SewerGEMS: Why I have zero overflow volume (Hydraulic Reviewer manhole table) in manholes that show "True" in the "Is ever overflowing" field? I'm using the explicit SWMM solver and I don't have bolted covers. Thank you very much.
Hello Ana,
Are you using any surface storage like ponded area for manholes? Is it a free outfall or tailwater elevation is defined?
In case of explicit solver overflow is determined by the total inflows minus the total outflows which is based on the enforced node elevation.
Do you see any flooding in SWMM calculation summary reports as below?
How do the dynamic solvers calculate overflow from a catchbasin or manhole?
If required you may upload model files for our review, Sharing model files.
Regards,
Sushma Choure
Bentley Technical Suppport
Hi Sushma,
We have "No storage" in the "Surface storage type" field for all of our manholes.
All the outfalls are "Free outfall" type
I see flooding in SWMM calculation summary report, but it is not equal to the overflow volume reported in the Hydraulic Reviewer:
The model is uploaded here: https://drive.google.com/file/d/11UH8ob_L3i-ZmMBSg7XsZd_y65QBi0DC/view?usp=sharing (I tried to upload it into the message but it showed some kind of error). The scenario I'm reviewing is Escenario11_Usuarios30_IDF2021_TR10_HuffLocal.
Thank you very much!
Hi Ana Maria,
Thank you for providing a copy of the model. From a quick glance at manhole 6061407 (the first one on the list), it shows the maximum HGL does reach the rim elevation, yet the graph of HGL does not show this. Looking at the "Time to maximum hydraulic grade" result field, the max HGL happens at 642.252 minutes, which is between the reporting timesteps of 640 and 645 minutes (because you have the Output Increment set to 5 minutes yet the model calculates every 0.1 seconds). So, it could be that the HGL spikes up above the rim elevation for a very short period of time which happens between the reporting timesteps so you don't see it in graph view (and some other places).
I will try re-running the model with a shorter Output increment and will get back to you.
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
I did this with a report timestep of 1 minute and I got the same results... maybe the report timestep should be even shorter?
Yes I tried with 0.5 minutes and although the sum of overflow in the hydraulic viewer then matched what was shown in the calculation summary, I did not see it in the graph for the same manhole. I suspect you'll need to use an even smaller timestep, or perhaps just consider it to be a momentary overflow. I will investigate further on Monday.
I tested this with the Routing Step set to 1.0 seconds and was able to observe the spike in HGL that is causing the momentary overflow:
I see that with the increased routing step, the flow continuity error increases though. It appears that this momentary spike in HGL and overflow is caused by instability. The following article has further guidance on how to address this. You may need a combination of the reduced Routing Step as well as other adjustments and a close look at the data input (which can also cause stability problems): Troubleshooting unstable SewerGEMS and CivilStorm model results using the Explicit SWMM Solver
I will plan to take a deeper look at the model to see if I can locate any specific recommendations to improve stability and avoid these "spikes", and will let you know if I find anything.
Ana Maria,
Try the following calculation options:
With this, the %non-convergence is 3%, the Flow Continuity Error is 0.4% and the overflow volume in the calculation summary matches pretty close to the sum of the overflow volume in the hydraulic reviewer. Also, I was only able to find a few manholes with "is ever overflowing?" set to True, without overflow showing in the overflow graph (8576963, 6066226, 6066059, 6060055, 6065070)
I was able to further improve the results by reducing the SWMM Output Increment to 0.1 minutes. With this, I was able to view positive values in the overflow graph for all elements with "is overflowing ever?" set to True.
I have updated this article based on the outcome of this conversation.
Hi Jesse. I tried the calculation options you proposed and the results are very good. The surcharge method "Slot" is the Preissman Slot? What is the difference between Preissman Slot method and Extran method?
Thank you very much!!!!! Very helpful advice!!!
The Preissmann slot method is used for simulating pressure or surcharged flows for gravity conduits, by adapting the conceptualization of pressurized flow to fit a free surface model. The slot extends vertically from pipe crown to infinity and over the entire length the pipe, and the width of the slot is usually 1% of the characteristic pipe dimension (diameter for a circular pipe) but not large than 0.02 ft. Read more about this in help documentation - search for "Pressure (Surcharged) Flow and Overflow (Street Flooding)".
EXTRAN is the original SWMM method to simulate pipes under pressurized conditions. When the nodes at both ends of a pipe have a water stage higher than the top of the pipe at that node, EXTRAN switches to equations for pressurized pipe flows.
Also you can read more about the comparison in this below discussion on SWMM forum.
Surchage method.
Thank you very much for the explanation, Sushma!!
We have added this information in the form of wiki which can be beneficial for other users in future.
What is difference between surcharge methods - Preissmann slot method and Extran method?