Closed conduit modelling

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Closed conduit modelling

ggdm
Hi everyone,

I'm modelling a closed conduit which has an ovoidal-like cross section with hec-ras. I'm interested in steady flow, open-channel (not pressurized, at least not for the most part of it) analysis. I modelled the conduit as a lidded cross section. At the moment I just put 3 identical cross sections along a line with a constant slope (simulating a uniform flow) to test the model behaviour and do some roughness calibration. However I encountered a couple ofquestions:

- Do I need Preismann slot? (I don't need unsteady computation);

- I'm having trouble using high flows (relatively to the conduit hydraulic capacity); the conduit's shape has a "turning back" rating curve (just like a circular open channel) so the maximum flow capacity is reached at about 90-95% of the total height of the section. It seems that when I use a flow higher than the capacity of the conduit at 100% (which is not the maximum possible) but lower than the maximum possible (as from separately computed uniform-flow rating curve) hec-ras doesn't compute the correct WS height but flows water up the lid (pressurized flow down the lid) even though the conduit would be able to take that flow as open-channel.

P.S.: I can't use culverts, bridges, etc.,  I need to know results at various cross sections.

Thank You in advance!
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Re: Closed conduit modelling

Chris G.
Administrator
Hi ggdm.  When using lids in steady flow, I believe RAS uses the energy equation to solve for energy from one cross section to the next (just like with regular cross sections).  If the answer you are getting does not meet what you believe to be the true capacity of the tunnel, your only recourse is to alter the n value as a calibration knob.  Keep in mind that RAS, even with lids, is not the best tool for modling tunnels or pipe flow.
Chris G.
@RASModel
www.therassolution.com
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Re: Closed conduit modelling

ggdm
Thanks for the answer Chris.

I finally ended up using a rating curve BC, from zero flow to maximum flow capacity (so not for the entire height of the section). That gives me the results I expect, even though I'll have to edit it if I decided to change roughness. All in all it works.
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Re: Closed conduit modelling

GoSparky
In reply to this post by Chris G.
Hi Chris.  I found this archived post when searching for info on cross sections with lids.  Why is RAS not the best tool for modeling tunnels or pipe flow?  And what would be better?  SWMM?  
(I'm trying to model a 'basin/weir/box culvert/outflow weir' in a WTP, so a simple culvert editor like HY-8 is not sufficient.)

thanks,
Lisa

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Re: Closed conduit modelling

ghaith
In reply to this post by Chris G.
plase, i represent siphon   by use cross section with lid,but surface of water surface is parallel and located above , did it correct