| Product(s): | SewerGEMS, CivilStorm |
| Version(s): | 08.11.XX.XX and higher |
| Area: | Calculations |
When using the Implicit or Explicit (SWMM) dynamic solver in SewerGEMS or CivilStorm, you will see a value for "Not Converged" in the Calculation Summary that appears after computing a model.
What does this represent, how is it calculated, and what can be done to reduce it if it is too high?
The "not converged" is the percentage of timesteps during which at least one element did not converge within the maximum number of trials per timestep. This is documented here as it is from the SWMM standard.
So for example if you see a large value such as 100%, it does not necessarily mean there is a problem with all results as it could be for example one specific element that happens to not converge every timestep. Therefore there will be a tendency for a large not converged percentage for large models because the chances will be higher that one element somewhere does not converge at a given timestep.
If you are seeing the high value for the percentage not converged, first check if the Flow Continuity Error" (mass balance error) is also high. This is shown in the Calculation Summary and the following article has guidance for how to pinpoint problem areas in the model and address them: Troubleshooting unstable SewerGEMS and CivilStorm model results using the Explicit SWMM Solver
To increase the chances of achieving convergence, you can also try increasing the "Maximum Trials per Timestep" calculation option (for example try 20, 40 or 100 instead of the default of 8, for sensitivity). However if you find that you are still seeing a high percent not converged, the problem may be elsewhere. Try looking for pipes with very short lengths (for example under 1 foot), or try a smaller Routing Step (calculation option) such as 1 second or as low as 0.1 seconds.
Note that a future version of SewerGEMS may utilize the updated SWMM 5.2 solver, which includes details of specific elements with the highest percent not converged, to help you pinpoint problem areas with convergence.
Troubleshooting unstable SewerGEMS and CivilStorm model results using the Explicit SWMM Solver