GPU 'normal flow' boundary - how does it work?

[dsheehy 0]

I've set up downstream boundaries in GPU models a number of times now using a 2d_bc HT boundary where a fixed water level below ground level is used. According to the documentation, this causes the model to apply 'normal flow' at this point, effectively behaving as an automatic HQ boundary.

However, the results usually look like water is simply pouring out of the boundary, with water level and depth decreasing as they approach the boundary. This does not look like normal flow as I would have expected it and can have effects quite some distance upstream.

The attached image illustrates the point.

Is someone able to expand on how exactly this boundary works?

 

[Sam Marginson 0]

x2 on this question, except I have a slightly different problem - I don't want to apply an HQ boundary, I want to apply a constant water level all around the model perimeter

However, as I'm running climate change simulations & don't want a different active area for each climate change scenario, in some locations in some runs the level's below ground

will this turn the whole line into an HQ boundary, or only the cells that are below the level?  It looks like the whole thing's turning into an HQ boundary

[Sam Marginson 0]

actually I'm wondering if this is some sort of TUFLOW_to_GIS artifact.  When I look at the XMDF in QGIS it looks OK...

[Sam Marginson 0]

actually I take that back.  I'm getting similar results at the D/S end of my results in the XMDF as well

[TeeganB 0]

Apologies for the delayed response here. Daniel please see Item 62(b) of the TUFLOW Build 2016-03-AC Release Notes providing the following correction to the 2016-03-AA Manual:

"A normal flow boundary in the GPU Solver is not activated by having the HT boundary level below the ground level. A normal flow boundary is invoked in the GPU Solver by specifying an automatic HQ boundary as per TUFLOW Classic. This is done by assigning a blank Name attribute in the 2d_bc layer, and the “b” (slope) attribute needs to be non-zero, although the slope value is not used by the GPU Solver. The GPU Solver assumes uniform flow based on the ground slope of adjoining cells. This feature may also be invoked by specifying a HT boundary with a water level of -9999. Note: the assumed water levels at these boundary cells are not presently output and the cells may appear as dry or with very low levels. Also the water levels at the common cell corners of adjoining active cells may be incorrectly low in the output. Improved output at automatic HQ boundary cells for the GPU Solver is planned for the next update."

Sam I'll contact you through TUFLOW Support, I might need a bit more information on your problem.

Cheers,

Teegan

 

[dsheehy 0]

Thanks Teegan. That explains a lot!

[par 0]

Please note for the 2016-03-AD there have been a number of changes / enhancements to the GPU boundaries.  Please refer to this post and the release notes: