Weir vs terrain in 2D connection

[Anonymous]

I am reviewing a model from a consultant where they modeled a proposed impoundment using a 2D area with an internal connection (weir) for the embankment. They edited the terrain to include the embankment across the whole downstream side of the impoundment. Then they also put a 2D connection with a weir on this embankment. However, in this weir, they have a narrow V notch that goes below the terrain elevation of the embankment. The model is doing its weird glitch thing where it won’t show the average elevation of the cells on the upstream and downstream side of the weir in the 2D connection editor that it does, sometimes, but the cells on either sideare aligned to the weir as a break line. I had thought that the model would not allow a weir to go below the terrain elevation, but that doesn’t appear to be the case. Due to that, I don’t know if the model will use the V notch or the terrain for the flow over the weir and/or through the notch. Their outputs are currently too coarse for me to see when it starts going through the notch vs over the weir, so I can’t tell. I can rerun the model with finer outputs, but it is large, and I can do some experiments, but I thought I’d check with this group to see if anybody knew the answer to this question about how the terrain and the V-notch that goes below it would interact here. I could see the model getting a water surface elevation for the cell on one side, using that for the V notch to calculate a flow through the structure, then giving the new WSE to the cell on the downstream side. This is what I think was intended. However, I could also see it not flowing through the structure until the water surface exceeds the terrain level at the structure. Anybody know the answer to this?

[Anonymous]

The user entered weir data can be lower than the terrain along the centerline of the weir. The user entered weir data cannot be lower than the lowest point in the adjacent cells. If it is, RAS will print out an error message and will not even run.

The flow from one cell to the next is controlled by the faces. The user entered weir data overrides the terrain data for the face. It doesn’t matter whether the user enters weir data that is higher or lower than the terrain, it uses the face data. So the flow will based on the notch.

The cell data is used to compute the volume of water that is in a particular cell. The cell volume tables are computed from the terrain and do not include the user entered station/elevation data along the centerline.

Modifying the terrain to include the embankment would generally make this more accurate. However, the error from not raising it is usually pretty small in the big picture.

If they raised the terrain to include the embankment, the notch won’t be included in the cell volume. However (with the usual caveat about not knowing the specifics of this model), I would think that raising the terrain would generally be more accurate. Raised cell data without the notch would be more accurate than no raised cell at all.

I think the only concern would be if the cells are very tiny compared to the notch, it might cause it to be more unstable.

[Anonymous]

Thanks very much. That was the info I was wondering about. I’ve done some tests and found that it is obeying the V-notch, but I think it’s only because the cell size is large enough to have both the toe and top of the berm in the same cell adjacent to the structure. So, that way as water touches the tow, the V notch overrides the berm, and it gets through. I would think that with a smaller cell size such that the cell adjcanet to the structure did not reach the toe, the water surface would need to get high enough to touch that cell before it would go through the V notch. If that cell only reaches partway down the berm, that would block flow from entering the V notch until that the min elevation of that cell was reached. Does that sound correct?

[Anonymous]

If the low point in the adjacent cell is higher than the low point in the notch, RAS will print an error message and your data set won’t even run.

You can try this. Enter a notch with a really low elevation.

If you have small cells and the low point of the cell is just barely at, or below, the notch, there may be instability.

If you don’t get an error message, and you don’t have stability issues, I would, generally, think you should be ok.

[Anonymous]

Scott, just to add to the other replies, this may not be an issue but, you may need to make sure that the cells downstream of the notch are not higher then the notch invert otherwise you will have back up flow until the d/s embankment level is exceeded. the cells touching the v-notch might be ok but if cells further downstream are higher then you will have incorrect results. It’s always best to have the terrain at the correct level and not impeding the flow in these situations.

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