Reliability evaluation of different procedures and techniques for measuring the scouring process induced by a rotating ship propeller

The scour hole and the deposition mound are the two main effects caused by the impact of a ship propeller jet flow on a mobile bed: the former can induce the instability of the quay structure in the harbor basins, whereas the latter can compromise the ship maneuvers. In recent years, several researchers focused their studies on the propeller scouring process both in the presence and in the absence of a vertical quay wall in front of the propeller face. The temporal development of the bed longitudinal profile in correspondence of the propeller axis was studied extensively. A common practice during the laboratory experimental tests is switching the propeller rotation on and off at each bed elevation measurement, overlooking the effects of this kind of practice on the scour hole and deposition mound formation. This paper shows why these effects are not negligible: albeit the scour hole does not appear deeper than that formed leaving the propeller switched on during the bed elevation measurements, it becomes more elongated and shifted toward downstream, with a reduced deposition height.