open TELEMAC-MASCARET The mathematically superior suite of solvers

Hi everybody,

I was testing some cohesive sediment transport in a channel or flume. I first ran the hydrodynamics with TELEMAC-3D and then used the results to run SISYPHE stand alone. I noticed that advection scheme 13 and 14, recommended when having tidal flats present, are not working. If I use scheme 4 I get nice advective transport.
I just wanted to report this. Could it be that it is caused by running SISYPHE stand alone?

kind regards,

Sven

Hello Sven,

Sure it is the reason. To run the NERD scheme (13-14) you need to have the fluxes between points computed by Telemac-2D or 3D. Generally speaking to have a conservative advection when solving the non-conservative form of the advection equation you need to have the discrete continuity equation available (old and new depths, fluxes between points). This is not possible in stand-alone mode, especially if you do not keep the same time-step. Furthermore, in standalone mode Sisyphe updates the velocities with somewhat wrong assumptions (e.g. considering that the product h*u will remain constant). This latter approximation may produce strange patterns of sedimentation, which are totally artificial. This is why we think that we should stop the standalone mode and do not recommend it.

This does not explain why you seem to have good results with scheme 4. With this latter scheme, which does not work with tidal flats I programmed an option in the case where the velocity field does not obey the continuity equation. This saves mass conservation but spoils the monotonicity. This option is also used when there is a velocity correction in Sisyphe, to take into account the fact that the sediment "sees" the velocity near the bottom.

With best regards,

Jean-Michel Hervouet

Thank you Jean-Michel for this clear answer.

Scheme number 4 worked in my small model because there were no tidal flats. In the large model it gives an error which leads me to the advection schemes.
This means that I can not use SISYPHE stand alone for our purpose.

The reason we would like to run only SISPYHE is that we want to start from an empty model (no sediments) and let only mud enter from the boundaries. We let the model simulate 1 year and compare beginning and ending to see if we can reach a kind of equilibrium state. Usually this takes 3 to 5 simulated years for the Scheldt estuary. This is the way we did it with Delwaq. Different scenarios were simulated in the same way and could be compared to each other to see the effects of the scenarios on the long term evolution on mud distribution.

So first we tried to couple T3D with Delwaq to use our Delwaq mud model. We finally made it work, but the telemac grid could not be aggregated in Delwaq resulting in very long simulation times. The best we got is a model that simulates as (fast)slow as reality! So we left this path to try to create a similar model in SISYPHE. But this gives the problems mentioned above.

So the only option available if we want to stay with TELEMAC is to simulate the 3D model coupled with SISYPHE (or sedi3d). But this makes it very unpractical to start from an empty bed. Simulation time would be too long.
Has anybody have any experience with this kind of modelling? The option we think of right now is to use expert judgement for effects on mud transport on the long term and only model short term effects.

Sven