If your model does not successfully calculate, try the following steps:

1. If you are running hydrology (rainfall on catchments) as well as hydraulics, check the outflow hydrographs from catchments to make certain they are reasonable.
2. Check the model for errors:
3. Use the Validate command and look at the warnings and/or errors that are reported.
4. When you Calculate the model, validation routines are performed that are not included during a Validate operation. Review the warnings and/or errors that are returned from both levels of validation.
5. Common data problems to look for include:
6. Incorrect channel or conduit slopes: Reasonable slopes are generally small and rarely negative. You can view slopes along a reach visually by using the Profiles feature. You can find unusually large or negative slopes through User Notifications and FlexTable reports. You can Color Code link elements by Slope and look for excessive values. If the model has parallel conduits connected by a flat (slope value of 0) conduit, try making that conduit Inactive.
7. Incorrect Channel or Conduit size: Look for unusual size changes along a reach. Color code drawing by Diameter to look for this type of discrepancy.
8. Very low flows: If flows are less than 0.01 cfs (0.001 m3/s), depths may fall below accuracy tolerances.
9. Look for areas displaying common modeling difficulties to verify input data is correct:
10. Flow splits at weirs and orifices that are dry at certain points during an extended period simulation.
11. Hydrograph rapidly changes within a short time (minutes).
12. Very sharp flood wave.
13. Near-critical slopes.
14. Significant and abrupt changes in the conduit size, shape and/or slope.
15. Looped networks.
16. Backwater up to a control structure.
17. Significant backwater conditions.
18. Flow control structures on relatively small storage nodes (ponds, wet-wells).
19. System inflows vastly exceeding the system capacity resulting in mass flooding.
20. Unusually small ponds compared to their inflow.
21. Many pumping stations in the system.
22. Look for a mix of very long and very small pipes, especially when using the SWMM engine. Eliminate or combine short pipes because their effect on routing is small. Break exceptionally long pipes into mutliple pipes that are each roughly the same length as other pipes in the network.
23. Examine the User Notifications that are displayed after calculating.
24. Examine Graphs and water surface Profiles. Create Flow plots at splits and at pump discharge areas and look for jagged peaks in the plot.
25. Default values for calculation options will work for the majority of cases, but some systems need small adjustments to converge. When the calculation is moving very slowly (you can observe that the model is stuck at certain times) or the results show apparent instabilities, it is an indication that the model is experiencing difficulties in converging to a stable and robust result. Try adjusting calc options in this order:
26. Initial conditions: Options include warm start or transitional start. Try both and see if one gives better results for your system.
27. Computational Distance, Calculation Time Step, and NR Weighting Coefficient: Loop through the following process:
28. Try an NR Weighting Coefficient value of between 0.9 and 0.99 with the default Computational Distance and Calculation Time Step.
29. Set NR back to default and try reducing the Computational Distance value.
30. Set Computational Distance back to default and try reducing the Calculation Time Step value.
31. Keep the Calculation Time Step the same and repeat the above steps.
32. Try increasing the the NR Iterations to 20.
33. Try increasing the LPI Coefficient to a value over 10.0. There is no absolute rule on whether the time step, LPI Coefficient, or the NR weighting coefficient should be changed or to what specific value; normally you should reduce the time step and increase the NR coefficient but sometimes the opposite can also help.
34. Isolate problems areas: Isolate the problem area by incrementally deleting small sections of your model and re-computing. This may help you narrow down the source data that the engine has trouble with. It may expose data entry issues or areas that are exhibiting common modeling difficulties.
35. Determine at what time step the problem occurs. Look for what is happening at that time. Is a weir beginning to overflow? Is it the first time a large pump comes on?
36. Switch to using the SWMM numerical engine. If there are problems when using the SWMM engine, try changing the Routing Method from Dynamic Wave (default) to Kinematic Wave or Uniform Flow. These methods do not handle backups as accurately as dynamic wave but they tend to be more stable.