Solitary wave-induced 2D seepage effects on sediment incipient motion

Solitary wave theory has been extensively applied to describe wave motions in very shallow water. Previous studies on sand movement induced by solitary waves have typically neglected the effects of seepage flow. This paper examines the effects of two-dimensional (2D) seepage on the Shields number using a semi-analytical model. A modified Shields number, incorporating seepage scaling factors, is derived to assess the influence of seepage forces on sediment incipience. It is found that wave-induced seepage can cause the maximum Shields number to increase by approximately 43.8 %, thereby promoting the incipience of seabed sediment. However, if only the vertical direction is considered, the downward "injection" force would reduce the Shields number. Ignoring the horizontal components of seepage may lead to an underestimation of seepage effects, potentially resulting in an inaccurate evaluation of sediment incipient motion. Parametric studies reveal that, under conditions of greater wave height, water depth, saturation, permeability, and lower Young’ s modulus, the maximum Shields number would noticeably increase due to the combined effects of horizontal and vertical seepage forces. The primary limitation is that the present results apply only to the laminar and partially transitioning regimes; further research is required for strong turbulence (R_e > 5 × 10⁵).