Many heterogeneous slurry flows can be modelled using the two-layer approach, wherein the particles' suspended weight is supported mostly by fluid turbulence in the upper portion of the pipe, and by inter-granular contact in the lower portion. The approach requires that the frictional force acting between the two layers be correctly evaluated. In early work, this friction was based on an equivalent roughness proportional to particle size. Recent experimental evidence indicates that for a wide range of flows, the interface between the two zones is several grain diameters in thickness, and the friction acting between the zones scales with this thickness, which is directly proportional to the shear stress. The shear-stress-dependent roughness allows calculation of more accurate values for the pressure gradient and the velocity at the limit of stationary deposition.