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Hi Everyone,

I am trying to simulate soil erosion in a small plot (5 by 2m and slope of 10 degrees) using Telemac-Sisyphe. I adopted the workings pluie example but now Telemac is coupled with Sisyphe. When Rainfall-runoff is activated in Telemac v7p2, the runoff generated is always too small to cause erosion even if I increase the value of RAIN OR EVAPORATION IN MM PER DAY to 10^6 mm/day. When rainfall-runoff option is deactivated (and RAIN OR EVAPORATION =yes), the model would would displays erosion when 5000 mm/day is applied.
Question;
1. Is rainfall-runoff option working okay? has anybody noticed the same and what can it be done?
2. How to transform daily rainfall (e.g 20mm) to mm/day in Telemac?

Thanks

Koskei

Hello

Interesting question :)
Yes rainfall-runoff works well but it depends on which criteria.
Theses features are validated for several simple and complex cases. What you should know is:
1- there is a full mass conservation (except for very few advection options)
2- the velocity field (which is critical for your applications) depends on a full range of options (advection scheme, fricton law and coefficient, bathymetry gradients, diffusion,...) and then can not be validated unless you have some measurements, so through a calibration excercise.

I suggest that before coupling with sisyphe, you need to validate hydrodynamics. Once, this is done, you have a zdditional task to play with sediment (choice of bedload law, calibrate grain sizes, etc). This step also should be calibrated using some measurments.
Anyway, I will be glad to have your feedbacks if you go on with this topic.

I hope that this helps

with my best regards

Riadh ATA

Thanks Riadh.
May I request you to comment on the second question, e.g assuming a daily rainfall of 20mm. Would it be applied in Telemac as 20mm/day?

Hi
For the second point you're right.
This also mean that those 20mm will be spread on the whole day...
If you need a more intense rainfall, you should change the value...

Regards

Dear Riadh and Coulet,

I sincerely thank you for your response and i am still puzzled by the Telemac results. When I try to compare simulated with the experimental data, I find Telemac results to be way too small. We have constructed erosion plots 5 by 2 and slope of 10 degrees. we mimicked rainfall using modified drip lines and running experiment for 20 minutes under constant pumping rate yields cumulative rainfall of 80mm and eroded sediment mass of approx 40kg. The mean diameter of sediment is 1mm (dry sieving) and density of soil is 1.2 g/cm3. Simulation could only yield less than 1 kg of eroded soil and there is unrealistic deposition of sediment near boundary.Please find the attached folder of my workings and I seek your direction in correctly set up the model for this case.

Hello kipkos,

First of all, as Riadh suggested, it seems that the first step is to check the hydrodynamics since you might be dealing with small water depths which can be tricky when coupling with sediment transport.

A second point that is worth checking is if you should use finite elements or finite volumes. At the last User Conference we got a presentation (by Florent TACCONE from EDF) on a similar topic and it was highlighted that finite volumes are better suited for such applications (erosion due to surface runoff). I don't know when the proceedings will finally be released but you might find useful information in this article.

Otherwise, as Riadh mentionned, the new rainfall-runoff model comes with new options for defining the rainfall, please have a look at the test case pluie which contains two application examples dealing with the runoff model.

Best regards
PL

Hello again,

I had a very quick look at your model and I found waht is likely a mistake in your cas file :

FRICTION COEFFICIENT =25
LAW OF BOTTOM FRICTION =2

With a coefficient of 25 you might want to use Strickler's friction law (LAW OF BOTTOM FRICTION =3).

I would also start by taking a simpler turbulence formulation instead of Smagorinsky.

I did not look at the Sisyphe part.

Best regards
PL

Hello

You can have a glance on subroutine prosou.f to see how it is implemented.
As Christophe said earlier, the rain is converted to m/s.
If you use the last version v7p2, you will see that there are new features dealing with rainfall-runoff in which you can introduce a complex hyietograph and manage the time and the intensity of rain. If you are interested you can read more in the user manual and look at case Pluie in folder Examples/telemac2d

I hope that this helps

with my best regards

Riadh

Hello

An other remark, about:
1- water depth which are very small
2- bathymetry gradient which is very high more than 18%

These are extreme cases for the validity of shallow water equations and for the algorithms that handle tidal flat in the finite element kernel. You can try also with finite volumes, but in this case you have to use the trunk version, since these feature has been added recently to the trunk.

with my best regards

Riadh

Thanks again.
I've been trying to run using finite volume without success. I am getting the following error message even after activating tidal flats in telemac and sisyphe;
'prosou: tidal flats option must be activated with scs cn runoff model'