Comparative Study on Scour Protection Methods for Pile Group Foundations Under Wave–Current Coupling

Wave–current coupling significantly increases the vulnerability of underwater pile foundations to scour, making effective protection measures essential for waterborne structures. In this study, a group pile of a sea-crossing bridge in Zhejiang province, China, was selected as the research object, and fifteen scour simulations were conducted using FLOW-3D v11.2 to investigate the effects of water depth, flow velocity, and wave action on the scour characteristics of pile groups. Furthermore, the performance of two protection strategies (riprap and sacrificial piles) was comparatively analyzed. The results showed that the presence of waves altered the scour pattern, while variations in water depth influenced the scour morphology through changes in the downflow structure. Under wave–current coupling, variations in flow velocity affected the distribution of the scour depth profile. Without protection, the maximum scour occurred between the second and third rows of upstream piles, with an average scour depth exceeding 7 m. When protection measures were applied, square riprap exhibited a superior performance compared to elliptical riprap. A 2 m thick square riprap layer effectively reduced the maximum scour depth by 89%. Sacrificial piles alone provided limited protection, reducing the maximum scour depth by 25%. These findings provide valuable insights for optimizing scour protection design under complex wave–current conditions.