Normal 2D vs. Weir Equation at Lateral Structure

[Scott Miller]

When I used the Normal 2D Equation Domain to convey flow between 2D and 1D areas at lateral structures there was no problem, until a 50-year peak entered through a tributary. The tributary is modeled in a 2D flow area, and is relatively steeper (@ 0.035) than the valley-run terrain (@ 0.002). Flow from the tributary super-elevated the water surface elevation at the 1D cross-sections that it discharged to by 3.5 feet. Apart from being anomalous, this produced an adverse hydraulic gradient that destabilized the water surface elevation at the entrance a 1D culvert, immediately upstream from the tributary.

I thought I had the problem solved by changing the overflow computation method at the tributary lateral structure to use the Weir Equation. That solution caused its own problems. (see image below)

Now the lateral structure blocks (at least some of the) flow from entering the 1D reach. My understanding is that flow across the lateral structure enters (or leaves) the 1D cross sections at the lateral structure embankment elevations regardless of the bank elevation of the cross-sections. http://hec-ras-help.1091112.n5.nabble.com/Rendition-of-Particle-Tracing-at-1D-2D-Boundary-td5737.html

The solution I consider using is to move the external boundary condition hydrograph to a 1D cross-section, simply backwater the tributary channel, and hope for the best. Let me know if there may be a better way. It would be preferable to deal directly with the 1D WSE getting superelevated, while using the Normal 2D equations.

[cameron]

was the 1D/2D iteration option turned on?

For the lateral structure what did you use as the weir coefficient?

Did you try breaking up the lateral structure into more pieces?

[Anonymous]

If you switch the really steep tributary from 2D to 1D, the 1D will probably have problems with that. I guess you can always give it a try. If you converted it to a 1D reach, you could try modeling it with the ModPuls option.

Can you make the entire model 2D? That would probably be more stable.

Having the steep 2D part dump into a lateral structure is probably not ideal. Instead of using a lateral structure to model the junction, you could extend the steep 2D section so the junction is entirely 2D. The 1D reach would have to broken into two parts. The 1D reach would directly connect to the 2D area upstream of the junction and the second part of the 1D reach would directly connect just downstream of the junction.

Finally, if the tributary is that steep, do you even need to model it? Remove the steep 2D tributary. Remove the lateral structure. And just attach a lateral inflow to the 1D reach to model the flow from the 2D tributary.

“My understanding is that flow across the lateral structure enters (or leaves) the 1D cross sections at the lateral structure embankment elevations regardless of the bank elevation of the cross-sections.”

Yes, that is correct.

[Scott Miller]

Yes, Cameron, the lateral structures are iterating up to 20 times. The 1D/2D flow error is frequent during high flows. Most of the errors do not exceed the 20 cfs threshold by much, but there are numerous lateral structures. Next time I run the model I can look for what is happening at this particular lateral structure. The peak flow from the tributary is about 160 cfs.

A weir coefficient of 0.2 is applied.

The lateral structure where problems are occurring is 30 feet long, long enough to meet the high points abreast a constrained channel. I could shorten the lateral structure, but any break would be in the flow. Would that be a problem?

[Scott Miller]

Thank you, Jarvus. I was thinking the model might be better with the tributary modeled 1D, but it sounds like the 1D/1D confluence would have its own problems. I’m not familiar with it. It doesn’t sound like a geometry rebuild would be worth the time spent.

My understanding is that bridge and culvert modeling in 2D is not explicit as in 1D. I may need to look further into that, but it is part of why I kept the construction alternatives part of the model in 1D.

Overflow across the road goes into the tributary channel when the crossing/culvert does not have adequate capacity. While it would be straight forward to move the tributary hydrograph to 1D as a lateral inflow, I could not get rid of the lateral structure outright.

It seems, as Cameron may have been getting at, that the 1D/2D flow error may just be too high. This would explain why the WSE is so different between 2D and 1D at that particular lateral structure, at least when modeled with the weir equation. I’m not sure why the 1D gets superelevated when normal 2D equations are applied.

Rather than put the tributary hydrograph entirely into 1D, I’ll split it 50/50, maybe 67/33, and expect that that will balance flows over the lateral structure.

[cameron]

I would say that the 0.2 value for weir coefficient might be too low. The value the HEC-RAS manual uses is for a lateral type of flow that is spilling out of a channel. It is not meant for channel type flow. Think of more like an inline weir which can have values up to 2.6 to 3.1.

[Anonymous]

“My understanding is that bridge and culvert modeling in 2D is not explicit as in 1D.”

2D areas do not support bridges. But the internal 2D area connection can have culverts and you may be able to model your bridges as culverts and get reasonable results.

Having culverts inside of a 2D area can be a source of instability. But assuming it converges to an answer, I think the answers for 2D culverts are as good or better than 1D. The flow and water surface around bridges and culverts is not 1D, so 2D does have an advantage.

It is hard to know if culverts inside of your 2D area would cause a problem without trying it. Shorter time steps generally help. Having somewhat larger cells upstream of the culvert can also help.

[cameron]

I have found that culverts in 2D do not like backwater and can cause instabilities so be careful. I agree that smaller time steps help or having the culvert cover only one grid cell

[Anonymous]

Yea but 2D to 1D connections do not transfer water to the overbank. It goes directly into the channel.

Its kind of annoying, but if you connect a 2D area to the overbanks of a 1D cross section, where flow is coming from the 2D area and transferring water perpendicular to the 1D cross section, water will not travel in the 1D overbank. It just will magically be put into the channel. This tell me that the placement of my lateral structure is much more important than just connecting previous models w 2D areas. It doesnt work like that and doesnt make sense to me why the software would do this. It was always this way in full 1D in older versions of the software

[Anonymous]

A 1D cross section really is only 1D. So it is going to put the flow in the lowest location that can convey flow. It is not capable of keeping the flow in the overbank when the channel is lower. For that, you need to use 2D.

[Author]

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