**1. Introduction**

During the berthing and deberthing processes, the quay structure is often present in close proximity to the ship propeller, and the resulting local scour hole that forms the base of the quay wall is a growing concern, as it may cause structural instability or even failure. The Permanent International Association of Navigation Congresses [1] reported that ship propeller-induced jet flow has been recognized as the main cause of scour around the quay structures. With reference to the impact of propeller jets, Sumer and Fredsøe [2] stated that the quay structures can be categorized into two principal types, closed and open quays. The former is characterized by a vertical solid wall that is constructed in the berth front to resist the horizontal load from the landfill and any other live loads. The latter, on the other hand, consists of a slope foundation, above which the quay slab is supported by a group of piles, columns or lamellar walls.

The scour problems caused by propeller jets in both types of quay have received extensive attention over the past decades. For the closed quay, Stewart [3] and Hamill et al. [4] systematically investigated the scouring action related to a stern propeller whose axis is perpendicular to the quay wall. Their results suggested that the maximum scour depth was significantly augmented when compared to that in the absence of any quay structures. Furthermore, they reported that the maximum scour depth exhibited a monotonically decreasing trend with the increasing wall clearance (=the longitudinal distance between the propeller and quay wall). Following a similar experimental configuration to that of Stewart [3] and Hamill et al. [4], Ryan [5] further extended their work by introducing the effect of a rudder in his study. As for the open quay, Sleigh [6] and Cihan et al. [7] experimentally investigated propeller-induced scour around an open quay with the focus on erosion on the quay slope itself. More recently, Wei and Chiew [8] and Wei et al. [9] examined the characteristics of the local scour hole around the toe of the quay slope, showing that the maximum scour depth first increases and then decreases with increasing toe clearance (=longitudinal distance between propeller and slope toe) until the quay effect becomes insignificant. To date, various empirical equations have been proposed to determine the maximum scour depth for both the closed and open quays. Although of important practical use, these studies have shed limited light on the underlying scouring mechanisms due to a lack of detailed flow field data within the scour hole, which is crucial to understanding the effect of the flow structure and the type of quay on the development of the scour.

In the case of the open quay, Wei and Chiew [10] made the first attempt at investigating the mean flow and turbulence characteristics within the developing scour hole, in which the scouring mechanism was found to be subjected to the jet diffusion and quay obstruction effects. Their relative dominance, which is dependent on the magnitude of the toe clearance, dictates the characteristics of the final scour hole. For the closed quay, Wei and Chiew [11] detailed the flow properties associated with the impingement behavior between the propeller jet and a vertical quay wall but in the absence of an erodible sand bed. Their results evidence the complex flow nature of the impinging jet. Thus far, few, if any, studies have been carried out to quantify how an impinging propeller jet flow induces the local scour hole around a vertical quay wall, and therefore a comprehensive understanding of the underlying physics remains elusive. For this reason, the main objective of the current study is to furnish a complementary investigation on this subject, which is of grea<sup>t</sup> importance for both the fundamental understanding and potential practical application to scour protection.

In what follows, the experimental setup and methodology are first introduced. Then, the asymptotic scour profiles and the associated flow patterns are discussed with four different wall clearances. Moreover, the temporal development of the vortex system within the developing scour hole is presented, together with a comparison of the circulation associated with each individual vortex. Furthermore, an energy spectra analysis of the time series of velocity fluctuations is conducted to examine the turbulent energy dissipation and its implication for the scouring process. Finally, near-bed flow characteristics are also discussed in terms of their correlation with the scouring development.
