What happens when the water level (HGL) exceeds the top elevation of an open channel?
The solution depends on the numerical solver you are using.
At cross section and channels, the program will extend the elevation at the element *vertically*, something like a wall going up from the either side of the cross section. Thus when the HGL gets to a level where overtopping would occur, the HGL will increase above the ground elevation. Because this extends up, basically to infinity, there are no overflows reported and the HGL can be above the ground surface. Note that no overflow (lost flow) occurs in this case. Overflows can occur at manholes, catch basins, wet wells, and ponds, though.
The topic on this subject in the Help is saying that if the HGL is above the ground elevation at a cross section, overflows will not occur. If the channel ends at a manhole though, overflow can occur there.
You will still get a user notification at a cross section stating that the cross section is overtopping (it will also be evident in profiles).
With the SWMM solver, if the water surface exceeds the top elevation of the cross section, the hydraulic grade is "capped" (truncated) and overflow will occur. The top elevation is the "Elevation (Invert)" plus the "Height" field. When the water depth exceeds the "Height", computed overflow is based on the cross section characteristics at the top elevation.
For the StormCAD and SewerCAD solvers, cross sections are treated as manholes. The hydraulic grade line will reset to the ground elevation at a cross section or manhole when the HGL is above the ground surface elevation at that node.
Why is the HGL reset to the rim elevation for flooded structures?