Full-scale tsunami-induced scour around a circular pile with three-dimensional seepage

Abstract

Seepage flows induced by geophysical tsunamis play a significant role in the tsunami boundary layer dynamics and associated sediment mobility. However, its impact on sediment transport and seabed morphodynamics in the presence of coastal structures remains unclear. In this study, we conduct rigid-bed and morphological simulations to investigate the role of seepage response in full-scale tsunami-induced flow features and sediment transport around a circular pile. The expressions for the onset threshold of sediment transport and the bed load motions are derived considering both bed-slope modifications and three-dimensional seepage forces, which are implemented in the coupled hydrodynamic, morphological, and soil models. The rigid-bed simulations demonstrate that the seabed suction response to the elevation wave can reduce the bed shear stress amplification underneath the contraction of streamlines alongside the pile and lee-wake vortices. The seabed injection response to the depression wave increases the stress amplification. It advances the position of boundary layer separation over the height of the pile, which further changes the lee-wake vortices. Note that the size of the horseshoe vortex and the upward-directed pressure gradient within it are less affected by the seepage flows. In morphological simulations, suspended load transport dominates the scour around the pile. Seabed suction during the elevation wave can slightly reduce the sediment transport rate, decreasing the scour depth, especially at the pile side. Seabed injection during the elevation wave causes exacerbated suspended load transport, leading to a more rapid and severe scour at the back of the pile. This study advances the understanding of seepage effects on tsunami-induced sediment transport and scour around a monopile foundation.