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Monica Macias Jimenez

Inexplicable outputs

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Hello modelers,

I am having difficulties with a nested model. I have a 1D2D model and then in the channel of my watercourse I have a nested 2D domain. The aim is assessing scour in the banks therefore velocity is the most important factor here.

I have the same roughness all around in the channel therefore, I would expect higher velocities where the LiDAR points out at deeper areas and also at the outside bank of the channel curves. However, I am not observing that and I am trying to think out of the box to find an explanation. I would say apart from roughness, flow (which should be fairly constant as there is no much flooding) and DTM profile I have not other variables which would affect to my results.

The model is unstable after the peak but looking at the peak and before, I do not see what I would expect (a "similar pattern of the DTM for the velocities according to depth"). Maybe I am missing something and any help will be very appreciate it.

Thanks,
Monica

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Hi Monica,

Sounds like an interesting problem for you! Could you post a couple of images to illustrate your setup and the results you're seeing please? I think that'd probably help with trying to think about what might be happening. E.g. What is the distribution of velocities you're seeing? How have you linked the in-channel 2d domain with the surrounding 1d-2d modelling?

All the best,

PHA.

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Hi PHA,

Please see attached two screnshots from one area of interest in our model. This part of the channel is nested as you can see from the results and the dots are the 2D2D connections.

As you can see from the DTM there are deeper areas in the left bank in this case around the area sticking out however, the velocities are higher in the center of the channel.

In terms of linking, the 2D2D is linked at the top of bank (yellow line) this was done in a previous model which has been verified so I did not modify that. You can see from the results that the connections are in a white area not being flooded. At the top I have a dummy downstream boundary with a spill in Flood Modeller. The spill has the same cross section as the joining point from 2D. The connections are done by SX line. Anyway, independently from what is coming into the nested model I would expect that after more than 1km downstream that is not that influential and would expect patterns following the DTM depth.

Any ideas?

Thanks,

Monica

2018-04-23 15_28_06-QGIS 2.18.13 - P3-2D_Model_MMJ_V02.png

2018-04-23 15_29_17-QGIS 2.18.13 - P3-2D_Model_MMJ_V02.png

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Hi again,

Sounds like your linking is all good, so leaving that and moving on...

It looks as to me as if in general you are seeing the effect you describe, with flows being slower at the sides of the channel and faster towards the centre where it's deeper. More local effects won't follow this same trend though and your depression (the red circle in the DTM image) is only very locally deeper; in this circumstance the approach and departure flows remain limited by the depth of the channel upstream and downstream, but that constant flow now has greater depth to flow through, and so gets to flow more slowly to pass the same flow- hence the reduced velocities at this location. Similarly, but conversely, where there's the lump in the side of the channel, water is having to squeeze around and over this and so is having to travel faster to get the water past. I would think it's this lump that's also causing the slower velocities on the east of the main channel, despite it being deepest there, as the lump disrupts and diverts what would otherwise be a nice flow path.

I hope that makes sense! I summary, I think it all looks fine. If you're looking at making alterations in the watercourse here for engineering works, then you could do worse than taking out the lump and filling in the hole! (probably if you took out the lump, the hole would fill itself in over time...)

Do ask further if I've not addressed your issue or haven't explained myself clearly. :)

PHA.

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Hi again PHA,

I have rechecked my outputs after reading your message, actually it makes sense. I totally understood what you wanted to explain. 

We were expecting higher velocities in the  bank than in the center as a consequence of the river bend there but thinking about it is not that accused.

Thank you very much for your help and clarifications!

Monica

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