Experimental investigation of current-induced local scour around composite bucket foundation in silty sand

• A laboratory testing program is conducted to investigate local scour around composite bucket foundation.
• The steady and bidirectional scouring characteristics around composite bucket foundation is described in detail.
• Observations of comparison between steady current scour and bidirectional current scour are made.
• A general relationship for scour depth in terms of combined Shields number and non-dimensional numbers is suggested.

In recent years, local scour has received attention because it may have a negative effect on coastal structures. Because of the large-scale arc transition and other special constructs, flow patterns around composite bucket foundation are complicated, and the local scour around composite bucket foundation has rarely been reported in the literature. In the present study, a detailed laboratory testing program on model with diameters of 150 cm and 75 cm embedded in silty sand was conducted in a flume. The scouring process and equilibrium scour hole geometry under steady and bidirectional current were investigated in detail. A procedure was suggested to predict the ultimate scour depth based on the observed variation of the scour depth over a limited time period. The equilibrium scour depth under a bidirectional current was approximately 16% less than that under a steady current, and there were two spoon-shaped scour holes in the steady current scour, whereas four saddle-shaped scour holes were discovered in bidirectional current scour. Based on these results, a functional relationship was suggested between the scour depth and other parameters, such as the diameter of the foundation, Shields parameter and median diameter of the soil bed.