Ship propellers cause high flow velocities near quay walls, jetties, and locks, which can lead to scouring of the bottom of these structures and potentially cause their instability. Therefore, the design of adequate bed protection has become a crucial factor. When designing bed protection, existing guidelines may not always result in optimal designs due to simplifications in jet-flow schematization and uncertainty of propeller-induced loads. To improve design guidelines and optimize bed protection, a joint research programme was initiated aiming at developing knowledge on propeller jet-induced scour by combining field measurements, scale model tests, and numerical modelling. The stability of erosion protection systems depends both on the ability to prevent soil loss underneath, as well as to maintain their integrity under the effects of hydrodynamic forces. Typical stabilization methods consist of penetration of rock with concrete, or using asphalt mattresses, which can become a costly and not so environmentally friendly solution. As a greener alternative, rock-filled gabion mattresses could be an attractive protection measure; this solution has the advantage of not requiring penetration of concrete/asphalt for stabilization since rocks are kept together in a wire cage (mattress). Rock-filled gabion mattresses have always represented a cost-effective measure for hydraulic engineering applications and have technologically improved over the years to meet the growing needs of the market. More recently, a special prefilled gabion mattress specifically designed to withstand turbulent actions caused by jet propellers has been developed. In this context, a physical scale model was built to investigate the flow patterns generated by confined propeller jets. A bed protection system by mattresses was tested in the scale model as an alternative to traditional bed protection methods, against the actions generated by one or two simultaneously activated thrusters. Flow velocities near the bed protected with rock-filled gabion mattresses were measured and compared with the velocities above other types of bed protection typically considered in the field, focusing on assessing the effect of the main relevant parameters on the magnitude and location of the maximum flow velocity near the bed. In addition, deformation tests were performed to investigate the effectiveness of the special prefilled gabion mattresses as a bed protection system, particularly in preventing scour caused by bow thrusters along quay-walls. The aim of the research campaign was to validate a novel interpretative model proposed for the design of special pre-filled gabion mattresses, introducing new and more complete resistance parameters. The deformation tests have shown that the specially designed prefilled gabion mattress bed protection system is effective in preventing scour near the quay-walls. The highest flow velocities were observed at the connection between the mattresses and the quay-wall; therefore, attention should be paid to this connection when designing bed protection system. The flow patterns generated by the bow thruster above the tested bed protection system were in line with the expectations for an intermediate bed roughness, between smooth and typical riprap grading. The physical model tests led to the validation of the new and more comprehensive approach for the design of special pre-filled gabion mattresses against jet propellers action.

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