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

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

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
>
>
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