Numerical investigation of KC number effect on wave-induced scour around pile groups

Abstract Wave-induced local scour threatens the stability of pile groups in coastal and offshore engineering. Existing studies mainly focus on twin piles under low KC numbers, while the scour mechanism for complex multi-pile systems under high KC numbers remains unclear. In this study, a three-dimensional numerical model is established using REEF3D, which solves the RANS equations with k - ω turbulence closure and captures the free surface via the level set method. After validation, the model is applied to three pile-group configurations (2 × 1, 2 × 2, 2 × 4) with a fixed gap ratio of 1.8, covering a KC range of 6.24–63.96. The results show that local scour around pile groups is dominated by gap flow and wake vortices rather than downflow and horseshoe vortices, with the maximum scour always occurring at the first-row piles. The scour mechanism transitions at a critical KC of approximately 46.2: gap-jet dominant for KC < 46.2, and wake-vortex dominant for KC ≥ 46.2. Increasing the number of pile rows reduces the maximum scour depth, but the mitigation effect diminishes with each additional row. This study fills the research gap for high- KC conditions and provides a theoretical basis for the anti-scour design of coastal bridge pile-group foundations.