HGL and EGL discontinuity and the "Structure Loss Mode" calculation option

Applies To 
Product(s): SewerGEMS, CivilStorm, StormCAD
Version(s): CONNECT Edition, V8i
Area:  Modeling
Original Author: Mark Pachlhofer, Bentley Technical Support Group


Why do I see a discontinuity in the Energy Grade Line (EGL) or Hydraulic Grade Line (HGL) in profile view?

What is the difference between the Hydraulic Grade (HG) and Energy Grade (EG) 'Structure Loss' calculation option when using the GVF-Rational (StormCAD) or GVF-Convex (SewerCAD) Solver? Which is correct to use?

Background - Structure Loss Mode

If you choose the EG option for the structure loss mode the headloss across the structure is the EGL (In) - EGL (Out) and the HGL is then calculated based off the EGL. On the other hand if you choose the HG option the headloss across the structure is the HGL (In) - HGL (Out) and the EGL is then calculated based off the HGL. 

We offer the two options because some engineers prefer that the headloss corresponds to the change in depth across the structure, while others prefer the headloss correspond to the change in energy. Note that when you choose the HG as your option the EGL at any given point is computed by adding the velocity head to the hydraulic grade and because of this energy discontinuities may occur, such as when a pipe size decreases in the downstream direction or a pipe slope increases. If you choose the EG as your option you can see the opposite effect where there could be discontinuities in the HGL. Flow discontinuities can also be responsible for energy discontinuities, since a structure is analyzed based on a different system time than a pipe, a direct comparison of energy grades is not reasonable. 

User Notification "The calculation option Structure Loss Mode must be set to EGL in order to use the HEC-22 Energy (Third Edition) Headloss Method."

For the HEC-22 2nd edition it is acceptable to use either the HG or the EG option, although technically speaking the EG option is more correct. This is because the head loss by definition is the energy loss.  

When using the HEC-22 3rd edition, however, an engineer should only use the EG option, as this option uses the EGL procedure in strict accordance with the FHWA HEC-22 standard (PDF here)

The solution to this user notification is to either change the Structure Loss Mode calculation option to Energy Grade, or change the node Structure Loss Method to something other than HEC-22 third edition. 

Discontinuity with the GVF-Convex or GVF-Rational Solver

When using the StormCAD or SewerCAD solver, you may notice a jump or discontinuity in the hydraulic grade line. In addition to the explanation above, this could also be due to a frontwater analysis as explained in the following article: Hydraulic grade discontinuity at steep pipes or across nodes (frontwater analysis)

Discontinuity with the Implicit and Explicit (SWMM) solver

When using the Implicit or Explicit solver in SewerGEMS or CivilStorm, the structure loss method is always HGL. Normally a HGL-based loss calculation will retain good EGL continuity but sometimes it cannot guarantee local energy conservation so occasionally an EGL jump may be seen across a structure. This is because the calculation is not fully based on the energy equation. Ensure that your model results are stable - review graphs of results like flow in the vicinity of the area of interest, and check the Continuity Error in the Calculation Summary (and the "%Not converged" for the Explicit (SWMM) solver) Use the following articles for guidance on stabilizing hydraulic results:

Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver

Troubleshooting unstable SewerGEMS and CivilStorm model results using the Explicit SWMM Solver

See Also

Hydraulic grade discontinuity at steep pipes or across nodes

What are the differences between the HEC-22 2nd edition and the HEC-22 3rd edition?