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 have a couple of questions:

- 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, see image) 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.: culverts, bridges, etc., are not an option, I need to know results at various cross sections.

Thank You in advance!

 

[gbam]

Why don't you set up a direct step spreadsheet for your sections. You can develop a rating table for various depths against area, wetted perimeter and velocity.

 

[ggdm]

Unfortunatly I don't have time at the moment for troubleshooting a new spreadsheet. Furthermore graphing and analysis capabilities of HEC-RAS are exactly what I need for my project, so this is not an option for me in this case.

However I'll add some more information I discovered. Actually this could be an HEC-RAS bug.

Problem seems to be in boundary conditions computation. What I need is downstream normal depth.
However If I put a "known WS" Boundary condition, run a steady computation and then again a "normal depth" BC, I get a WS under the top of the Lid, with the same flow and BC as before, where I got pressurized flow.
The problem is that after this "trick" WS is not even the same if I choose "known WS" differently! Quite crazy and not promising for my analysis.

Anyone has ideas why is this happening? Shoudn't "normal depth" BC give always the same WS and profile? (no geometry change, of course)

Thanks

 

[ggdm]

Moreover it seems that if you put a known WS BC between the critical depth and the maximum capacity height (as from rating curve, see image in first post) and after change it to normal depth BC (even without running a computation) I'm able to get correct and consistent WS in my lidded section.
It seems that doing the "trick" sets HEC-RAS to internally compute normal depth from the known WS height.

 

[beej67]

If different boundary conditions are giving you grief, and you don't know which one best expresses the true boundary condition of your model, I suggest extending the model further downstream, to a point where changing the boundary conditions doesn't affect the depth in your culvert.

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