The effect of bottom roughness in two-layer flows down a slope

Laboratory experiments were conducted to study the effect of locally enhanced bottom roughness in a density-stratified two-layer flow down a slope. Three bottom roughness configurations were investigated, a smooth bed and two artificially roughened beds, employing sparse and dense roughness elements, respectively. The bottom roughness elements have shown to affect differently the generation and collapse mechanisms of large-scale two-dimensional structures observed at the interface between the two layers: dense bottom roughness inhibits the generation of large-scale structures while sparse bottom roughness inhibits their collapsing mechanism. Both bottom roughness configurations cause a reduction of the size of the large structures at the interface as compared to the smooth case. Two main sources of entrainment have been identified, namely the observed large-scale structures at the interface and enhanced bottom roughness. Sparse bottom roughness gives the lowest entrainment coefficients among the three cases due to the low interaction between the bottom boundary layer and the interface.