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Claire

QT Boundaries

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I am using QT boundaries to model inflows from two rivers entering a tidal estuary and they are misbehaving!

I have set the boundaries using the bc database and approprately named csv files giving the two QT boundaries constant inflows of 18.6 cumecs and 37.3 cumecs respectively.

As shown in the attached image at the first timestep the boundary is trying to remove flow from the 2D domain at both boundaries which ultimately results in the failure of the model after 1 minute of simulation time.

Can anyone enlighten me as to what Tuflow may be up to at the boundaries and how I can resolve the problem?

Claire

post-2339-1273672674_thumb.png

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Using an instant large inflow will cause model instability because flow will jump from one edge of the cell boundary to the next quicker than your given time step.

Try adding the flow slowly, ie starting at 0 cumecs, and slowly ramping up to 18.6 and 37.3 cumecs respectively.

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I have now tried ramping the inflows up from 0 cumecs at time 0 to their respective totals over a period of 1 hour.

This has made no difference to the stability of simulation. I have after the first timestep, a dV of -28266 with a Qi of 1 and a Qo of 0.

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If TUFLOW is saying that Qo is 0, then no flow is leaving the model. I agree that the flow should be gradually introduced, rather than immediately added as a large lump. Some other suggestions are:

1. Have you checked the boundary inflow in 2d_2d_bc_tables_check.csv?

2. The large instability at the first timestep suggests a problem with the initial conditions. Is the IWL set correctly? The extent of inundation shown in the SMS image looks unusual.

3. Have you checked the Z levels in the model at the boundaries, to make sure there are no -9999 metre values?

Hope this helps.

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Thanks for the suggestions so far.

1. I and my colleges have checked the boundary and check files and everything is as it should be.

2. The IWL in the model is set using a 2d_IWL polygon the level is taken from the tidal boundary which is at the other side of the model extents (not shown in the SMS plot). Does the QT boundary have an initial water level? And could it be something to do with the fact that the two IWL's do not match and if so is there a way of setting the QT IWL?

3. I have checked and there are no -9999 zpt values in my domain.

Claire

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Hi Claire,

I believe QT boundaries still operate on a 'hidden 1d node' basis. This means there are a bunch of 1d nodes you can't see, and TUFLOW adds your inflow to those. These have a volume (equivalent to one row of cells I think), so upon the flow being added, they've got an appropriate head, and they are linked to the 2d using an HX boundary. It may be that these hidden 1d nodes have the wrong IWL set, so your tidal water is moving from the 2d to fill up these 1d nodes at the start of the run.

Unfortunately, I don't know how you might go about setting the IWL in these hidden nodes! So if that is the cause of your problem, then you may just have to start with them dry.

So try with your tide nice and low, and ramp up your tide and your inflows and see how it all goes!

Could someone from the TUFLOW team confirm that this is how things work and if appropriate comment on the IWL in the hidden nodes? Thank you!

Please let us know how you get on Claire,

PHA.

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Hi Claire

Looking at the image in your first post, there is something not quite right somewhere. First check your boundaries by using the 2d_bcc_check.mif layer and the 2d_bc_tables_check.csv file. If everything looks all right, please email these files plus your 2d_bc input layer to support@tuflow.com and we'll check things out.

Cheers

Bill

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As speculated the problem was the IWL in the hidden 1D nodes.

I had set an overall IWL of -999 to keep low lying inland areas dry initally and then used a IWL polygon to set the IWL in the Sea to 0.14m.

Swapping these two round and setting a global IWL of 0.14 and using a polygon IWL to keep the land dry solves the problem :)

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