Hi,
Can you explain why the maximum hydraulic grade line is increasing when additional cross-sections are added and keeping all other parameters unchanged?
In current drainage model comprising of open channels & culverts the maximum hydraulic grade level (HGL) is observed to be 8.5m (approx.) with cross-section distance varying between 100-120m. But the observed HGL is on higher so inorder to control the HGL, we reduced the cross-section distance to 15-25m by adding additional cross-section nodes. In both cases Implict (SewerGEMS Dynamic Wave) solver is used.
Keeping all other parameters same by making the above changes, HGL increased by 2.0m (approx.). But based on my experience with similar kind of network models when developed in HEC-RAS model we never observed such steep raise in HGL (results still not compared with 1D unsteady HEC-RAS).
Can somebody explain me the reasons leading to increase in HGL.
Thanks
Satish
Hyderabad India
Hello Satish,
I tried to reproduce the issue mentioned by you, but max HGl rather reduced slightly in my testing.
Can you please upload the model files for our review?
Sharing model files
Regards,
Sushma Choure
Bentley Technical Suppport
One factor to consider is the continuity error. if the model is hydraulically sensitive, it could be that the change in physical parameters results in a large enough change in convergence to cause the unexpected change in hydraulic grade.
Check the continuity error figure in the calculation summary before and after the change. Also, graph flow in the general area and make sure the graphs appear stable. If you were comparing the Maximum HGL figure, you might find that a short-lived instability (or spike in HGL) is causing the difference, whereas the rest of the results match.
If the problem appears to be with stability, first check your user notifications, review all data entry for issues, make sure you have the latest version (we tend to improve the numerical solver stability with each version) and adjust the advanced calculation options if necessary. You might also try using the Explicit (SWMM) solver. You can read more here: Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver
If this does not help, please upload a copy of the model as Sushma mentioned, then reply to the thread to let us know.
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.