Mstn v8i SelectSeries3 - v08.11.09.357
InRoads v8i SelectSeries2 - v08.11.07.566
I'm having a problem with the EGL/HGL calculations for a storm sewer system. That SDB, a DGN, and some support files are attached in a zip file for review. When pipes G-7-P-4 and G-7-P-5 are set to 18" diameter, then HGL and EGL are calculating as expected. However, when the same two pipes are changed to 15", it appears that the EGL is being used as the HGL at the upstream end of the pipes. That makes the system look like it is surcharging. Can someone from Bentley take a look at this and let me know if it's a bug, or if there is an error in our SDB?
When I tested G-7-P-5, the HGLup was the invert + depth of flow for both the 18" and 15". What were your settings in the Network Design dialog?
Toggle on "Enable Time of Concentration." Once that is on, my settings match what you have shown.
OK... that toggle did the trick for me to reproduce it. With the toggle ON, the flow increases to an amount large enough for the pipe to be Subcritical. With it toggled OFF, the flow is smaller, and pipe is Supercritical.
Here is the problem, since the outfall slope is so steep, the EGL is high at inlet G-7-IN-6. The HEC-22 Energy Grade Line Evaluation Procedure step 5K (page 7-43) states that losses in the inlet do not apply when supercritical occurs in two successive pipes. So when the program evaluates pipe G-7-P-5 and sees that it is subcritical, it thinks it is OK to carry the loses upstream (because it is only the second pipe in that branch), so it sets the HGL up in the pipe equal to the elevation of the EGL (high) minus the velocity head. This is obviously not correct.
The scenario in this dataset is one that I thought we had fixed in the 566 build (D-08386), but this dataset is showing an aspect of the problem is still around. If the outfall pipe is supercritical, the loses should not be carried upstream. I have filed D-128303 with development to fix this problem. As a workaround until it is fixed, you could get a pretty good idea of what the HGL/EGL elevations would be in reality if you temporarily ran a model with a less steep outfall (say 1% or 2%, whatever the smallest slope is that still yields supercritical flow in the outfall).
Answer Verified By: David Lanham