I am modeling a vegetation patch. I need to assign roughness values separately for the bed and vegetation. When preparing the grid by multifunction grid generator, I assigned the roughness of the bed as 0.012. Then I add a polygon group for the vegetation patch and gave the roughness as 0.05. Once I execute the attributes mapping, the bed roughness automatically changes from my initial value (0.03). Why? Can anyone guide me on this? Thanks!