Experimental investigation on local scour evolution around a circular monopile in the silty bed under wave-current combinations

Abstract

Silt is widely distributed in offshore regions where numerous marine structures are located. Compared to sandy bed, silt exhibits lower cohesion and consolidation characteristics, which can significantly influence sediment transport processes. However, there remains a lack of research on local scour and its temporal evolution around circular monopiles in silty beds. This study presents an experimental investigation of equilibrium scour depth and scour process around a slender circular pile in a silty bed ($d_{50}$ = 0.075 mm, ${\rho }_d$ = 1.64 g/cm³) under steady current and wave-current combined conditions. The normalized scour depth and the dimensionless scour time scale $T^{\ast }$ in the silty bed were compared with those in sandy beds under corresponding hydrodynamic conditions, and the effects of parameters e.g. relative water depth, Keulegan–Carpenter (KC) number, relative current strength $U_{cw}$ and Shields parameter were analyzed. Results show that the maximum scour depth around a monopile in the silty bed primarily occurs at the pile sides. Wave superposition significantly influences the scour hole size and the slope angle. Under steady current, the normalized scour depth of the silty bed is generally lower than that of sandy beds and increases with the relative Shields parameter and relative water depth. Conversely, $T^{\ast }$ in the silty bed is significantly larger than that of the sandy bed and decreases with increasing relative Shields parameter and decreasing relative water depth. Under combined wave-current conditions, the scour depth in the silty bed exhibits a non-monotonic increase-decrease trend with the increase of $U_{cw}$, peaking at $U_{cw}$ = 0.8–0.9. For a given $U_{cw}$, due to weaker inter-particle interlocking force, larger sediment transport capacity and reduced bedload supply applied in this study, partial scour depths in the silty bed exceed those in sandy beds with larger KC numbers. $T^{\ast }$ in the silty bed is approximately 10–100 times greater than that of sandy beds under similar hydrodynamic conditions. As $U_{cw}$ and wave-induced Shields parameter increase, $T^{\ast }$ shows a decreasing trend. The effects of $U_{cw}$ and wave-induced Shields parameter on $T^{\ast }$ gradually diminishes with increasing combined wave-current intensity. New empirical equations with improved prediction accuracy were proposed for estimating scour depth, scour hole sizes and scour time scale in the silty bed.