Pressure Flow Criteria

[mlc3n3]

Hi,

I am modeling a bridge where the 100 year storm event approaches the low chord of the bridge. There is a risk of pressure flow at this storm event.

The design of the bridge is dependent on the hydrology, so several different structural members have been modeled with varying low chords.

In HECRAS there is an option for pressure flow criteria: energy gradeline or water surface elevation. Is there a recommendation of what option to use and under what circumstances?
If using the energy gradeline, when the low chord meets the gradeline, the water surface will essentially jump and cause pressure flow. Is there any reason why?

Thanks in advance!

[cameron]

The default in the model is set to energy gradeline. This basically makes the bridge think that the energy gradeline is the wse and causes pressure flow.

The link below has some discussion on this in the comments section.

http://hecrasmodel.blogspot.com/2009/08/problems-with-bridges-and-culverts.html

EGL is used because of the conservative assumption that any disturbance in the flow (debris, etc.) could bring the HGL up to the EGL level. If you have a supercritical reach, or if you suspect that the HGL is so far under the deck that using the EGL is not appropriate, then change the pressure flow criteria. Its really just up to your own judgment.

[mlc3n3]

Hi Cameron,

Thank you so much for the speedy reply! This isn’t the first time you’ve helped me out!

Alright, that makes sense and clarifies things.

I’m doing some more googling. I can update if I find anything more detailed.

Thanks again!

[Author]

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