hydraulic grade line

Question

Hello Folks , Kindly i have a question related to Sewer-gems Software , my question is about the hydraulic grade line in a sewer networks that modeled throughout Sewer gems or sewer cad . 
 
 The Hydraulic grade lines have a small jump before manhole entrance , even-though the inlet pipe and outlet pipe has the same : diameter , flow , slope , partial full flow percent my question why this small jump happens ??

Jesse Dringoli · Accepted Answer

Khalid, 
 Please check the Profile Description of these two pipes. Most likely you have two supercritical pipes in series, which will cause a discontinuity because of the frontwater analysis performed, as described in this article: 
 Hydraulic grade discontinuity at steep pipes or across nodes (frontwater analysis) 
 Basically the upstream pipe tries to approach normal depth on the downstream end, and the next pipe needs to start at critical depth on the upstream end, which is higher than normal depth because the pipe is hydraulically steep.

Daniel.Bowdler@Bentley.com · Answer

We do not support an option to turn off the hydraulic jump that is modeled when a supercritical reach is interrupted by a manhole. In your case, I expect the software to yield consecutive S2 profiles. Alternatively, if you deem the junction between your two pipes as hydraulically insignificant (lateral inflows, junction losses, etc), then you could model your reach as one continuous pipe instead of two.

Scott Kampa · Answer

Hello Khalid, 
 Is there any inflow at this node? Is there some sort of control structure or flag gate assigned to the conduit upstream of the node? If so, this might account for this. 
 If not, we may need to see a copy of the model. To send this, please use the steps in this link . 
 Regards, 
 Scott

Yashodhan Joshi · Answer

Hello Khalid, 
 This hydraulic jump depends on the termination of your network at the outfall. Is the downstream conduit terminating in an outfall? 
 If so have you set the outfall to a specific tail-water elevation or a rating curve? By default the outfall is a "free outfall". 
 In "Backwater Analysis" the engine calculates the water surface profile based on the normal depth at the downstream outfall. That's why the resulting profile might be resulting in a hydraulic jump. 
 Also from the image above, there is a drop in EGL (energy grade line) suggesting a super-critical to sub-critical flow transition. This is often accompanied with a hydraulic jump. 
 You can share the model files by uploading them in the link Scott provided for testing if none of the above conditions are present in your model. 
 Hope this helps.

Jesse Dringoli · Answer

Khalid, 
 The "jump" here is not the same as a hydraulic jump that would occur along the length of pipe when a hydraulically steep pipe transitions to a hydraulically mild sloped pipe. 
 The "jump" here is due to a jump from near-normal depth up to critical depth, and is an artifact of the way that the calculation is performed - it performs a "frontwater" analysis with steep pipes, where it starts at critical depth on the upstream end. See the previously sent article for more information. 
 So, although this may look strange, it is expected and you can essentially ignore the "jump" that occurs, or combine the two pipes together.

hydraulic grade line

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