Overflow reported at catchbasin even though HGL below rim elevation

Product(s): SewerGEMS, CivilStorm
Version(s): V8i, CONNECT Edition
Area: Output and Reporting


The "Is Overflowing?" field is shown as "True" and a positive value is seen for "Flow (Overflow)", yet the HGL is below the rim elevation of the catchbasin.


This can happen if there is no gutter downstream of an on-grade inlet to capture inlet bypass flow and the flow is lost to the system. You will see a user notification about this:

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

If you graph "Flow (Overflow)" and "Flow (Bypass)" for one of the catchbasins in question, you will see that the overflow is equal to the bypass flow. Because the bypass flow has nowhere to go (no downstream gutter link), the flow is lost to the system in the same manner as overflow and is counted in the Overflow result figure (and mass balance). Hence, the "is overflowing" will show as "true" because of the positive value for "Flow (Overflow)".

The solution to this is to lay out gutters downstream of the offending catchbasins, so that bypass flow will route to the gutter instead of becoming overflow. If you have multiple instances of this situation, right click on one of the related user notifications and choose "select all elements with message...". You can then create a selection set, filter a flextable, use the highlight button in Network Navigator, and other things to help you locate and address each case.

See Also

Difference between a "surcharged" and an "overflowing" node

User notification: "There is no gutter leaving this 'On Grade' catchbasin. Bypassed flow is directed to the subnetwork outfall."

Zero overflow at manhole despite HGL above rim.