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Hi everyone.

I am setting up a simple test case (see initial bathymetry in attachment). The western, northern and eastern boundaries are closed. The southern boundary is open and a tidal flow is imposed. The initial bathymetry is flat and above mean sea level almost everywhere, but on the southern part where a bottom slope ensures that there is no wetting-drying at the open boundary (at least at start).

After some time (3 M2 tidal cycles, corresponding to 3 geomorphic years, thanks to morphological factor), a creek channel network has been shaped by erosion (see final bathymetry in attachment), as can been observed in real tidal marshes and mudflats.

I only consider one type of cohesive sediments for now, and no bed load transport. The concentration is set to 0 at the open boundary. So, in theory, sediments can only leave the computational domain.

However, looking at the initial and final bathymetries, it seems that the sloping area has been filled up. An integration of the layer thickness confirms it, as it goes from 38.4e6 m^3 (initially) to 38.63e6 m^3 (final step).

I would be very happy if someone could explain me what I am missing here...

I suspect the added sediment results from your cli settings for the bed level. They are set at 5, i.e fixed levels. However, in order to get fixed bed levels the gradients in the transport have to be 0. This causes a tricky combination of the boundary concentrations being 0 (so no transport) and 0 gradients in that transport, but the next row of nodes having transport predicted. So one of the two has to be broken.

I suspect there is some transport into your domain through the boundary. You could check the sewdiment fluses through the boundary to be sure.

Hello,
It seems that your problem comes from the set up of the boundary conditions. As open boundary a Neumann BC-type is needed. Please try it by imposing a flag = 4 in the 'tracer' column of your cli file.

Best wishes,

Pablo

Thank you Michiel and Pablo for your answers.

I made a smaller version of the test case so that it can be run faster. The distance between western and eastern boundaries is smaller (see input files in attachment).

Unfortunately, changing the tracer flag from 5 (fixed) to 4 (free) in the CLI file did not change anything (see figures in attachment). But wouldn't it be a bit surprising that putting the boundary free would allow to impose 0 concentration?

Michiel, you are speaking about boundary setting for bed level. I thought the fourth flag in the CLI file was for tracers. I'm not sure to understand where I defined fixed bottom levels...

Finally, could you confirm that with my setting, there should be no bed load transport?

Thank you for your help.

Hi.

That is the problem. Yu have made no specification for the bed load transport. Add BED LOAD = NO to your SISYPHE steering file to turn it of. As can be seen from the sisyphe.dico file, the default is YES, so by not mentioning it in the cas file, bedload transport is calculated.

I thought that there was no bed load transport for cohesive sediments, by default. I read that in the manual.

Anyway, I added BED LOAD = NO, but it did not change anything...

Hi everyone,

The figures in attachment show the bed load transport, the suspended transport northward, and the suspended transport southward. They are all averaged over the entire simulation.

As I was expected, there is no bed load transport (even without BED LOAD = NO, because only cohesive sediments are taken into account).

The suspended transport figures show that the only transport through the open boundary (south) is southward, meaning that some sediments are leaving the domain, but nothing is coming in. That is exactly what I wanted to simulate.

So I really don't understand why the volume of the bottom layer increases over time... I hope someone can help me to understand what's happening here. Am I doing something non conventional somewhere?

The only thing left, that I can think of is the consolidation model. Is the density of the settling material the same as that of the bed that is being eroded? Generally fresh deposits will be less dense than consolidated mud layers. So the volume of the bed might be different with the same mass.
But I don't know how that is in your model.

Thank you Michiel. This is a very good suggestion, indeed.

Please find the CAS file for SISYPHE in attachment.

I am not defining the density of suspended material. If I'm correct, the default value is 2650 g/l. For the bottom material, the mud concentration of the single layer is 500 g/l. So there might a problem here, I guess.

However, using SEDIMENT DENSITY = 500 did not solve the problem. It actually did not change the results.

I also tried MUD CONCENTRATION PER LAYER = 2650. The results were different, but there was still an increase in bottom volume.

Do I need to do something else to ensure that bottom and suspended material have the same density?

Hello,
Please try a numerical simulation switching-off the consolidation processes.
Then run a simulation with models 1 and 2 of consolidation.
That can give us some tips on what could be happening.

Best wishes,

Pablo