I need to model filling/emptying patterns of a sanitary sewer tunnel whose outflow will be controlled by a pump station at the downstream end. How can I specify a no-flow outfall so that the wastewater starts filling the tunnel? Or can this not be done in SewerGEMS? I tried setting the tailwater elevation of the outfall higher than the tunnel crown, but the model still simulated flow out of the network as the sum of the inflows.
We may need some additional information for this, such as a diagram of what is being modeled and/or the model files you are working with. Included in this information would be what the outflow of the tunnel is. For instance, is the pump station directly connected to the tunnel? If not, what elements are between the tunnel and the pump station?
For the outfall, you might consider an elevation-flow curve.
Other options for modeling the tunnel would include using the pond element. There are options for the pond to be modeled as a pipe volume. Ponds require outlet structures, such as culverts, risers, etc. Pumps are not allowed as part of the outlet structure, so if the pump station is directly connected to the tunnel, this may not be the best option.
If the pump station is directly connected to the tunnel, you may want to consider modeling this as a wet well.
If this doesn't help, more information will be needed on what is being modeled in this system and the general setup. You can also send the model you have using the following information:
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If you upload the model using the confidential sharefile link please let us know in this post so we know to retrieve the file.
Regards,
Scott Kampa
Bentley Technical Support
Thank you, Scott. I uploaded the model to Sharefile.
Basically, we have a 20,000 LF, 21.3-ft tunnel, with 6 dropshafts. The tunnel will be used to divert excess flow from a sanitary trunk sewer and store it until it can be pumped back to the trunk sewer. I have modeled the dropshatfs as bolted manholes. The d/s end of the model has a wet well (approx. 25,000 gal) and a pump. I'll be looking at filling patterns first, so I added a pump curve that basically has the pump off all the time. The model is sending the excess flow as an overflow to the wet well, so I cannot see the backwater effect on the tunnel. Thanks again for looking at this.
Hello,
With the current setup, the downstream hydraulic grade will basically be the value at the wet well. With the pump not being on, the wet well fills to the maximum level.
If you want more control over a case where the pump is not on or is in some other way inactive, you may consider creating a scenario similar to the screenshot below.
In this, I created a new outfall (called O-5 above) with a conduit connected to this. I made the wet well, pump, and the original outfall inactive (thus they appear as gray in the screenshot). The advantage of this is it can allow you to have some additional control over the boundary condition. With the wet well governing this, the hydraulic grade will be defined by the dimensions of the wet well and and flow going into it. With the new outfall, you can try some of the options within the outfall instead.
Once you have the setup of the filling patterns set up how you want, you can then use the wet well and pump for further analysis. Please see if this will help at all with your analysis.
Regards,Scott
Thank you for your response. I had actually started modeling the system with an outfall instead of the wet well/pump combination, but I was not able to get the model to simulate a zero outflow condition (which is what will happen when the pump is off) --I uploaded the model to Sharefile--
I tried several options for the outfall boundary condition:
- Tailwater above crown of tunnel (325 ft) --> it causes the last section of pipe to be immediately under pressure, which shouldn't be the case since at such low flows, the pipe is not full. Also, the flow out of outfall is the sum of inflows rather than zero.
- Outfall elevation of 325 (like a vertical wall that blocks flow) --> model gives negative/positive random flows/depths
- Elevation-flow curve (table with zero flow for all elevations from 300 to 430 ft) --> outfall flow is sum of inflows, same answer as if the boundary condition is set to free outfall.
Is there any other boundary condition I can use to simulate no outflow?
Let me step back and ask what you are expecting in terms of results. Are the pipes supposed to slowly fill over time? Should they become surcharged? If the conduits are only expected to reach a certain elevation, you might be able to try a place a stop control structure on one of the downstream pipes with a weir just above the expected elevation. This could conceivably keep flow from reach the outfall, as long as the results are well-balanced and the inflows correct.
For the model layout itself, are the elevations and pipe sizes accurate to what you have in the field? Also, you have hydrograph inflow at all of the entry points (modeled as manholes). I just want to make sure the input there is correct as well.
Lastly, was there a reason that you wanted to try to model this in two steps (zero outflow first, then include pumped flow)? If it is possible to include the pumped flow as well, you might end up with reasonable results in comparison to trying to model a case where there is no allowable outflow.
Scott