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When using the Implicit or Explicit (SWMM) Solver for analysis there is often a result in the manhole properties; “Is Surcharged” or “Is Overflowing”. This article explains both the terminologies and the difference between them.
The “Is Surcharged” and “Is Overflowing” are the properties attributed to the results of a manhole or a catchbasin element.
When we say that a manhole/catchbasin 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/catchbasin. 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 manhole/catchbasin is overflowing it means that, the hydraulic grade of the incoming flow is greater than the rim elevation of the manhole. This indicates that water is no longer contained in the conveyance system and is overflowing out of the system through the manholes/catchbasins. It should be noted that these results are exclusive to the Implicit and the Explicit (SWMM) Solvers only. In the rest of the Solvers viz. GVF-Convex and GVF-Rational, these results are not considered.
The below image provides a better understanding of both the concepts.
Note:
1. The 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.2. In pre Select Series 4 versions the terminology used was “Is Flooded?” which has got 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 manholes are surcharged or overflowing is to assign annotations of “Is Surcharged” and “Is Overflowing” to your manholes and see which of those values returns as “TRUE”. Those are the manholes 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 manhole/catchbasin is ever surcharging or overflowing in the entire period of simulation (Extended Period Simulation). Another way is to color code the manholes/catchbasins as per the values for “Is Overflowing?” and “Is Surcharged?” which would give a quick view of the number of manholes/catchbasins for which these criteria is fulfilled in case of a large network.
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