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

Product(s): SewerGEMS, SewerCAD, StormCAD, CivilStorm
Version(s): CONNECT Edition, V8i
Area: Output & Reporting


What is the difference between the fields “Is Surcharged” and “Is Overflowing" when using the Implicit or Explicit (SWMM) Solver?


The “Is Surcharged” and “Is Overflowing” fields are properties attributed to the results of a manhole or a catch basin element. Note: Starting with CONNECT Edition Update 2, cross section elements will also remove "Is Overflowing" and "Is Ever Overflowing."

When we say that a node is surcharged it means that, the hydraulic grade of the incoming flow is greater than the downstream pipe's soffit (top/crown) but is less than the rim elevation of the manhole minus the free-board for the manhole or catch basin. The rim elevation is the level at which the manhole access cover is located. Most of the times it is flush to the ground level but sometimes it may be slightly above or below the ground elevation depending on the designer. The free-board height of the manhole is the distance between the rim elevation of the manhole and the water surface level.

When we say that a node  is overflowing it means that, the hydraulic grade of the incoming flow is greater than the rim elevation of the manhole (or ground elevation of the catch basin or cross section). This indicates that water is no longer contained in the conveyance system and is overflowing out of the system through the nodes. It should be noted that these results are exclusive to the Implicit and the Explicit (SWMM) Solvers only. In the GVF-Convex (SewerCAD) and GVF-Rational (StormCAD) solvers, these results are not considered.

The below image provides a better understanding of both the concepts, for a non-bolted structure.


1. The above assumes that the structure is not bolted. For a manhole with "is bolted?" set to "true", no overflow/flooding will occur and the structure will be surcharged with a hydraulic grade above the rim elevation.

2. The pipe soffit (top/crown) is determined by adding the Rise (Height) of the downstream pipe to its upstream invert. If the downstream pipe is set to use the invert of the upstream node, the invert will be the node's invert elevation. The information is used for the scenario being analyzed from its assigned physical alternative.

3. In pre-SELECTSeries 4 versions the terminology used was “Is Flooded?” which has since changed to “Is Surcharged?” and similarly “Is Ever Flooded?” has changed to “Is Ever Surcharged?” starting with the Select Series 4 release.

An easy method to check if your nodes are surcharged or overflowing is to assign annotations of “Is Surcharged” and “Is Overflowing” to your nodes and see which of those values returns as “TRUE”. Those are the nodes and conduits the user can focus to remedy. In addition to this there are result the result fields of “Is Ever Surcharged?” and “Is Ever Overflowing” which tell you if a node is ever surcharging or overflowing in the entire period of simulation (Extended Period Simulation). Another way is to color code the nodes as per the values for “Is Overflowing?” and “Is Surcharged?” which would give a quick view of the number of nodes for which these criteria is fulfilled in case of a large network.

See Also

Zero overflow at manhole despite HGL above rim.

Overflow reported at catchbasin even though HGL below rim elevation

Why is the HGL reset to rim elevation for flooded structures?

How do the dynamic solvers calculate overflow from a catchbasin or manhole?

Notifications about conduits operating under pressure or being in the Preissmann slot

The result field "Depth (Flooding)" is listed as N/A