Observations in the field suggest that the existing guidelines for bed protection design may lead to overly conservative protections at berthing structures of inland vessels. In this study, physical scale model tests have been performed using particle image velocimetry to assess the near-bed velocity and the velocity decay of jets generated by transverse bow thrusters. The effect of the bed roughness, under-keel clearance (UKC) and quay wall clearance on the resulting near-bed velocities are discussed here. The results show that the maximum near-bed velocity occurs in the vicinity of the quay wall and results from an interplay between jet diffusion and the blockage effect of the nearby boundaries (e.g., quay wall, bed, and vessel), where the roughness of the confining boundary plays a role. The existing design guidelines predict fairly well the maximum near-bed velocity for smooth beds and for relatively large UKC's. However, the guidelines tend to overestimate the maximum near-bed velocities for rough beds and for small UKC's. This study proposes a new method based on measurement data to estimate the maximum near-bed velocity for confined bow thruster jets.

During berthing operations of large vessels, the bow thruster jet deflecting on the quay wall and the bed can lead to high flow velocities near the bed. This may scour the bed when it is left unprotected, causing instability of the adjacent quay wall. Due to the complex flow field of the reflected jet, the decay in near-bed flow velocities perpendicular to the quay wall is unknown. This results in uncertainties in the design of bed protections, especially in the required width.

In this research, the decay of the near-bed flow velocity perpendicular and parallel to the quay wall induced by a 4-channel bow thruster is studied. Field measurements have been conducted in the North Sea Port of Gent with one of the largest Dutch inland vessels. The near-bed flow velocities have been measured at multiple distances from the quay wall.

For the flow velocity measurements four main parameters have been varied: the applied bow thruster power, quay wall clearance, number of thrusters, and the lateral distance between jet axis and measurement sensors.

The highest flow velocities were measured near the quay wall, rapidly declining while moving away from the quay. Comparison of the measurement results to the Dutch and German guidelines generally leads to the conclusion that these guidelines are conservative. In addition, the dependency of the velocity on the total travelled distance by the jet as given in the Dutch method is not reflected in the measurement results. Furthermore, fundamentally different outcomes on the influence of the quay wall clearance on the near-bed flow velocity have been found.

When the measured near-bed flow velocities are used as the sole input to calculate the required bed protection, significantly smaller rock sizes and asphalt mattress thickness would be necessary to withstand the hydraulic load of the jet in comparison to current guidelines. Further studies with different vessels and direct measurement of the efflux velocity of the thrusters are recommended.