Toe erosion in front of the harbor structures due to ship's propeller action

Abstract

Many of sheet-pile bulkheads and gravity type quay structures of various harbors have failed because of the toe erosion due to ships' propeller during berthing/de-berthing operations. Toe erosion for quay walls is becoming more frequent because of the increase in the number of motor-ships and engine power. On the other hand, roll-on/roll-off ships always use the same berth and this may increase the scour depth. Once the local flow velocity (or bed shear stress) increases above the critical value for sediment movement, scour at the toe starts and will continue until the flow velocity (or shear stress) within the scour hole falls below to the threshold values. Velocities in propeller jets and from bow thrusters can easily be up to 10 m/s and more with resulting bed velocities from 3 to 4 m/s. These velocities can impinge directly on the seabed producing local erosion especially when the bed clearance is small. The prediction of the maximum depth of the scour in front of the harbor structures is an important design problem. In other words, the design of quay structures requires a special attention for the ultimate scour depth in front of such harbor structures. It is intended here to give methods to predict the velocities in the jet flow of main propellers and to estimate the scour dimensions.