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

[Gareth D. <gar...@gm...>]

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... <mailto: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
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