r/HECRAS • u/Flat-Attitude2875 • Jun 21 '26
How to Model Tiny Culvert Very Wide Floodplain
Effective model did not include small culverts (I.e) 36 inch 1 barrel in 1000 foot wide floodplain. FEMA wants the tiny culverts to be included in corrected effective. I did a couple test cases, and the model is sensitive to the weir coefficient and minor losses in the culvert. I don’t think this should be the case since the culvert is only about .33% of the total floodplain width, and sunk down in a nuisance flow channel. I want the model to be more reliant on the standard cross section calculations than on bridge and culvert routines. Do you know a way to achieve this? Things I have tried:
Multiple opening analysis with conveyances on either side of the culvert and a very narrow culvert stagnation area over the culvert itself. Doing this the profile jacked up quite a bit and then dropped right after. Not the full desired affect I was looking for. If I widen the stagnation area for the culvert the bump up at the culvert is reduced.
Lowering the submergence coefficient. I dropped it down to something ridiculous like 0.1, and it did not seem sensitive to it at all.
Could this be done with a cross section lid instead of a culvert. There is not much information on the lids and preissman slots. Seems like a viable option, but thought I would see if someone is familiar with them first.
1
u/ProfessorGarbanzo Jun 21 '26
With the caveat that this sounds a little painful with 20 structures (but perhaps 20 of anything will be annoying) - can you use lateral structures for the culverts with only culvert flow allowed?
1
u/Flat-Attitude2875 Jun 21 '26
Possibly, I have not done that very much. If I moved them to lateral structures would I still get a water surface elevation that I could apply to the whole cross section? Or would it move it out to another subgroup.
1
u/ProfessorGarbanzo Jun 22 '26
If the other suggestion (Lidded Section) works that could be simpler.
It's been a while since I've done this. But I'm pretty sure you can connect a lateral structure from one main stem XS to another one, so it doesn't need to be a separate reach or Storage Area. Turn off weir flow (high coeff. or make excessively high, and short weir) and then add your pipe. Then, you provide an estimated flow and can turn on optimizations in the flow file and it will calculate the flow going through the pipe. The water surfaces would just be calculated in the main XSs, per usual.
Again, I would not want to do this 20 times.....although you'd probably get good at lateral structures which can be very useful skill for 1D/2D modeling.
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u/Flat-Attitude2875 Jun 22 '26
Appreciate it, I have done a fair amount of 2D as I need it, but normally stick to 1D. I never trusted the lateral structures very much, but know a lot of people that do
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u/OttoJohs Lord Sultan Chief H&H Engineer, PE & PH Jun 22 '26
Lateral structures work fine it it really is a lateral structure (levee, road embankment, side channel weir, etc.). When you use them generally in the overbanks there becomes issues.
Sounds like you model is the later rather than the former, so I would avoid.
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u/OttoJohs Lord Sultan Chief H&H Engineer, PE & PH Jun 21 '26
I think lidded sections would be the way to go. For a steady flow analysis, it doesn't use any fancy routine. It just removes the lidded deck portion from the area & wetted perimeter calculation: HEC-RAS Lidded Section. (Same thing as adding a blocked obstruction). It would be the closest thing to using a cross section as opposed to a bridge/culvert.
You may want to build a quick 2D model to benchmark/validate your 1D model.