Can a Parshall Flume be modeled as a Conduit Control Structure? Would you recommend modeling as a Depth-Flow Curve or a Weir?
My thought is to use the Depth-Flow Curve for specific flume geometry rather than approximate as a weir. Where is the freeboard depth and head across structure measured from? Typical depth/discharge tables for Parshall flumes use depth at the "Ha" location in the crest of the flume (in this case Ha is located 9" above the invert of the 30" incoming sewer, in the standard 24" Parshall Flume).
Thank you for your help!
Hi Brooke,
I assume you are using CivilStorm or SewerGEMS.
At what level of detail do you need to analyze the flume? What ultimately will you use the hydraulic model for? (what questions will it answer or what problems will it help you solve?)
Since there are several interrelated components in close proximity in a Parshall flume and since it sounds like you have obtained rating table information, using a depth-flow control structure makes sense.
If the depth/discharge rating table that you have is measured from the "Ha" crest of the flume, you would set the control structure crest elevation to the crest elevation at the "Ha" location, then the depths in the depth-flow table would be measured from that crest elevation as the datum. You could include the detail of the incoming pipes first sloping downward, then sloping upward to the crest, then back down again. Or, you could simplify and just use a single upstream and downstream conduit, if the hydraulics of the whole flume is accounted for in its depth-flow rating table and if you do not need to see all the detail of the different-sloping pipes across the flume.
For example see the below layout where conduits and transitions are used to model the uphill slope to the crest, with a stop control structure to model the depth-flow curve, where the crest (invert) elevation of the depth-flow control structure is set equal to the crest elevation at the transition node location.
See related articles for more information:
Using Start and Stop Control Structures for conduits and channels
Cross section vs Transition for modeling slope or size change
When in doubt, it is usually best to perform a sensitivity analysis, whereby you try a few different estimated approaches and check to see if the results are sensitive. If they are not, then it may be acceptable to adopt a simplified approach. So in this case you could try the depth-flow approach and compare it to, for example, a simplified assumption using a weir control structure.
Regards,
Jesse DringoliTechnical Support Manager, OpenFlowsBentley Communities Site AdministratorBentley Systems, Inc.
Answer Verified By: Brooke Hamberger
Hi Jesse,
I am using SewerGEMS to model a sanitary sewer system. The existing Flume is located on the downstream end of the system prior to discharge to another municipality. It seems the flume impacts the upstream trunkline based on available flow data and visual observations, so we do not want to ignore the impact of the flume on upstream sewers. We are using the model to predict basement backups with the HGL as we model raising overflow weirs throughout the system.
Your below approach sounds like it will work, checking the sensitivity.