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Hello Toby,

Sorry, it seems that you have found a bug dealing with culverts and LIPS.

Can you try to change the advection scheme and switch LIPS to NERD e.g. (13).
I have tried with TELEMAC-2D test case siphon and it seems to better work.

In addition to tracers, you may also need to change LIPS for velocity components, I have not tried.

Unfortunately, our LIPS scheme specialist will not be able to answer for months and it may take me some times to understand what the issue is with LIPS (and not in a close future).

Hope this helps,

Chi-Tuan

Hi Chi-Tuan,

Thanks for your response - didn't catch it before my last reply.

Will try with the NERD scheme for advection of velocities and tracers. I guess a 'SCHEME FOR ADVECTION OF VELOCITIES' = 1 would be required to avoid LIPS?

Do you know if there is any major difference between scheme 13 or 14 as I haven't used them a whole lot?

Hi Chi-Tuan,

Can confirm the tracer transport is working correctly now with Scheme 13

Thanks very to both yourself and Christophe for the assistance in debugging this.

Regards,

Yes, I would use characteristics (= 1) if LIPS does not work for velocities.

If I remember, schemes 13 and 14 can give different results if there are dry areas. I have not investigated deeply but I think there are not so many basic differences of implementation.

Hope this helps,

Chi-Tuan

Looking into the source code of DIFSOU.f on line 329 or 332 I cannot quite work something out.

Writing the value of MASSOU(ITRAC) immediately after it is defined for a source point along the culvert I am getting an output value of zero. However, computing the actual mass of tracer according to the below I am getting a non-zero value

Not really understanding why this is happening as my Fortran skills are limited, although if it is indeed zero then I would assume that no tracer mass but an increase in cell volume may cause such an unrealistic dilution.

Hi Christophe and Chi-Tuan,

Was wondering if either of you had carried out any sensitivity analysis on the model calculated flow rates relative to the cell size at the upstream and downstream nodes before?

What I am seeing is that the model calculated flow rates are substantially smaller than those I estimated by hand using the same Bodhaine equations but using modelled water levels without the culverts in place.

It seems that for my larger culverts, changes to the water levels locally at the culvert nodes are more pronounced which may be artificially influencing the head. I can imagine that with larger cell volumes changes to the WL at the culvert nodes would be less pronounced.

Regards,
Toby

Hi Toby

No sensitivity analysis conducted on my side but totally agree with your analysis.
The culvert functionality had been developed to take into account the effect of existing and potentially in charge connections in the flow when those structures are smaller than the mesh size without refining the model.
Small structure implicitly means small discharge then small velocities and by consequence allow to consider free surface as head...

Using this capability in case of large structure could leads to erroneous results due to violation of these assumptions.
In case of significant velocity there is probably some adaptation to envisage (enlarge artificially the structure, modify buse.f file to use head instead of free surface)

Hope this helps

Hi Christophe,

Thanks - once I have some time I may try to modify BUSE.f to take water levels from nodes 1 or 2 points away from the culvert node and see if that improves the results.

Alternatively, as you mentioned, artificially increasing the culvert size may work, however its difficult to know a priori what flow rate to be targeting.

Regards,
Toby