Question regarding HGL

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Brett Robinson
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Joined: Tue Oct 15, 2019 12:22 pm

Question regarding HGL

Post by Brett Robinson »

I'm noticing that when running the storm analysis for a pipe system. Sometimes the pit HGL level is above the D/S pipe HGL. Is this because the HGL line in the pit is the expected water level - not the actual HGL? I noticed that it is mentioned that's the case back in the 2007 model. Is this still the same in v14?

Also, is a pretty sharp gradient of the HGL an expected result when a taller pipe connects with a shorter culvert? I'm assuming this is causing some form of turbulent flow showing that Manning's should no longer be used, does 12D model the expectation of turbulent flow? Or is it just a matter of something in the Manning's equation causing a discrepancy.
Owen Thornton
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Re: Question regarding HGL

Post by Owen Thornton »

The HGL in the pit is higher that the HGL in the upstream pipe, when the Kw factor is greater than the Ku factor. Sometimes Kw=Ku, sometimes Kw>Ku.

...

Re your question about a steep HGL, is it within the pipe, or above the obvert?

Manning's formula only covers friction losses, but it gives best results when the flow is fully turbulent (like most stormwater design flows).

The additional pressure changes and energy losses at nodes, are covered by the Ku and Kw factors.

Please see:
viewtopic.php?f=7&t=984

Especially:
downloads/Owen.Thornton/12d_Drainage_An ... aulics.pdf
downloads/Owen.Thornton/12d_Drainage_Analysis_Ku_Kw.pdf
Brett Robinson
Posts: 7
Joined: Tue Oct 15, 2019 12:22 pm

Re: Question regarding HGL

Post by Brett Robinson »

I appreciate the response, Owen.

Thanks for clarifying that, those resources helped. To answer your question: it is above the obvert. Changing it to Coleman made it more realistic.

I am not authorised to access the link you sent, however: "viewtopic.php?f=7&t=984."

Thanks again,
Brett
Owen Thornton
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Joined: Tue Oct 04, 2005 12:50 pm
Location: Brisbane
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Re: Question regarding HGL

Post by Owen Thornton »

You need to register and log in to the forum to gain full access. (I didn't notice your post was in the general section.)

A pressurised HGL that is much steeper than the pipe, indicates that the pipe may be too small for the flow. If you are still achieving the necessary freeboard at the upstream node, it might not be an issue.

But changing from Manning n roughness, to Colebrook k (mm) roughness, without changing the roughness value, is not correct.

If n is 0.013, k is likely to be in the range of 1 to 1.5 mm.
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