Flow Change at Bridge

[AMRooke]

I am needing to make some revisions to an existing model, but there is an odd setup and resulting warning that I would like to review with y’all. My proposed revisions are discharge only, and I would like to submit a clean model (no errors or major warnings), so if corrections are needed to the existing base model, I would like to start at that point.
Field condition: There is a bridge that is undersized culvert that is overtopped by both storm profiles. There is a storm drain that outfalls into the creek, inside of the culvert, and the roadway drains out of the watershed after minimal overtopping of the culvert.
Model setup (1D steady flow): There is a flow change location at culvert XS2 (immediately DS of structure), which increases Q approx 35% (from 3,776 to 5,110 cfs in the smaller storm profile). The spill out of the model is via a lateral structure that begins also at XS2, continuing thru 6 downstream XS.

Model results (RAS 5.0.3): There is a warning that the flow optimization failed to converge at the lateral structure, and tweaking the iterations and tolerances will not make the optimization converge.
My questions:
1) I have always tried to avoid putting flow change locations within an inline structure section (bridge/culvert XS 2 or 3), is there any potential issue with doing such? The existing model was approved by the community and FEMA, but that is no guarantee of a good model.
2) How about having a lateral structure that falls within an inline structure section (XS 1-4), is that a potential source of concern?
3) You may note in the XS and profile that the channel cross-sections appear to miss the actual thalweg (the culvert invert appears buried), but I have always understood that the culvert routine will still work with the full flow area in the culvert, and that the small flow area that was missed by not picking up the exact thalweg likely has little impact on low frequency storm events, so this should not be an issue.
I welcome any input. Thanks in advance.

[ldguethle]

Hey Andy,

Couple thoughts:

– Change the internal culvert xsec to represent the culvert (would typically do this in unsteady, unsure how much benefit it would provide in steady) or if you really think the channel is not going to get washed out in these events consider a using a blocked culvert. Roughly 1/3 of the culvert area is lower than the xsec invert, which a lot of times would cause major issues.
– Consider a lateral/distributed inflow along the lateral structure instead of a single point inflow. Or consider moving the point inflow to xsec 1.
– Start the lateral structure after xsec 1. basically give xsec 2 a break, there’s too much happening at the culvert
– Confirm that the ridge in your left bank should not be blocked/leveed
– Lower your ineffective areas at your deck a bit below the deck. Having them match the deck exactly is begging for trouble (at least in unsteady – maybe not such a big deal in steady).
– Use HEC-RAS 1D-2D unsteady! ok ok I guess that is not an option w/ FEMA or your project…

Good luck,

Lee

[Lonnie A]

Andy,
Here are a few things I would try.
-Why are the ineffective elevations set so high at this crossing? Is there something downstream that would make the effective flow area this narrow and stack the water that high? With out seeing the downstream topo I would put the ineffective as single points using the elevation and stage of the ones set at the culvert face. And then expand the effective flow width going away from the crossing.

-suggest editing the internal cross-section in the bridge editor so the section flowline matches the culvert opening. What you have typically works for steady state but I know in unsteady I have to often do the internal XS edit for stability.

-there are a lot of changes happening at the outfall, you have large change in flow area and added flow at XS 2. If the first two suggested edit doesn’t help you might add a XS between 1 and 2 to insert the flow a little further downstream.

The lateral structure location isn’t a issue. Another thing to consider is the weir coefficient you are using on the lateral. Is there a true weir flow condition or is it just sheet flowing away? If it isn’t a weir flow condition I’d lower the coefficient to values like are recommended in the 2D manual. Maybe something in the 0.5 to 1.0 range. With a high coefficient it could suck off a lot of flow in one iteration which then drops the WSE significantly so it can’t iterate to a solution.

[Author]

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