Zero overflow at manhole despite HGL above rim.

Applies To 
Product(s): SewerGEMS, CivilStorm
Version(s): CONNECT Edition, V8i
Area:  Calculations
Original Author: Jesse Dringoli, Bentley Technical Support Group


With the Implicit or Explicit solver 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?

Or, why is "is overflowing" or "is ever overflowing?" not showing "true", when the hydraulic grade is above the rim elevation?


Check for a Bolted cover

First, for a manhole, make sure you do not have a bolted cover (Bolted Cover? = True). If it is set to true, this causes the model to model a surcharge condition where the HGL can rise above the rime without overflow. Change this to False if the manhole can overflow.

Adjust LPI coefficient or use the SWMM Solver

If this is not happening at a bolted manhole and you are using the Implicit numerical solver, then the issue is likely caused by tailwater. In some hydraulically challenging situations, the Implicit solver may ignore overflow in favor of stability. 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. However, this will likely come at the cost of stability (and you may notice a larger continuity and less stable model results). So, you may need to adjust other advanced calculation options to compensate or consider using the Explicit (SWMM) solver which tends to be better suited for models that have this situation.

Check for Gutter Overflow

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. In other words, if the overflow is spilling out of the catchbasin and into a downstream gutter, the program does not count this as "overflow" since it is not lost to the system, and therefore you will see zero for the calculated overflow result, and "is overflowing" and "is ever overflowing" will show as "false".

Check Reporting Timestep Granularity

Lastly, if you are seeing "True" for "Is Overflowing Ever?" and a positive value for the "Overflow (Maximum)", yet the graph of HGL never reaches the rim and the overflow graph is zero for the whole simulation, the problem may be due to the timestep. There may be a brief increase in HGL and corresponding overflow that occurs between the reporting timesteps. For example with the Explicit (SWMM) solver, the Routing Step calculation option indicates how often the model calculates results, but the SWMM Output Increment indicates how often the results can be reported. The overflow could be occurring between the Output increments causing it to not display in graphs. For example check the "time to maximum overflow", it may show for example 642 minutes when the output increment is 5 minutes, so the graph only shows results for 640 minutes and then 645 minutes. If the overflow occurred for a brief period of time between these output increments, it would not display in the graph. In this case, try using a smaller Output Increment.

See Also

Troubleshooting unstable SewerGEMS and CivilStorm results using the Implicit solver