Downstream rise with larger bridge opening

[mlc3n3]

I’m modeling a proposed bridge larger than an existing bridge. The proposed is about 30ft upstream of the existing. Lowchords match, although top elevation of the road increases 1.75ft. Original opening is around 430 SQFT, new opening is an additional 140 SQFT (rough calcs). I am looking for a “no rise” condition. Right now both bridge models overtop during certain storm events. Obviously not ideal but this isn’t my problem.

Problem is I have a 0.8ft rise downstream of proposed bridge. The rise takes place consecutive stations all within 300ft downstream of the bridge. Upstream sees no rise.

Overall this seems really odd to my colleagues and I.

Any ideas, suggestions, thoughts?…

[GoSparky]

I’ve seen this before in bridge models and it’s been caused by the ineffective stations on the downstream cross sections. If the proposed bridge has a wider opening and you keep the same flow expansion angle as you used for the existing bridge, then the downstream ineffectives get moved out, which increases the active flow area and you get a small rise downstream of the bridge where you’d think there should be no changes in a subcritical model (Q=VA).

You’ll also get a rise if the proposed channel is bigger. When you make the area bigger, the velocity goes down and thus the WSEL has to go up. I’ve never had the time to investigate and find out at what point the area gets big enough to not get a rise (and probably every site is different anyway).

Also, if the bridge is overtopping, then you often will get a rise upstream because the proposed bridge is larger (longer or greater deck thickness) and blocks more of the flow than the existing bridge did, which results in an upstream rise.

[mlc3n3]

GoSparky,

That is helpful information! It makes sense. Almost like a hydraulic jump, or a gradual hydraulic jump. My critical depth matches my WSEL right after the bridge where the rise is. I did some research about a rise downstream of a bridge and found info similar to what you provided:

https://www.fhwa.dot.gov/engineering/hydraulics/pubs/hif12018.pdf

Yes, I think its a case by case thing. For this project I’ve adjusted the bridge length, opening and low chord, but this only helps upstream while downstream doesn’t improve.

Thanks you!

[mlc3n3]

*Update*

Through lots of troubleshooting an error was found in the model.

At one of the downstream xsec the bank flow lengths were too long. Channel flow length was correct (around 185′), but the left and right bank flow lengths were wrong (around 850′).
I believe this happened when my flow lengths were off, I reassigned flow lengths in GeoHECRAS, clicked a wrong box, and this changed my overbank flow lengths.

Correcting this error corrected the model!

What I think was happening was HECRAS saw the long overbank flow lengths, interpreted a large area, and produced a slow down in the water. This therefore created a rise.

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