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paul_ollett

User defined HQ Rating Curve Unstable

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Hi,

I'm having problems with an HQ rating curve causing my model to blow up one minute into the run. For some reason the HQ downstream boundary is becoming very unstable. There seems to be a slight instability at startup, which causes an increase in discharge, which results in an increased H, then the instability seems to feed back on itself and gets worse and worse.

The rating curve was developed from a catchment-wide 1D hydraulic package, by running a series of steady state discharges through the reach of interest, then measuring off HGL for the various discharges. See attachment.

At the downstream edge of the 2D_code layer, the cross section along the HQ line has a varied RL. In other packages one approach involves lowering or digging out grid cells, due to problems with wetting and drying. Actually I tried this to no avail - even though I know it should not be necessary in TF. I lowered the cells across the width of the HQ line to RL-2 for a distance of 30m upstream. The HQ line was initially snapped to the 2d_code edge, but I have also tried shifting it upstream 20m.

My suspicion is that it may have something to do with the IWL of RL 2.28m at zero cumecs flow. The creek I am modelling is a tributary to a larger river. The RL 2.28 results from a 20-yr storm tide level, and a 2-yr flood in the larger river downstream.

My time step is 0.5s, and grid size 2m. I need a fine grid size as I plan to also run the morphological module at some point. I have the same blow up no matter which version of TF I use. I've also attached a cross section at the boundary, and the HQ rating curve being used. I've just started a run at 0.1s time step, so will find out how this went tomorrow. If it runs through, 0.1s still seems too fine a time step to have to use.

Does anyone have any ideas?

Regards,

Paul.

HQ_details.xls

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Hi Paul,

Is this a 2D HQ boundary? Also what kind of flows are you putting into the model when it is going unstable?

I have had similar problems with 1D HQ boundaries with ratings derived from 1D packages, early on in the model. To take your rating as an example, my baseflow might start at 10 cumecs, in which case I would fudge the bottom end of the rating so that any flows below 10 cumecs have a stage of 2.335mAOD. You may also need an IWL layer to set this level too. This smooths everything out at the start of the model (obviously the results will be a bit skewed, but not an issue if you're interested in higher flows).

The other thing I notice is that the left bank of your cross section is a fair bit lower than the upper extent of the rating - I think if the left bank confined the range of flows this may clear things up as well?

Not sure whether your issue is in any way related, but hopefully this may help!

Cheers

Richard

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Thanks for your reply Richard.

Yes, it was a 2D boundary, and at the time of the instability no upstream flow was entering the model. I reduced the time step to 0.2s and things became stable, which then showed negative flow was occurring at the down stream boundary.

The problem was due to the way I digitised the downstream boundary. It was not perpendicular to the flow direction and did not extend to the entire flow width. I've seen negative discharges before on two other occasions, both when the downstream boundary was very close to a structure. It seems as though something I did with the boundary caused similar behaviour. Everything is fine now.

Cheers,

Paul.

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