the roughness is very important because the interactions between flows and surface is reduced to a region very near to the surface(boundary layer). the boundary layer thickness and the average height of rugosities decide how the interaction occurs. it is not the same when rugosities emerge outside the boundary layer than when rugosities are embedded in the boundary layer.

The fundamental results about wall roughness definition and control were obtained by German Johann Nikuradse. He plotted dependences which connect wall roughness, Re number and Darcy friction factor from Darcy–Weisbach equation. These dependences take into account position of the rugosities in a boundary layer and very usefull. Please, search for "Nikuradse curves" and you will see the necessary graphs and criterion equations.