Uncontrolled overtopping during flood events can endanger embankment dams. Erosion of the downstream slope and scouring of its toe due to the high energy of the overflow can lead to breach formation and complete failure. Solution to overcome these dangerous overflows lies in overlays which dissipate flow energy along the downstream embankment slope. In this context, an experimental research of flow on different systems of macro-roughness overlays for 18.6 degrees and 30 degrees stepped dam slopes was conducted. Aerated flow characteristics (air entrainment, depth, velocity) were measured for the three nappe, transition and flow regimes. Results highlight the notion of drag form structures, effective roughness and homogeneous aerated flow. They also stress the different definitions of flow depths. Finally a set of empirical laws to describe the hydraulic behaviour of aerated turbulent flow over macro-roughness could be derived. They can be used as a basis for the design of an optimal protection overlay for embankment dams in order to allow controlled overtopping during floods.