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RE: Error when modelling with mixed sediments

Bas Gradussen, modified 21 Days ago.

Error when modelling with mixed sediments

Keen Forecaster Posts: 1 Join Date: 10/28/20 Recent Posts
I am studying the morphodynamic effects after widening a certain reach. Directly after the first time step, I get the following error:
'You are getting close to the centre of the earth since you run out of substrate material. Ask V about input.mor.ThUnLyrEnd to solve this problem. Come with coffee please emoticon'.

Unfortunately no real-life coffee for now emoticon

The location where the Ls of the top-substrate layer approaches 0 is the same location where a sedimentation shock wave rises. I believe this is a common error (based on a previous conversation with Liselot). What is happening here? Is - at the location where we observe the sedimentation shock wave - a new substrate layer created on top of the previous top-substrate layer for which a very small Ls is registered and it therefore looks like the new top-substrate layer is almost fully eroded? Is there a way we can solve this (using input.mor.ThUnLyrEnd maybe)?


Bas Gradussen 

Edit: I tried running with input.mor.ThUnLyrEnd = 4 (2 times input.mor.ThUnLyr). This did not solve the problem.
Victor Chavarrias, modified 19 Days ago.

RE: Error when modelling with mixed sediments

Famous Fortune teller Posts: 45 Join Date: 4/24/20 Recent Posts
Hi Bas, 
Great to know that you are using Elv! The error message indicates that the bed level is below the lowest level of the substrate. The lowest level of the substrate is the bed level minus the sum of all the substrate layers. Worded differently, the error message indicates that the thickness of all substrate layers at one point is 0. If, for instance, initially you have 10 substrate layers of 0.5 m plus the active layer of 1 m, in total you have a 6 m substrate. In this example, if somewhere you degrade more than 6 m: ERROR. 
It is strange that you say that the error appears where a sedimentation wave arises. As explained above, the error is related to excessive erosion. There must be erosion somewhere.If this happens in the first time step it indicates that from the start there is a huge amount of erosion. That is problematic and probably unreallistic. The first question I have is: are you solving steady or unsteady flow? If it is unsteady, maybe the initial flow condition is causing the large erosion. In this case, yu could consider using 'input.mor.Tstart' for preventing morpodynamic update during the start of the simulation (i.e, spin-up). 
If flow is steady, there is no initial flow condition. Then, it must be related to the schematization. Indeed, by using input.mor.ThUnLyrEnd you can create a very thick last layer for preventing the error. I would suggest that you use an extreme value (1000?) just for computing a few time steps and be able to plot the output. Then we can see what is going on. There is no computational extra cost for having a very thick last layer, so it is always a good idea to have a thick one. It will not prevent the results being unreallistic, but it will allow you to prevent the crash and see what is going on. 
How is your flow? Do you impose a constant discharge or a hydrograph? How large is your space step? If you impose a hydrograph with a small time step, expect some issues to arise. Let me know and we discuss it.