Dear sir / madam,
I am having a problem in CivilStrom regarding pipe losses calculation. I have a sewer system that is running full (pipe are always 100% full) because this system drains to a pond that has a water level higher than the pipe crown level. Pipes are running flat with no slope.
The issue I am facing is that the pipe losses calculated in CivilStorm using the Implicit (SewerGEMS Dynamic Wave) solver differs dramatically from the losses I calculate manually using manning's equation for the same flow and roughness coefficient. I am assuming that a 100% full pipe is designed as a pressurized pipe and I have already chosen in the Calculation Options Manning's equation to be used as the "Pressure Friction method".
I have attached:
1- a PNG file with my question rephrased in more clarity
I have tried to load the CivilStorm files but failed to do that because of their size exceeding the maximum allowed. If there is another way to upload the files, please let me know.
Thanks in advance for your support,
Can someone help me figuring out why CivilStorm pipe losses do not match the manually calculated ones using manning head loss equation? I am really confused !
It is expected to have different results when comparing the dynamic wave solvers (Implicit and Explicit SWMM) to the GVF-Convex and GVF-Rational solvers. If you are only designing a sewer system then using SewerCAD or SewerGEMS would be more appropriate using the GVF-Convex solver, if you are using the Manning's equation for analysis. See the article below for details on why the results are different in different solvers;
Model results are different between the GVF-Convex and the Implicit/Explicit solvers
Additionally, please see the below article on which is the best solver to use considering your use case;
Differences between solvers: GVF-Convex vs. GVF-Rational vs. Implicit vs. Explicit (SWMM)
As per Manning's equation;
V = k/n * (R^0.67) * (S^0.5).
Here S = hf / L where hf is the headloss and L is the length of the channel or reach. For uniform steady flows, the energy grade line = the slope of the water surface = the slope of the bottom of the channel.
Using this equation you can try computing the headloss manually.
Let me know if this helps.
Bentley Technical Support
I checked the links provided but still don't have a clear answer for my question. As per my previous question screen shot attached, you can see that I have entered a manning's roughness coefficient in the software conduit properties and therefor I am assuming that manning's equation is used for calculating the pipe friction head loss.
My question now is, if the (SewerGEMS Dynamic Wave) solver uses manning equation for calculating pipe friction losses, why does the program head loss results is completely different than the manually calculated results.
In the screen shot attachment in my previous email, I have indicated clearly the equation I have used with an example for more clarity. The equation I have used for calculating the pipe losses is exactly the one you are suggesting but I have modified the general equation you suggested V = k/n * (R^0.67) * (S^0.5) by isolating the hf term. As explained in my previous email, the pipes are always filled with water which allows more simplification for the equation.
I tried my best through my previous email to prove that manning equation, that you agreed with me that it can be used for manual checking, gives completely different results when compared to the (SewerGEMS Dynamic Wave) solver results and my question is why???
As per my humble opinion, regardless of the Solver used, a pipe with a fixed size, flow, roughness coefficient and that is 100% full should always have the same pipe friction loss if manning's equation is used by all Solvers.
I would appreciate if you can go again through my previous email and this one as well and provide more clarification because I am really hesitating using CivilStrom software if I can't defend and explain confidently the results to our client.
Thanks in advance,
Our FlowMaster product is another way to check the headloss for a pipe flowing full. The below screenshot shows that it matches your hand calculation for CO-10.
However, the calculations in SewerGEMS can be more complex since it uses a fully dynamic solver (you mentioned you are using the Implicit solver). There are a number of factors that may be influencing the headloss figure shown. For example even though the flow through the pipe exceeds its "capacity" figure, since that figure is based on normal depth, if the pipe is not flow at normal depth, it may not yet be full (see this article for more)
There could also be other dynamic effects influencing the calculations (such as tailwater effects). If the pipe is flowing full, the Implicit solver uses the Preissmann slot method - you can read more about the related calculations in the Help topic "Pressurized Flow".
Lastly if the model is not stable (typically indicated by a high mass balance error in the calculation summary, or unstable hydrographs) then the results will also be impacted. The following article can help resolve such cases:
Troubleshooting unstable SewerGEMS and CivilStorm results using the implicit solver
As a quick example check I set up a new model with one pipe (configured as your CO-10) and an inflow of 5505 m^3/h, and the pipe was not flowing full when assuming free outfall at the downstream side. I then set a user defined tailwater depth of 1.0 meters and the headloss matched what you expected (0.14 m).
If this does not help, please provide a copy of the model by going to File > Save to Package, then upload the resulting ZIP file using the ShareFile method described in the following article: Sharing Hydraulic Model Files on the OpenFlows Forum
Jesse DringoliTechnical Support Manager, OpenFlows ProductsBentley Communities Site AdministratorBentley Systems, Inc.
Thanks for your prompt reply !
I have gone carefully through your email and I have checked that @ time 12:15 minutes (Highest flow time slot):
1- The pipes are surely running full
2- I do believe that the model is stable because I am not getting any of the errors that you have mentioned
I have attached the model for your reference, I would appreciate if you can have a look and let me know your opinion.
Thanks for providing a copy of the model. The reason why the headloss is not expected in this case is because of other influences on the dynamic numerical solver. It appears that it is omitting part of the headloss component in favor of stability (which is more likely to happen in surcharge situations)
In order to tell the numerical solver to apply more dynamic effects, the LPI coefficient can be increased (in the calculation options). For example if you increase it to 5.0, the calculated headloss will be very close to the expected value (for me it was 0.133 m).
See related article: Zero (or near-zero) headloss or flat profile appearance with pressurized conduits
Answer Verified By: AMR BAZARAA
Thank you very much for your prompt and comprehensive reply !
I also do appreciate the effort that you and the support team are offering for assisting Bently users.