraj, 
 When a pond's water surface elevation reaches the top, overflow occurs which is lost to the system. It does not get routed anywhere and it accounted for in the "overflow volume" in the mass balance seen in the Calculation Summary window. 
 If you need to direct the overflow somewhere else, you will need to use a separate pond outlet node and direct the discharge using a conduit connected to an outfall, connected to the downstream element. See this article for more.

Flow out of detention pond decreases with less storage

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