Hi,
I have a model in which a pond goes to an outlet structure to go directly to our storm system. However, we want to discard the water quality volume from that same pond to go to arbitrary outfall so it does not affect downstream of our system. I know what the volume that I need to discard is.
Therefore, I was wondering if it would be as simple as having two outlet structures with pipe attached to each of them, one for the water quality volume and one for the rest of the flow? For the water quality volume the pipe after the outlet structure would end in an outfall.
How would I make sure that only the amount of XXX of volume is taken to that pipe?
Thanks
Paola,
In general you will want to model the system as close as possible to the real system. What in the real system limits the outflow based on volume? (how is that accomplished?)
You can indeed model multiple outlets leaving the same pond. See the "Pond" section of this article: Modeling a flow split (diversion) in SewerGEMS or CivilStorm
With the pond outlet structure component in SewerGEMS, you can control outflow based on the available outlet types: orifice, weir, riser, vortex valve, user defined rating table. If you want to assume a specific, constant outflow that yields a certain volume, you could use the user defined rating table pond outlet type, assign it to a second pond outlet node attached to the pond, then connect that to a free outfall via a conduit.
Or, if this specific volume you're looking to "discard" is actually infiltration (water soaking into the ground around the pond), you can use one of the available seepage methods.
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Jesse, would probably looking through the question and answer that I found on this thread (in bold below) help me walk through how I take into account the seepage infiltration for that pond?
Seepage (infiltration) is a property of the pond element. You will see the attribute called "Pond Seepage Method" in the "Infiltration/Inflow & Seepage" section of the pond properties. You can choose between a few different methods depending on which numerical solver you are using.
Prior to today, more information could be found in the Help topic "pond flow loss". I have now documented information about seepage in our Wiki here: Available Pond Seepage (infiltration) and evaporation methods per solver
If none of these methods are suitable for your situation, you can indeed use the approach of modeling the infiltration/seepage with a user defined rating table (varying flow as a function of headwater elevation) on a separate pond outlet structure.
Okay i found the "Pond seepage method" in the section and it looks like we are using the explicit so my only option is either Green Ampt method or discarding in an infiltration outfall with user defined rating table. Are those the only two ways to account for infiltration flow not going into our system?
I am also looking into the water quality section and possibly doing a pollutant graph for the TSS. But right now i do not seem to have all the pollutants parameters.
I have tried to look online but still a little confused on what is the difference between explicit and implicit. And why do we use one over the other?
Paola Espinoza-Youn said:Are those the only two ways to account for infiltration flow not going into our system?
For a pond with the Explicit solver, yes. It should be fairly straightforward to set up the secondary pond outlet with user defined rating table, or adjust the Green-Ampt parameters until you get the desired infiltration rate.
Paola Espinoza-Youn said:I have tried to look online but still a little confused on what is the difference between explicit and implicit. And why do we use one over the other?
See: Differences between solvers: GVF-Convex vs. GVF-Rational vs. Implicit vs. Explicit (SWMM)
For a model with ponds where you need to route a hydrograph, the Explicit (SWMM) solver tends to be much more stable than the Implicit solver, and accounts for more dynamic effects compared to the more simplified GVF-Convex solver.
So the Gregg-Ampt Parameters are all soil properties, correct? That is what it seemed to be when I looked it up online. And then after I run the model, is that when I see what infiltration rate resulted from those soil Gregg-Ampt Parameters that I input.
Nevermind i see that I answered my own question regarding if they are soil properties. I did not see that it explains what the parameters are underneath when you click on it.
However, to get the desired infiltration rate. Is that something that I would see if I got the desired infiltration rate after I run the model or is that somewhere I can look someplace else before running it?
Thanks for helping!
Paola Espinoza-Youn said:However, to get the desired infiltration rate. Is that something that I would see if I got the desired infiltration rate after I run the model or is that somewhere I can look someplace else before running it?
You would need to compute the model first and graph the pond seepage result field.
Jesse, after I have run the model that I did with the seepage method approach would I be graphing the flow(seepage loss)? Would that tell me how much flow was lost due to infiltration?
I looked at it and supposedly we are trying to get 3.5 cfs discarded and my model resulted in higest result of 1.3 cfs for the flow(seepage loss). Therefore, would it be best to mess with the soil information until I get my desired cfs or try the approach of discarding flow with another outlet from the pond with a rating table.
Should I also be looking at the seepage rate results? I looked at that and was getting 0 in/hr which does not seem right.
Thanks,
Paola Espinoza-Youn said:Jesse, after I have run the model that I did with the seepage method approach would I be graphing the flow(seepage loss)? Would that tell me how much flow was lost due to infiltration?
Yes, the "Flow (Seepage loss)" would show you the seepage flow rate over time. For the total volume, see the Hydraulic Reviewer. This is mentioned in the section "Reporting flow or volume loss from seepage" of the seepage article provided earlier.
Paola Espinoza-Youn said:I looked at it and supposedly we are trying to get 3.5 cfs discarded and my model resulted in higest result of 1.3 cfs for the flow(seepage loss). Therefore, would it be best to mess with the soil information until I get my desired cfs or try the approach of discarding flow with another outlet from the pond with a rating table.
Either approach could be considered. There are pros and cons to both. With the pond outlet structure approach you have more control over it, but you'd see some extra elements that aren't really in the real system (pond outlet node, pipe and outfall).
Paola Espinoza-Youn said:Should I also be looking at the seepage rate results? I looked at that and was getting 0 in/hr which does not seem right.
Can you clarify a bit more? Are you referring to the "Flow (Seepage loss)" field in the pond properties. It may show zero in the first timestep - make sure you graph this result to see how it varies with the pond water surface elevation.
Hi, I am getting two different results when doing the seepage method vs. the pond outlet structure approach. Is that common?
Also for the seepage method i am geting practically same "Flow(seepage loss)" numbers for all my three storm event i am evaluating, 2 yr, 10 yr, and 100 yr. Is that common too? I would think they would be drastically more different.
Paola Espinoza-Youn said:I am getting two different results when doing the seepage method vs. the pond outlet structure approach. Is that common?
Different results are to be expected (as these are two different methods) unless you were to configure the user defined pond outlet to exactly match the configuration of the Green-Ampt method, using trial-and-error.
Paola Espinoza-Youn said:Also for the seepage method i am geting practically same "Flow(seepage loss)" numbers for all my three storm event i am evaluating, 2 yr, 10 yr, and 100 yr. Is that common too? I would think they would be drastically more different.
The degree to which the seepage results change depends on the difference in pond water surface/wetted area and the Green-Ampt parameters.
I did a quick test in a sample file in the latest version (10.04.00.158) and in a scenario where the pond water surface elevation is higher, I see a high seepage flow. I used the SWMM solver with made-up numbers for the Green-Ampt parameters. If you are not seeing a larger difference, check the pond elevation results and Green-Ampt parameters. For example I see a much higher seepage as I increase the conductivity or initial deficit fields.