I am designing an underground detention system consisting of 3 x 1131LF 60 inch pipes. The reviewer is asking for a profile of the HGL in the pipes with each inlet into the UGD (see his requirements/questions below).
My question is that profiling the detention pipes accounting for inlets/lateral flows doesn't seem realistic. If this was a surface pond the max WSE would be used as the downstream for each of the laterals. Is there a printout I can provide to him that would answer his concerns? Per his comment #3, isn't the underground detention already modeled as a pipe hydraulic element since the model has the pipe size, slope, and tailwater information?
UPDATE - The reviewer is only asking that I show that the Hydraulic slope of the UGD with the flow in the pipe (as conveyance) is so flat that it doesn't affect the volume of the pond. We are providing a profile showing the HGL as we would any conveyance pipe with an extremely flat slope (almost zero) to satisfy him.
Hello Cheralyn,
If you are modeling the underground detention as a pond set as Pipe Storage or Storage chamber, the hydraulic grade will be uniform along the length of the pipes. This is because the pond element, including for pipe storage or storage chambers, does not account for resistance from the sides of the pipes. For this reason, there is no hydraulic gradient to show, just the single water surface elevation over time.
If you need to account for the slope of the HGL (hydraulic gradient) across the pipes, you would need to model this explicitly as conduit elements in CivilStorm or SewerGEMS, whose dynamic solvers do account for both the storage effects and the hydraulic resistance, and can generate profiles.
The wiki article on storage chambers has been updated with this information.
Regards,
Scott
Answer Verified By: Cheralyn Armijo
Scott,
When using SewerGems, if I use Pipe Volume as a pond element (instead of 3 individual conduits), will the model incorporate the friction slope in the model?
Hi Cheralyn - please see previous reply from Scott:
Scott Kampa said:If you are modeling the underground detention as a pond set as Pipe Storage or Storage chamber, the hydraulic grade will be uniform along the length of the pipes. This is because the pond element, including for pipe storage or storage chambers, does not account for resistance from the sides of the pipes. For this reason, there is no hydraulic gradient to show, just the single water surface elevation over time.
The pond element only knows about the volume at a given stage (elevation). Even though there's a "pipe" option, that is essentially a shortcut to developing the stage-storage curve behind the scenes - there is no facility for accounting for the resistance of the walls of the pipe and thus no friction slope. As mentioned in the earlier reply, you only get a single HGL for the pond element. In cases where friction losses need to be accounted for, you will need to use the conduit element.
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
If you think about it for a second, the pipes used for storage are very large compared with the flow through them. Therefore, the velocity will be close to zero and the flat surface model is correct.
If the pipes ae so small that there will be a significant velocity, they should be modeled as pipe elements.