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In SewerGEMS or Civilstorm, why do I see zero overflow for a manhole or catchbasin during a time step in which the HGL at that node is above the rim?Problem Number 32696
First, for a manhole, make sure you do not have a bolted cover (Bolted Cover? = True). If you do not, then this is likely caused by tailwater. In such a case, you can overcome this issue by increasing the LPI coefficient in the advanced calculation options. For example, try a value of 5.0 to 12.0. This allows the calculation solver to have more dynamic effects and account for the overflow from backwater. Note that this will only work for circular conduits.
For a catchbasin, if there is a downstream gutter, the "overflow" will be accounted for in the downstream gutter and would not be overflow in the traditional sense in that it is not lost from the system.
Troubleshooting unstable SewerGEMS and CivilStorm results using the Implicit solver