Re: [Anuga-user] Problem with unrealistically high stage (Part 2)

[Kyle A W. <kyl...@ut...>]

Hi Gareth,

I can create a more quantitative plot with surface slopes through time if
that helps, but if you just want to see the time evolution of the depth, I
have the following GIF of depths through time (the color axis was chopped
to [0 1], but it shows the relevant problem clearly):
https://utexas.box.com/s/pbc46qvxcdctbdp0qos8us1ie2rq96fo
None of the channel banks north of the ~3,275,000 line should be inundated
at all. They aren't even normally inundated at twice this discharge.

Do you have specific recommendations as to what I could change about the
boundary conditions? The topography is identical to what others have used
in other models (Delft3D, XBeach) without experiencing this problem.

Thanks
- Kyle

On Wed, Jun 19, 2019 at 8:04 PM Gareth Davies <gar...@gm...>
wrote:

> Hi Kyle
>
> It might help if you can also send a plot showing time-series of stage at
> a set of points along the channel (from right upstream, through to the
> downstream tidal limit, at reasonably regular intervals). Also, perhaps you
> could be more specific as to where you think the "water spilling out" is
> unrealistic.
>
> **Is there a constant I'm unaware of in the code that I need to change?
> What is the shallowest water surface slope that ANUGA can simulate? I'm
> hoping this isn't a model compatibility issue, but it seems like a
> possibility. **
> -- There is no "hard limit" to the slope that can be simulated. If your
> results are convergent (i.e. not changing significantly under mesh
> refinement) then I think it's not very likely that the problem is due to
> limitations in the solver -- it's more likely some issue with the
> topography or the manner in which boundary conditions have been imposed.
>
> Cheers,
> Gareth.
>
>
> On 20/6/19 10:45 am, Kyle A Wright wrote:
>
> Hi all,
>
> I've been having a recurring problem with unrealistically high river stage
> values in the upstream portion of my domain. I've tried a number of things
> to fix this, but the end result is always the same: water spilling out of
> the channels in areas where it certainly doesn't spill out of at
> half-bankfull discharge. I've tried all of the things I can think of (as
> well as the suggestions from my previous email to the list-serve) to
> alleviate this issue and nothing has worked, so I'm hoping someone here
> might have some ideas. Is there a constant I'm unaware of in the code that
> I need to change? What is the shallowest water surface slope that ANUGA can
> simulate? I'm hoping this isn't a model compatibility issue, but it seems
> like a possibility.
>
> I've uploaded a simplified version of my code along with inputs in the zip
> file at this link, in case it is of interest to anybody:
> https://utexas.box.com/s/l1u01fgrtqjeqsjmi5pgde6c1ktmzczy
> It also contains a figure showing the stage issue.
>
> Below is a list of the things I've attempted to see if it would resolve
> this issue:
> 1) Different boundary conditions up and downstream (water does not appear
> to be building up in the model)
> 2) Clean breaklines in the mesh at the channel boundaries to reduce
> numerical drag
> 3) Increasing the grid resolution in and/or around the channel (down to
> grid sizes much finer than the "shallow-water" approximation should be
> valid)
> 4) Changing the vertical datum
> 5) Carving a deeper inlet and upstream channel
> 6) Artificially raising the banks in areas near the inlet to allow the
> flow directions to initialize before flowing into the relevant area of the
> domain
> 7) Starting from a fully dry bed
> 8) Starting from a static constant stage over inundated areas
> 9) Changing the flow algorithm to the least spatially diffusive
> 10) Storing cell vertices uniquely
> 11) Increasing the minimum_allowed_height
> 12) Changing the interpolation algorithm used to set the elevation of the
> cell centroids
> 13) Turning off all friction
> 14) Turning off channel friction with very high bank friction
>
> Any help would be greatly appreciated.
>
> Thanks and best regards,
> - Kyle Wright
>
>
> ----------------------------------------------------
> On Mon, May 13, 2019 at 10:02 PM Gareth Davies <gar...@gm...>
> wrote:
>
>> Hi Kyle,
>>
>> Here are a few ideas to consider:
>>
>> Is there a growth in the downstream waterlevel over time, beyond what you
>> would expect from the tides (e.g. maybe the dirichlet boundary is not
>> allowing enough water to flow out of the model domain)? If that is the
>> case, then you might try using a
>> transmissive_n_momentum_zero_t_momentum_set_stage boundary in the
>> downstream region.
>>
>> Otherwise, maybe your model has too much "numerical drag", even in the
>> refined mesh test cases? In that case you might get better results by using
>> breaklines in the mesh, so they follow the channels -- combined with
>> setting elevation data at centroids, rather than vertices, so there is a
>> clean discontinuity between the bank and the channel bed. This is good
>> practice in general -- I've seen models with poorly resolved channels that
>> were heavily affected by "bump banks" and associated numerical drag. The
>> benefits of "cleanly defining" channels in the mesh can be large.
>>
>> You're right to also wonder about the flow algorithm being too diffusive.
>> ANUGA type numerical methods are motivated by modelling flows with rapid
>> spatial variations, shocks, etc, and they tend to bleed energy more than
>> one might like for very quiescent subcritical flows. The latter can still
>> be modelled, but it might demand more resolution than you'd like. The
>> limiter could certainly play a part in that. You can check this by
>> repeatedly refining the mesh -- if the result is not convergent, be
>> suspicious.
>>
>> A few other things: You should probably test the "DE1" algorithm (it
>> might be less diffusive than "DE0" which I think is the default);
>> definitely double check your datum! Also, check if your upstream boundary
>> realistic (i.e. is the lake level getting too high)? This might suggest a
>> problem at that end of the model.
>>
>> Good luck,
>>
>> Gareth.
>>
>> On 14/5/19 11:31 am, Kyle A Wright wrote:
>>
>> Hi all,
>>
>> I've been getting very unrealistically high depth/stage values in the
>> upstream end of my model domain, and I'm not sure how to figure out what is
>> causing it.
>>
>> Currently, my model is a river delta with one upstream channel (constant
>> discharge inlet + reflective BC inside an upstream lake) and a downstream
>> bay with a constant tidal water level set by a nearby tide gauge (Dirichlet
>> BC). Even for flows significantly lower than bankfull discharge, the water
>> is spilling out of the channel and flooding the surrounding landscape in
>> places that definitely do not normally flood. This happens even if I make
>> the bed completely frictionless, increase the grid resolution, or start
>> with different initial conditions.
>>
>> Is this something anybody else has experienced, and do you have any ideas
>> for how I might fix it?
>>
>> The only ideas I have left are that it is perhaps related to (1) the
>> datum (I am using NAVD88 in the vertical, which I believe differs from the
>> one assumed by the model, but I wouldn't expect that to matter much over a
>> few km), (2) slope limiting (which I don't understand well, but it is a
>> very flat landscape), or (3) the flow algorithm (too diffusive?).
>>
>> I've attached pictures of the depth and a quiver plot of the flow
>> velocities for context. Happy to send the code if helpful.
>>
>> Thanks & best regards,
>> -- Kyle
>>
>>
>> This message is from an external sender. Learn more about why this
>> matters. <https://ut.service-now.com/sp?id=kb_article&number=KB0011401>
>>
>
>
>
>
>
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-- 
*----------------------------------------------------------------------------------------*
*Kyle Wright*
The University of Texas at Austin
Department of Environmental & Water Resources Engineering
(512) 712 - 8688