xs lid giving bad data

[mtpisgah]

Hello,
I am working on a couple of models that keep blowing up. Same stream reach, but FEMA thinks it is a straight 4×6 box the whole way, but is actually a 4×6 box, with two bends, and a 36″ round culvert at the end.

Model 1
Straight box modeled as a culvert – this works and gives data we can believe.

Model 2
Saved as version of Model 1, updated for the straight box modeled as a lidded xs – this gives drastically different data which seems erroneous. The water surface starts to over top the road which is not good, but the weird thing is that where the culverts (lidded sections) connect, there is a spout of water since the sections cannot be on top of each other. The water surface drops at the connection and the rest of the roadway is not over topped.

Model 3
Bent box modeled as lidded xs + the 36″ pipe modeled as a pipe or lidded xs – gives similarly erroneous data as Model 2.

We have tried to run it with and without the Preissmann Slot, to not real effect.

We are under inlet control so that may be the issue since lids are not designed for this use, but how else would you run the model?

Any thoughts?

I have read through a lot of lidded cs posts here and cannot find anything that really talks about the issues, just that some random model instability that is not described.

Thanks

(edited to add data on inlet control)

[Jarvus]

I don’t know what it would take to satisfy FEMA. However, if you are sure the system is under inlet control, then hydraulically speaking you are done. If it is under inlet control, the two bends and the 36″ pipe at the end will not have any effect on the WSE and EG upstream of the culvert. Just model it as the 4/6 box with inlet control.

If you aren’t sure about inlet control, or FEMA requires more:

Yes, you could try and model it all as cross sections with lids but that won’t compute an inlet control answer. However, you could compare the answer with lids to the inlet control of the 4X6 box and go with the higher of the two.

You could also try and model it as a combination of culverts and cross sections with lids. If this is a steady flow FEMA floodway analysis, that is probably what I would try. You mention instability, so I’m presuming this is unsteady? Doing a combination of culverts and XSs with unsteady is possible, but instability can be an issue.

For your “Model 2″, you say there is a spout of water. I’m not sure what you mean by this. For the cross sections with lids, the lid needs to be thick enough (that is, the top of weir elevation needs to be high enough) that you don’t get water showing up on top of the lid. So maybe you just need to make the top of the lid higher. Otherwise, I’m not sure what you mean.

For steady flow, you don’t need the Preissman slot. For Unsteady flow, yes you definitely need it.

For a combination XS and culvert:

To connect the 4X6 to the 36″, add a culvert for the 4X6 box and add a second culvert downstream for the 36”. You will need two cross sections in between the culverts. I would probably make one cross section approximate the 4X6 box and the other cross section approximately a 36″ circle. Make sure the top of the lids are high enough that water doesn’t appear above these lidded cross sections. The upstream 4X6 will still compute an inlet answer and the dual 4X6-36″ culvert will compute an outlet control answer.

There are various ways to handle the energy loss between the connection. If you want to handle it as a minor loss: set the exit loss on the 4X6 to zero, set the entrance loss on the 36″ to zero, and set the contraction/expansion loss on the two lidded cross sections to zero. Then add an additional energy loss to one of the two cross sections using the coefficient multiplied by the velocity head. This would be the minor loss coefficient. Alternately you could allow the lidded cross sections to compute the contraction/expansion loss if the transition is gradual enough.

For the bends, the 4X6 box could be broken into 3 different culverts with lidded cross sections between them and then add a minor loss for each bend as described above.

I think that should be straight forward for a steady flow answer. But all this could be tricky if you are having unsteady stability problems.

But running a steady flow model with a range of tailwaters and a range of flows might give you a better understanding of how the culvert system is behaving and what needs to be taken into account for unsteady flow. And again, if you can demonstrate that it really is inlet control over the flows and tailwaters you are trying to model, you only need to deal with the upstream part of the 4X6 box.

[Jarvus]

I haven’t tried the cross section with lids between culverts for unsteady flow. Because of the bridge/culvert HTAB curves, you might need three cross sections between the culverts instead of two. But if you put them close together, I think that would be fine.

[mtpisgah]

Jarvus,
That is a lot of good advice, we will start working through it. We are running it as a steady flow model (tried unsteady and it was not better). The instability I referred to was more of a model instability rather than flow, but the model may not be unstable.

Since the 36″ pipe is the limiting factor, we are considering submitting it as the existing conditions and ignoring the 4×6 that is upstream. A complete 36″ has the same effect in real life as half 36 and half 4×6, but not sure if FEMA will accept it.

Regarding the ‘spout of water’ i mean in the profile view, some of the models show water shooting up between the cross sections that delineate the 4×6 and the 36″. When the road over tops, the over topped water stops at that cross section and joints the water between sections.

I will post again when we get better results.

[Jarvus]

If you are modeling a culvert as a cross section with lid and the culvert is pressurized, the profile plot will show the water above the top of the culvert. However, if you look at the cross section specific output, you can see that the area and velocity is correct. It is just a confusing output issue where the water surface elevation is being reported as the hydraulic grade line instead of being reported as the top of culvert.

However, as I mentioned several times before, it is important that the lid is thick enough that the water does not get above the lid. For cross sections with lids, the area above the lid is connected to the area below the lid essentially allowing flow to move back and forth. So this is why you need a thick enough lid to prevent this from happening.

If you are modeling a culvert where the road is over topped and you want to use cross sections with lids for the culvert, it is a little more awkward. The best I have come up with is to create a junction on either side of the road with two separate reaches. One reach for modeling the flow over the road and the other reach for modeling the flow through the cross sections with lids.

[mtpisgah]

We are modeling a reach with the potential for flow over the road. I am a bit confused as to how you are modeling two reaches for the same thing. Is this in parallel? How would that work exactly?

[Jarvus]

If you model a road crossing as a culvert and weir, I believe RAS computes the culvert flow and the weir flow separately. It iterates back and forth until it gets the correct flow going through the culvert and the correct flow over the road to balance. If you run enough data sets for enough years, you will probably eventually see a message along the lines of, ‘the weir/culvert flow didn’t converge’.

Cross sections with lids are not like this. I believe that the wetted area above the lid is combined with the wetted area below the below the lid (and the same thing for wetted perimeter). It then does a steady flow backwater on this combined hydraulic area.

If you have a very long culvert you model with cross sections with lids, you could start out with flow above the road at the upstream end but as you go downstream and the energy drops below the road, all the flow disappears from the road and is transferred into the culvert. This obviously isn’t how most culvert work.

So for the culvert part of the flow being modeled with lids, the lid needs to be thick enough to not have flow above it.

If your model has a 4X6 with a couple bends and also a 36″ pipe, I would presume this is not a short, simple road crossing but instead the culvert is probably underground for some distance.

So upstream of the culvert you could add a junction. And you would have a second junction downstream of the culvert. Yes, there would be two parallel reaches that contain cross sections. The reach that represents the flow over the road would be normal cross sections. The reach that represents the flow in the culvert would have cross sections with lids. It is a little awkward to set up. And it can be even more awkward to explain to a client. However, given the limitations and assumptions that are already built into a 1D, steady flow model, I think you can get perfectly fine answers.

You would also want to turn split flow optimization on for the upstream junction in order to get Steady flow to iterate back and forth and balance the flow/energy between the two reaches.

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