Re: [Anuga-user] Problem with unrealistically high stage

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

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
>
> *----------------------------------------------------------------------------------------*
> *Kyle Wright*
> The University of Texas at Austin
> Department of Environmental & Water Resources Engineering
>
>
>
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