The present study investigates local scour downstream of a sill followed by a rigid apron, due to a wall jet and a surface jet, related to the development over the apron of a hydraulic jump and an undular hydraulic jump, respectively. Eight laboratory experiments were run, for variable discharge and downstream water depth. A transition jump, which changes intermittently between a hydraulic jump and an undular hydraulic jump, was observed. The different characteristics of the scouring process due to different kinds of jumps are presented, together with the velocity fields inside the scour hole, due to both the surface jet and the wall jet. A stability analysis of the undular hydraulic jumps shows their intermittent behaviour. Agreement between the mathematical model and the laboratory results was found.
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