Calculating Fluidized bed height

Hello,
I am trying to simulate Fluidized bed with DEM model. But I was not able to calculate the fluidized bed height. From vtk, I was not able to get anything. I have attached the most basic 2D DEMM fluidized bed tutorial case.
Can anyone please help calculating the bed height?

fluid_bed_dem_2d-tutorial-case.zip (8.8 MB)