I am getting a -21.0% Flow Continuity Error in my Sewergems model so I exported it and ran it in the SWMM 5.1 software and got a -0.16% Flow Routing Continuity Error.
I'm wondering what could cause such a drastic change in results.
A few notes. The model has two ponds modeled and one outfall. One inlet that is upstream of the pond has a -800.76% continuity error for it's surface node (in sag inlet - "Node 828f_Surface). Obviously this is causing the most problems but I'm not sure how to fix it since the inlet is not overflowing or surcharged. The rest of the nodes listed in the "Highest Continuity Errors" section within Sewergems are in the 9-36% range.
I am using Sewergems CONNECT Edition Update 1 (10.01.01.04) within Microstation V8i SS4 and EPA SWMM 5.1.012. I will try to upload the "Outfall D6_NEW" file for reference.
To add, I just tried making the orifice in my pond outlet structure 2" smaller and it reduced the overall "flow continuity error" in Sewergems to -9.3%, but the "Node 828f_surface" error actually increased to -1061.70%.
This might help the overall error, but I'm still wondering why the SWMM program would solve the same model and have such a smaller error as well, with my original input values .
Hello Jared,
Have you tried setting the SWMM calculation options to reduce your continuity errors?
Often when designing large or complicated models it would give somewhat higher continuity errors or unstable results.
Here is the relevant article in this regard;
Troubleshooting unstable SewerGEMS model results using the Explicit SWMM Solver
Hope this helps.
Regards,
Yashodhan Joshi
I have looked at this article before and adjusted quite a few of the SWMM calculation options. None have reduced the error at the one inlet's surface node however. Additionally, I believe the same options are exported to the SWMM file so when I run it within SWMM 5.1, the results shouldn't really be that different..
We are looking into this issue. I will let you know when I have some more information for you.
Scott
There were improvements made to the Explicit solver in relation to the the SWMM solver with regards to pond outlets. This is the reason that the operation and results are a little different. The results tend to be better in SewerGEMS for cases like this.
There are a couple of things you can try to improve the continuity error in SewerGEMS. If you set the the time step to 5 seconds and the output increment to 120 seconds, there is an improvement to the continuity error. Setting the time step to 10 seconds and the ouput increment to 5 minutes improves it further.
Please let us know if you have any questions.
In addition to the information above, the developers noted something else that may impact the stability of the results. It appears that the two ponds in the model have composite outlet structures that include elements where the elevation is outside of the operating range of the ponds. Each of the two ponds have an orifice as part of the composite outlet structure, but the invert elevation of the orifices is below the lowest point of the associated pond. I would review these and perhaps other elements in the model to be sure that the input is reasonable.
Hi Scott - Thanks for the feedback.
Increasing the time step or output increment did not seem to help so I've gone with reducing the orifice to a size of 4" instead of 6" which reducing the error down to about 9%.
Regarding the elevations of the orifice, I wondered how the model would handle the situation so perhaps it is contributing to the problem, but what we were trying to do is model a sloped pond with an orifice that is a certain elevation (bottom slope of XX% toward the outlet structure) lower than the "bottom of the pond" per the model. Hence setting the elevation of the orifice and the FG at the outlet structure below the bottom of pond. It does seem to be okay in other models where we are doing the same thing, and I'm not sure how the pond and outlet structure export to SWMM, but again, the error in the SWMM 5.1 program was essentially 0%.