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Hi,

I have a 2D model with a river channel and a surrounding plain. During the run, negative water depths appear next to the channel and propagate in the plain.
Following indications in the forum, I defined the type of advection as 1;5 with discretization in space 11;11. The results didn’t show any negative depth. But the water surface wasn’t flat and smooth anymore. So I change the free surface gradient compatibility from 0.9 to 0.5. I couldn’t observe any significant changes in the results. With discretization in space 12;11 or 12;12 the computation doesn’t start.
I also tried to use type of advection 14;5 and treatment of negative depth = 2, then oscillations appeared next to my boundary condition. Reducing the time step, helped to stabilize the system at the beginning, but after a while the oscillations appeared again.
Do you have any more ideas how I can reduce the propagation of the negative water depths without generating oscillations or uneven water surfaces?
Thanks a lot for your help

Hello,

Negative depths, if they remain small, are not too much of a problem as mass is conserved in our algorithms. However the setting with TREATMENT OF NEGATIVE DEPTHS is more correct and ensures a mass conservation at machine accuracy (it can even be used for this if you have no dry zones). Then oscillations next to your boundary condition may reveal another problem, e.g. the fact that the flow is supercritical. If you have also dry zones in your entrance it may be the reason, try to avoid it,and also look at VELOCITY PROFILES , which with options 4 and even better, 5, may give a cleaner velocity profile at the entrance.

With best regards,

Jean-Michel Hervouet