What is the difference between the Hydraulic Grade and Energy Grade 'Structure Loss' calculation option?

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

Problem

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

Solution

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. 

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? 

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