Are you talking about a sliding bed in a slurry system or are you talking about a pneumatic system?

I am not sure the term has any relevance in a slurry system, the solids should be well dispersed and if they are not, there is very unlikely to be any definitive line between part of the pipe where particle to particle contact dominates and the upper section where the particles are dispersed in the fluid.

If you are talking about a pneumatic system, agin it is very difficult to answer your question. Whether solids settle on the bottom of the pipe depends upon the solid density, particle size, pipe diameter, whether it is before or after a bend, etc. The amount of and depth of deposition will vary along the pipeline. Even at a fixed point, if there is a layer, it is likely to flow past in dunes so the depth will vary with time as well.

I am sorry, I think the answer to your question is "No one knows". ■