Surface diffusion

Hi all,

I am using ASPECT 2.3.0 to simulate crustal deformation in 3D in presence of compression and extension, and for some of my simulations I would like to introduce erosion. As pointed out in the manual, erosion is reproduced by diffusing the surface, and the diffusion rate depends on a hillslope transport coefficient k that has a default value of 10e-6.

I have already run my models by adopting a free surface, and everything works as expected. However, when I switch from free surface to diffusion, I always obtain a flat model surface, regardless of the adopted value for k (I tested values ranging from 10e-4 to 10e-30). In addition, when I implement time dependent, compression-to-extension velocity boundary conditions, the solver does not converge anymore when the change of velocity at the boundaries occurs.

Can anyone tell me a little bit more about how the surface diffusion works, and why I do not see any relief in the model surface?

Thank you very much.

Hi Alessio,

Thank you for posting the question on the forum and happy holidays!

The short answer is that you will need to specify that the top boundary is both a free surface, which is subject to diffusion.

Here is an example of how to specify this in the PRM file:

subsection Mesh deformation
  set Mesh deformation boundary indicators = top: free surface, top: diffusion
  subsection Free surface
    set Surface velocity projection = normal
  subsection Diffusion
     # Diffusivity term. Increasing this value will result
     # in a smoother free surface and lower topography
     # amplitudes.
     set Hillslope transport coefficient = 1.e-6

I hope this is helpful and please let us know if you have any additional questions!

Dear John,
thank you very much for your quick reply. I modified the .prm file according to your suggestion, and everything seems to be working fine now. I will update the topic when I get the confirmation that the code also converge. Happy holidays to you!