Zero overflow at manhole despite HGL above rim.

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

Problem

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?

Solution

First, for a manhole, make sure you do not have a bolted cover (Bolted Cover? = True). 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.

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".

See Also

Troubleshooting unstable SewerGEMS and CivilStorm results using the Implicit solver

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