Stability of slope material affected by bow thrusters at open quay structures

Abstract

In the last decades shipping has increased significantly, resulting in larger ships with an increasing power in bow thrusters. Due to this increasing power, the hydraulic loads at bottom protection near quay structures has also increased, making a traditional bed protection more expensive. Therefore an alternative design method is suggested in which the bottom is not fully protected. As a result, scour could occur. The designer of the super structure should adapt his design on this scour. Consequently the total structure (bed protection and superstructure) could be designed more cost effective compared with the current design method of full bed protection. However, at this moment a method to predict scour at a sloping bed with piles affected by a bow thruster is not available, so it is impossible to apply the alternative design method. The objective of this master thesis is to set up a method to predict the equilibrium scour depth. In order to set up such a prediction method, a literature study was conducted. This literature study focused on the available knowledge of bow thruster induced currents and the effect of these currents on the instability and erosion of the bed material. The stability and mobility of the bed material depend on the ratio between flow forcing (flow velocities and turbulences) and the bed strength (particle size and density). When this ratio exceeds a critical value the bed is not stable anymore and transport will occur, leading to scour holes. Not only the thruster jet itself could induce scour formation, also the presence of a pile at the slope could lead to additional scouring. This is caused by the fact that a pile in a flow field causes differences in the flow field (local higher velocities and turbulence intensities). Consequently, extra scour is induced. This scour mechanism is called the pile obstruction mechanism. The total proposed equation consists of both the pile obstruction mechanism and the jet diffusion mechanism. The equation is validated for a horizontal bed. Due to absence of data for the sloping bed case the equation is not validated for the case of a slope yet. It is recommended to validate the proposed equation for the case of a slope with piles by conducting lab experiments.