Problem

When attempting to compute a model with the Explicit (SWMM) solver in SewerGEMS or CivilStorm, the calculation fails and the following User Notification appears:

"The SWMM engine failed during the calculation.  Go to the Report Tab on the Calculation Summary Report which can be accessed via Analysis/Administration > Summary."

In the SWMM Calculation Summary, the following message appears near the bottom:

Access violation in module execRouting at step ___, hour _ --- execution halted.

Solution

This message generally indicates that the numerical solver encountered an unexpected situation, causing it to fail during the calculations.

Situation 1 - on-grade inlets with no bypass gutter or inverse-sloped bypass gutter

In one case, this has been known to occur if there are catchbasins set to be on-grade but with no downstream gutter attached, and the numerical solver encounters a timestep during which bypass flow occurs. Run the Validate command first to see a list of such cases:

"The associated 'On Grade' inlet does not have a bypass gutter.  The bypass flow will exit the system."

Or, run the "Inlets on grade without bypass gutters" query within the "Network Review" section of Network Navigator.

In another case this has been known to occur if there is a bypass gutter attached to an on-grade inlet but the bypass gutter has an inverse (negative) slope. 

To fix this, change the offending inlets to in-sag, or add/fix a downstream gutter. For the inverse slope issue mentioned above, install Patch43 or later of version 10.03.04.53 or install a newer version when available. (reference # 788333 and 887501)

Situation 2 - extreme instability

This issue may also occur in cases where extremely unstable results occur. Try using a smaller Routing Step in the calculation options, or use the guidance in the following article:

Troubleshooting unstable SewerGEMS and CivilStorm model results using the Explicit SWMM Solver

In one example case, extreme instability was occurring due to an excessively large inflow due to a units mistake. Correcting this to prevent extreme overflow and instability resolved the issue. A more specific message may be generated in future versions (reference # 887501)

In another case, the selection of "True" for the calculation option "Use Bentley Transition Equation" caused extreme instability with excessive flooding. In some cases turning this off will help stabilize the results.