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Why is the HGL reset to rim elevation for flooded structures?
This article applies to SewerCAD and StormCAD. For information on how overflow and flooding is handled by SewerGEMS and CivilStorm see this article.
Flooding at manholes and inlets in StormCAD and SewerCAD occurs whenever the elevation of water is above the structure rim elevation. When this occurs, the backwater analysis will continue by resetting the hydraulic grade to the structure rim elevation or ground elevation, whichever is higher. However, if a structure is defined with a bolted cover, the hydraulic grade is not reset to the rim elevation.
In actual flooding situations, flows may be diverted away from the junction structure and out of the system, or attenuated due to surcharged storage. In StormCAD and SewerCAD however, even though the governing downstream boundary for the next conduit is artificially lowered to prevent the propagation of an incorrect backwater, the peak discharges at the structure are conserved and are not reduced by the occurrence of flooding at a junction.
Basically, when you see the hydraulic grade reset at the structure, the structure is flooding. This is an indication that the system is undersized. The program will account for the lost flow from flooding at the outfall element, and this flow can be seen in the outfall properties or flextables. You should however keep in mind that the results are skewed in this case compared to a real system where some flow would exit where flooding occurs.
Note: The program's algorithm is fundamentally based on backwater analysis. As a result, a continuous frontwater analysis is not performed through two or more consecutive steep pipe.
Hydraulic grade discontinuity at steep pipes or across nodes (frontwater analysis)
SewerGEMS and CivilStorm: How to find volume (such as overflow volume) through specific elements