Conduit slope set to zero

Question

Hello, 
 
 I have a scenario that has 2 above grounds basins both that have the same bottom surface elevation. Basin A is the "upstream" basin, while Basin B is the "Downstream" basin. Flow from basin A goes into Basin B through a pipe that has zero slope (Conduit CO-1, refer to below screenshot), basically acting as additional storage for the routing, or so that's my thought. Basin B has an overflow structure which is used to discharge all of the project flow rates to my point of compliance. 
 Here's where i need help. I'm trying to understand the results of the analysis. Intuitively, the pipe acts as a culvert or orifice and the flow direction in the pipe will depend on the headwater of each respective basin. If the upstream basin HGL is higher then the HGL of the downstream basin, that tells me that there's positive flow. If the opposite occurs at a certain time step, then its negative flow. The results of my analysis does now show that and i'd like to understand why. Additionally, i'd like to understand how the flow is determined for the conduit in this situation. 
 The below snippet shows the time step at 0.43 hrs, which Basin A HGL at 1267.55' and Basin B at 1267.47'. This would tell me that there is positive flow, and the direction of flow is going down the way i'd expect it to, but instead, the calculations suggest the flow is negative and its going upstream.

If anyone could help me with understanding this, i'd really appreciate it. 
 
 Thank you,

Jesse Dringoli · Accepted Answer

Hello Mike, 
 You logic is indeed what one would expect - flow in the conduit between the ponds should travel toward the pond with the lower elevation. However, Interconnected ponds in CivilStorm can take some finesse to achieve stable results and the unexpected flow may be due to a stability issue. Without seeing the model, here are a few suggestions to look into: 
 1) Check the pond outlet node attached to Basin A and make sure the headwall/tailwater settings are correct, as well as the Flow Direction of individual outlet components. If you have "has outlet structure?" set to False for the outlet node, consider changing it to "true" and entering a control structure for better stability. 
 2) Try adding a small slope to the conduit 
 3) If there is minimal restriction from the pipe between the ponds, and if the pipe is at the bottom of both ponds, consider combining them into a single pond with an equivalent stag-storage curve. 
 4) Use a smaller calculation timestep, adjust other parameters, or use the Explicit (SWMM) solver (if you are not using it already) with a small Routing Timestep. See more troubleshooting tips in the following article (make note of the parts that talk about ponds) - 
 Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver 
 Troubleshooting unstable SewerGEMS and CivilStorm model results using the Explicit SWMM Solver 
 5) Try upgrading. I see that you're using an old version, V8i SELECTseries 4. It is possible that improvements made to the numerical solver in more recent versions may improve stability. Plus, there are many other features and enhancements introduced in the CONNECT Edition. See more here: 
 What's new in CivilStorm V8i SELECTseries 5? 
 What's new in CivilStorm CONNECT Edition? 
 What's new in CivilStorm CONNECT Edition Update 1

Conduit slope set to zero

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